FORMULATION AND METHOD FOR INHIBITING CARBON-BASED DEPOSITS

Abstract

There is a formulation and a method for inhibiting carbon-based deposits on metal substrate. The method comprises the use of a formulation comprising at least one oxidizing agent and at least one etchant capable of forming free metal ions from the metal substrate, at least one sequestering agent having a ligand capable of forming a complex with the free metal ions and at least one chelating agent having a ligand capable of complexing with at least one surface metal atom.

Claims

1. A formulation for inhibiting carbon-based deposits on a metal substrate, the formulation comprising at least one oxidizing agent and at least one etchant to form free metal ions from the metal substrate, at least one sequestering agent having a ligand selected to form a complex with the free metal ions and at least one chelating agent having a ligand selected to complex with at least one surface metal atom, wherein the at least one etchant comprises 50 to 70 wt % of the formulation, based on the weight of the formulation, the at least one etchant being an acid selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, trifluoromethanesulfonic acid, fluorosulfuric acid, trichloroacetic acid, trifluoroacetic acid and combination thereof.

2. The formulation according to claim 1, wherein the weight ratio of the at least one oxidizing agent to the at least one etchant is 0.2:2 to 0.2:3.

3. The formulation according to claim 1, wherein the weight ratio of the at least one etchant to the at least one sequestering agent is 5:4 to 5.5:6.

4. The formulation according to claim 1, wherein the weight ratio of the at least one sequestering agent to the at least one chelating agent is 4:0.1 to 5:0.3.

5. The formulation according to claim 1, wherein the at least one oxidizing agent comprises 10 to 15 wt % of the formulation, based on the weight of the formulation.

6. The formulation according to claim 1, wherein the at least one oxidizing agent is selected from the group consisting of peroxides, peroxyacids, peroxyesters, permanganates, mineral acids, nitrates, persulphates, perborates, perchlorates, perchlorites, chlorates, chlorites, percarbonates, derivatives thereof and combinations thereof.

7. The formulation according to claim 1, wherein the at least one sequestering agent comprises 20 to 25 wt % of the formulation, based on the weight of the formulation; and wherein the at least one sequestering agent is an amino acid selected from the group consisting of glycine, threonine, aspartic acid, glutamic acid and serine.

8-21. (canceled)

22. The formulation according to claim 1, wherein the at least one sequestering agent is a hydroxycarboxylic acid selected from the group consisting of malic acid, gluconic acid, citric acid, tartaric acid, glycolic acid and lactic acid.

23. The formulation according to claim 1, wherein the at least one chelating agent comprises 0.5 to 5 wt % of the formulation, based on the weight of the formulation.

24. The formulation according to claim 1, wherein the at least one chelating agent is selected from the group consisting of ketone, phosphonic acid, sulfonic acid and an optionally substituted heterocyclic compound having at least 3 heteroatoms.

25. A method for inhibiting carbon-based deposits on a metal substrate, said method comprising: contacting said metal substrate with a formulation to inhibit carbon-based deposits on said metal substrate, wherein said formulation comprises at least one oxidizing agent and at least one etchant to form free metal ions from the metal substrate, a sequestering agent having a ligand selected to form a complex with the free metal ions and at least one chelating agent having a ligand selected to complex with at least one surface metal atom, wherein the at least one etchant comprises 50 to 70 wt % of the formulation, based on the weight of the formulation, the at least one etchant being an acid selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, trifluoromethanesulfonic acid, fluorosulfuric acid, trichloroacetic acid, trifluoroacetic acid and combination thereof.

26. The method according to claim 25, wherein the weight ratio of the at least one oxidizing agent to the at least one etchant is 0.2:2 to 0.2:3.

27. The method according to claim 25, wherein the weight ratio of the at least one etchant to the at least one sequestering agent is 5:4 to 5:6.

28. The method according to claim 25, wherein the weight ratio of the at least one sequestering agent to the at least one chelating agent is 4:0.1 to 5:0.3

29. The method according to claim 25, wherein the contacting operation is carried out at a pressure between 101,325 Pa to 506,625 Pa.

30. The metal according to claim 25, wherein the free metal ion is selected from the group consisting of Fe.sup.2+, Fe.sup.3+, Cr.sup.3+, Ti.sup.+, Ti.sup.2+, W.sup.+ and W.sup.2+.

31. The method according to claim 25, wherein the at least one surface metal atom is selected from the group consisting of Fe, Cr, Ni, Ti and W.

32. The method according to claim 25, wherein the surface metal atom is a surface metal ion selected from the group consisting of Fe.sup.2+, Cr.sup.3+, Ni.sup.+, Ti.sup.+, Ti.sup.2+, W.sup.+ and W.sup.2+.

33. A metal pipe treated with a formulation for inhibiting carbon-based deposits on a metal substrate, the formulation comprising at least one oxidizing agent and at least one etchant to form free metal ions from the metal substrate, at least one sequestering agent having a ligand selected to form a complex with the free metal ions and at least one chelating agent having a ligand selected to complex with at least one surface metal atom, wherein the at least one etchant comprises 50 to 70 wt % of the formulation, based on the weight of the formulation, the at least one etchant being an acid selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, trifluoromethanesulfonic acid, fluorosulfuric acid, trichloroacetic acid, trifluoroacetic acid and combination thereof.

34. The formulation according to claim 1, wherein the at least one etchant comprises 50 to 70 wt % of the formulation, the at least one oxidizing comprises 10 to 15 wt % of the formulation, the at least one sequestering agent comprises 20 to 25 wt % of the formulation and the at least one chelating agent comprises 0.5 to 1.5 wt % of the formulation, wherein the weight percentages add up to 100 wt % of the formulation.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0115] The accompanying drawings illustrate the disclosed embodiments and serve to explain the principles of the disclosed embodiments. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the present disclosure.

[0116] FIG. 1

[0117] [FIG. 1a] is a schematic drawing showing the carbon-based deposits accumulating in the conduit of a pipe or tube.

[0118] [FIG. 1b] is a photograph showing internal deposits that choke up a process part used in the oil industry such that the process part has to be shut down for unscheduled maintenance.

[0119] FIG. 2

[0120] [FIG. 2a] shows a flow chart depicting the present method for making the present formulation.

[0121] [FIG. 2b] shows how a surface defect on a metal substrate causes or encourages carbon-based deposits accumulation.

[0122] [FIG. 2c] shows another mechanism by which carbon-based deposits can build up on the surface of a metal substrate.

[0123] [FIG. 2d] is an SEM image at a magnification of ×500 showing the carbon-based deposits built-up on a spent pipe.

[0124] FIG. 3

[0125] [FIG. 3a] shows the experimental setup for treating a targeted pipeline.

[0126] [FIG. 3b] compares an original and treated surface of a metal substrate

[0127] FIG. 4

[0128] [FIG. 4] illustrates the generation of turbulent heat flux due to ditch-like pattern (see accompanying scanning electron microscope (SEM)) on the polished interior surface of a SS321 tube.

[0129] FIG. 5

[0130] [FIG. 5] shows a process flow summary of the present formulation treatment (chemical polishing) protocol.

[0131] FIG. 6

[0132] [FIG. 6] shows the general experimental setup for an oil encapsulation test.

[0133] FIG. 7

[0134] [FIG. 7a] is a schematic diagram showing the possible formation of stable 5- or 6-member ring with iron species.

[0135] [FIG. 7b] shows a possible formation of a cap-structure with the surface iron species.

[0136] FIG. 8

[0137] [FIG. 8a] is a SEM image at a magnification of ×2,200 showing the interior surface of an untreated SS321 tube.

[0138] [FIG. 8b] is a SEM image at a magnification of ×2,000 showing the polished surface of a SS321 tube using LHC treated for 30 minutes at 25° C.

[0139] [FIG. 8c] is a SEM image at a magnification of ×2,200 showing the polished surface of a SS321 tube using LHC and etidronic acid treated for 30 minutes at 25 ° C.

[0140] [FIG. 8d] is a SEM image at a magnification of ×2,200 showing the polished surface of a SS321 tube using LHC and NDSA treated for 30 minutes at 25 ° C.

[0141] FIG. 9

[0142] [FIG. 9a] is a SEM image at a magnification of ×2,200 showing the treated SS321 surfaces using SVA solution.

[0143] [FIG. 9b] is a SEM image at a magnification of ×2,200 showing the treated SSS321 surfaces using SHP solution.

[0144] FIG. 10

[0145] [FIG. 10a] is a photograph showing the degradation of aerospace lubricant with extended heating.

[0146] [FIG. 10b] is an optical microscopic image of the used oil (at a scale of 100 um).

[0147] [FIG. 10c] is an optical microscopic image of the heated oil from a polished tube (at a scale of 100 um).

[0148] [FIG. 10d] is an optical microscopic image of the heated oil from the crude tube (at a scale of 100 um).

[0149] FIG. 11

[0150] [FIG. 11a] is a SEM image at a magnification of ×2,200 showing a crude surface exposed to oil for 60 hours at 300° C.

[0151] [FIG. 11b] is a SEM image at a magnification of ×2,200 showing a LHC-polished surface exposed to oil for 60 hours at 300° C.

[0152] [FIG. 11c] is a photograph showing the photos of two sample tubes with the top photo corresponding to that of FIG. 11a and bottom photo corresponding to that of FIG. 11b.

[0153] FIG. 12

[0154] [FIG. 12a] is a SEM image at a magnification of ×2,200 showing the coking extent on a LHC(etidronic) polished surface.

[0155] [FIG. 12b] is a SEM image at a magnification of ×2,200 showing the coking extents on a SHP-polished surface.

[0156] FIG. 13

[0157] [FIG. 13a] is a SEM image at a magnification of ×2,200 showing the coking extent of the crude untreated SS321 tube after a five cycle heating process.

[0158] [FIG. 13b] is a SEM image at a magnification of ×2,200 showing the coking extent of the LHC-polished SS321 tube after a five cycle heating process.

[0159] [FIG. 13c] is a photography comparing the coking extent of the crude untreated SS321 tube (left) and the LHC-polished SS321 tube (right) after a five cycle heating process.

[0160] FIG. 14

[0161] [FIG. 14] is a graph of the Raman spectroscopy for carbon-based deposition on a SS321 pipe.

DETAILED DESCRIPTION OF DRAWINGS

[0162] FIG. 1a shows a typical process pipe in which a process fluid is channeled through. In FIG. 1a, oil 100 and 106 is flowed through the conduit of a process pipe. As the oil 100 and 106 flows through, hydrocarbon materials or carbon-based deposits present in the oil 104 may agglomerate and accumulate on the surface of the metal pipe. The formation of the carbon-based deposits may be caused by the heat soak 102 that is produced from residual heat or direct process heating. This heat soak may be at a temperature of 232° C. or 450° F.

[0163] FIG. 2a is a flow chart depicting a method for preparation of the present formulation, treating a metal substrate and characterizing the results of this treatment. The first step 200 begins by preparing the present formulation (chemical polishing solution). This solution is prepared by mixing the four components and optionally a stabilizer as described above (also see example 1). Subsequently, this solution is used 201 to treat a pipe (e.g. a metal pipe) by filing and contacting the pipe with the solution in the second step 210. The entire pipe may be immersed in the solution.

[0164] In the next step 220, characterization 202 of the treatment is carried out using an oil encapsulation test, a microscopic examination of the topography or morphology of the treated surface of the pipe and an analysis of the used oil sample.

[0165] FIG. 2b shows how surface defects 232 of a metal substrate encourage carbon-based deposits to accumulate on the surface 230. Surface defects 232 are the sites where carbon-based deposits can get trapped. As the initial carbon-based particles get trapped at these surface defects 232, the carbon-based deposits at these sites agglomerate and grow 234. Gradually, the agglomerates turns into a coke clog 236 and the coke clog continues to further grow 238 due to severe eddy flow which carries more carbon-based particles that are susceptible to being trapped by the grown clogs 236. FIG. 2d shows how the coke developed on a surface due to defects in a spent pipe. The broken arrow shows an area of heavy coke build up.

[0166] FIG. 2c shows another mechanism by which carbon-based deposits can form on the surface of a metal substrate. An oil molecule 520 present in a hydrocarbon fluid flowing through the conduit of a pipe in close proximity to the surface of a metal pipe 500 is shown. As more oil flows along 502 under heating, the oil molecule becomes cross-linked with other oil molecules 522. As the process continues 504 under extended heating, the cross-linked oil molecules grow into a coke embryo 524. As more of such coke embryos form 506, they flocculate together 526. The flocculated coke embryos 526 then coalesced together 528 in step 508. The coalesced coke embryos 528 then settle and deposit on the surface of the untreated metal pipe via sedimentation 510.

[0167] In FIG. 3a, an experimental setup of how the internal surface of a targeted pipeline can be treated is shown. The prepared formulation is fed through the inlet pipe into a buffer tank 4 containing the formulation (chemical polishing solution). This buffer tank 4 serves as a feed tank to downstream units and also for temporary storage purpose just in case of an over-feed. When the pipeline to be treated is positioned within the ultrasonification chamber chamber, the liquid pump 3 is switched on to channel the formulation into the pipeline. Between the liquid pump 3 and the ultrasonification chamber, a flow meter 2 and a flow control valve 1 is installed to regulate and monitor the fluid flow. Together, these devices may be use to control the contact time between the formulation and the pipeline. The formulation is then passed out of pipeline into a spent solution tank 5 which collects the used formulation for possible recycling or regeneration. A flow control valve (not shown) can be fitted between the outlet of the pipeline and the spent solution tank for the same purpose as mentioned above.

[0168] FIG. 3b generally illustrates how an original rough surface 310 of a metal substrate is modified 300 to have a new topography 320 after being treated with the present formulation (also called chemical polishing). The original rough surface 310 can be attributed to the original production methods of the pipe or tube, such as cold drawing.

[0169] FIG. 4 illustrates how the turbulent heat flux 410 is generated due the ditch-like pattern on the polished interior surface of a treated SS321 tube 400 that is exposed to a heat source 420. The accompanying SEM image shows the ditch-like pattern on the surface of the treated SS321 tube.

[0170] A summary of the formulation treatment (chemical polishing protocol) is shown in FIG. 5. Firstly, a SS321 tube is weighed and cleansed by rinsing with acetone in step 400. This ensures that the formulation is contacted with the surface of the tube rather than any unwanted debris that may be present on the surface. The tube is then further rinsed 402 with water and dried completely in step 410. The cleansed tube is soaked and contacted 404 with the present formulation (polished using the present formulation) at room temperature in step 420. After treatment 406, the tube is then rinsed and dried in step 430. The tube is then weighed and a weight loss of less than 8.5% is measured.

[0171] An oil encapsulation test is shown in FIG. 6. The oil is encapsulated in a SS321 stainless steel tube 600 and metal nuts 602 are used to seal both ends of the tube. The sealed tube 600, 602 is left in a furnace 610 to be exposed to a heat soak of about 300° C. The adjacent figures show the dissected tube for visual examination and the sealed tube for the present oil encapsulation testing. This testing is used to simulate the real situation where degradation of lubricant oil and coking happens during industrial processes. After being heated for a length of time, the metal tube is being dissected for visual examination and further analysis.

EXAMPLES

[0172] Non-limiting examples of the present disclosure and comparative examples will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the present disclosure.

Example 1

Preparation of the Formulation and Treatment Protocol

[0173] In oil industry, internal coke deposits in tubes are a common problem as they restrict flow to the extent where processes shut down. This is illustrated in FIG. 1a and FIG. 1b.

[0174] FIG. 2a shows the general flow chart describing the steps of the disclosed method and starts with the formulation of the chemical solution, followed by carrying out chemical polishing (CP). This is done by filling the pipe (or tube) with the formulation and the method finishes off by characterizing the treated pipe via an oil encapsulation test, a microscopic examination and an analysis of the used oil samples. This has been described above under detailed description of FIG. 6.

[0175] FIG. 3a provides a schematic drawing of the experimental setup. The experimental setup contains the inlet for the chemical polishing solution, a flow control valve, a flow meter, a liquid pump, the pipeline being treated with the chemical polishing solution and a vessel for the outlet.

[0176] The present disclosure shows the unexpected discovery of a modified microstructured surface obtained by the present formulation and method. It displays distinct coke-resistant capability by stimulating an eddy flow field over the interior surface because the turbulence facilitates agitation and hence reduces adsorption to the surface as well as thermal stress on the decomposition of oil molecules. The eddy fluid field is presumably induced by the turbulent heat flux that is formed when coming through such an intricate interior surface (FIG. 4). The coke-resistant effect was demonstrated by heating the selected aerospace lubricant oil encapsulated in the SS321 tube for a designed length of time. In short, this is simple and easily scalable and hence could be employed in many industries.

[0177] FIG. 5 illustrates an outline of the chemical polishing protocol, using an example a SS321 tube. Starting with a cleaning step using acetone, the tube is then rinsed with water and dried completely. The tube is then soaked in the chemical polishing solution comprising a mineral acid, an oxidant, a sequestering agent, a chelating agent and an optional stabilizer at room temperature. The weight loss after this cleaning step is less than 8.5%. The composition of the sample SS321 tube is described below in table 1b. The tube contains an alloy of carbon, manganese, phosphorous, sulphur, silicon, chromium, nickel, molybdenum, copper, nitrogen and a titanium complex. The specific values can be found according to the ASTM specification.

[0178] FIG. 6 shows a schematic drawing of an oil encapsulation test. The sample tube encases the oil and is sealed on both ends with a metal nut. It is being brought into a furnace and heated for a certain length of time, afterwards the metal tube is sectioned for examination.

[0179] Preparation of the Formulation (Chemical Polishing Solution)

[0180] The formulation (chemical polishing solution) used in the present disclosure invention is water based. The components consist of an oxidizing agent, an etching agent, an ion-sequestering agent or coordination ligand or/and a stabilizing agent (e.g. 1,5-naphthalene disulfonic acid and 5-aminotetrazole). The oxidizing agent is peroxodisulfuric acid (H.sub.2S.sub.3O.sub.2SO.sub.3) or hydrogen peroxide (H.sub.2O.sub.2). A mineral acid, such as HCl, HNO.sub.3 and H.sub.2SO.sub.4, or their combination or a superacid, such as CF.sub.3SO.sub.3H and HSO.sub.3F, is employed as an etching reagent. The ion sequestering agent is selected from highly water-soluble organic hydroxycarboxylic acids and amino acids, such as malic acid, gluconic acid, citric acid, tartaric acid, lactic acid, glycine, serine, etc. of which lactic acid is preferred because it gives the most desired anti-coking surface. This is because the chemical polishing itself is a cathodic reaction (M.fwdarw.M.sup.n+), the surface of substrate carries a layer of strongly adsorbed metal ions. Sequestering these metal ions from the surface is required for continuing the reaction. The metal ions adsorbed on the highly reactive surface sites, such as those described by the terrace ledge kink (TLK) model, are more difficult to be removed and hence an appropriate sequestering agent is required. However, with the use of a chelating agent that could form stable 5- or 6-membrane rings with iron species (FIG. 7a), these reactive sites could be protected from being polished and hence the ridges on the polished surface could be better reserved.

[0181] Similarly, a stabilizer defined as an amphiphilic compound with a planar shape, such as anionic 1,5-naphthalene disulfonic acid (NDSA) and cationic benzotriazole, was included to integrate with or substitute the chelating agent through the formation of the cap-structure with the surface iron species (FIG. 7b). The cap-structure could render both projecting and concave domains because the amphiphilic nature of the stabilizer favours aggregation on the metal surface. So polishing would be more significant in areas that are less covered by the stabilizer. The typical composition of the chemical polishing formulation for the present application is listed in Table 1a. It may be emphasized that as illustrated in FIGS. 3b and 4, the present formulation does not aim to achieve a smooth surface but rather to develop a microstructure that is capable in prompting turbulent heat flux as the medium between heat source and oil. This is further shown with a schematic drawing in FIG. 3b, wherein a rough surface is left behind by cold work and the new topography is created after chemical polishing.

TABLE-US-00001 TABLE 1a Composition of the lactic-hydrogen peroxide-hydrochloric acid (LHC) chemical formulation (polishing solution) Composition Amount Hydrogen peroxide solution (20 wt %) 3 mL Hydrochloric solution (37 wt %) 9 mL Lactic acid 5 mL Etidronic acid* 0.207 g Stabilizer** ~0.1-0.2 g The ‘*’ next to etidronic acid means that another suitable chelating agent is 1,5-naphthalene disulfonic acid (NDSA) while the ‘**’ means that the any organic compounds that are capable of forming a cap-structure with iron species on the surface of a metal substrate can be used as the stablizer.

[0182] The roles of these four different components are explained as follow. The etchant (e.g. hydrochloric acid (HCl)) removes surface rust (Fe.sub.2O.sub.3) from a metal or solid substrate. It may also capable of removing the iron component from a metal substrate (e.g. stainless steel surface). These may be illustrated by equations (1) and (2) below.

6H.sup.++Fe.sub.2O.sub.3.fwdarw.2Fe.sup.3++3H.sub.2O   (1)

2Fe+2H.sup.+.fwdarw.2Fe.sup.2++H.sub.2   (2)

[0183] Meanwhile, the role of the oxidizing agent (e.g. hydrogen peroxide) serves to convert ferrous ion (Fe.sup.2+) to ferric ion (Fe.sup.3+) (see equation (3) below) since ferric ion has a larger reaction equilibrium constant to combine with a sequestering agent (e.g. lactic acid) compared to ferrous ion. The formation of a ferric ion-lactic acid complex (see equation (4)) may be needed to stabilize iron in the aqueous phase. If not, the ferrous ion may undergo hydrolysis to form Fe(OH).sub.2 precipitate when the concentration of free ferrous ions reaches a sufficient concentration for this to occur. If the precipitates form, the substrate may become coated with the precipitated Fe(OH).sub.2 and the surface may not be treated with the present formulation effectively.

2Fe.sup.2++H.sub.2O.sub.2+2H.sup.+.fwdarw.2Fe.sup.3++2H.sub.2O   (3)

Fe.sup.3++xL.fwdarw.[FeL.sub.x].sup.3+  (4)

where L is lactic acid, acting as sequestering agent and x is about 1.5.

[0184] Lastly, the chelating agent (e.g. etidronic acid molecules) functions as an adsorbate by chelating or forming a ligand with the surface Fe atoms at morphologically protruded locations (see equation (5)). This may help to foster the development of the desired microstrucure as shown in the figures, thereby restructuring the surface of the metal or solid substrate to attain the above advantages.

Fe+E.fwdarw.Fe⊂E   (5)

where E is etidronic acid, acting as chelating agent.

[0185] In one particular instance, the respective amounts of these four components may be H.sub.2O.sub.2 (3 ml)/HCl (9 ml)/lactic acid (5 ml)/etidronic acid (0.21 g). These amounts may be applicable for treating a 5 cm.sup.2 surface. Particularly, the dose of the chelating agent (e.g. etidronic acid) may affect the surface topography. For example, an increase in etidronic acid from a dose of 0.21 g (0.78 wt %) to 0.4 g (1.5 wt %) may bring about the highest concentration of ridges and grain edges or highest average surface roughness. This may become counterproductive and heavy coking may occur on the surface on the metal or solid substrate since there is likely to be more protruding surfaces that may lead to the deposition of particulates.

[0186] In another instance, if the dose of the etchant (e.g. HCl) is too high, over-polishing or severe etching may occur which may lead to the same deleterious effect. It should be noted that the dose of the oxidizing agent (e.g. H.sub.2O.sub.2) may have to follow that of the etchant in order to transform ferrous to ferric ions as explained above. Lastly, the dose of the chelating agent (e.g. lactic acid) may have to be sufficient such that the amount present in the formulation suffices to chelate with the Fe.sup.3+ ion produced.

[0187] Observably, the etidronic acid may exert a significant influence on the formation of the microstructures of the treated surface but lactic acid may not. This phenomenon may be attributed to the different chelating selectivity between these two ligands. Etidronic acid may selectively adsorb on ridges and grain edges via forming chelates with Fe while lactic acid may form chelates with Fe.sup.3+ in the bulk of solution. Hence, it is possible that the chelating selectivity depends on, for instance, the size and charge between the ligand and the central metal, the Gibbs free energy released from coordination, the pH of the reaction medium and the solubility of the component used. Theoretically, in this particular system, the chelating agent must be able to function in a strong acidic medium with pH less than or equal to zero.

[0188] Accordingly, the use of the present formulation to treat a metal substrate is namely a cathodic reaction. An example of the cathodic reaction for a stainless steel alloy when reacted with the present formulation may be shown below.

Fe(Cr, Ni).fwdarw.Fe.sup.2/3++Cr.sup.3++Ni.sup.2+

[0189] An example of the chelating complex is shown below. Observably, the chelating agent is a ligand capable of forming a ring structure that encapsulates the metal atom or metal ion. The metal atom or ion in this instance is nickel.

##STR00001##

Ligand (Chelating Agent) Complex

[0190] Composition of a SS321 Metal Substrate

[0191] The composition of a stainless steel grade SS321 pipe is shown in table 1b below.

TABLE-US-00002 TABLE 1b Composition of alloy stainless steel SS321 Alloy C Mn P S Si Cr Ni Mo Cu N Other 321 0.08 2.0 0.045 0.03 0.75 17.0 to 19.0 9.0-12.0 0.75 0.75 0.1 Ti = 5 × (C + N) (mini) to 0.7 (max)

[0192] All values indicated are maximum values unless otherwise specified. Values are derived based on AMS and ASTM specifications standard.

[0193] Treatment Methods (Flowing Treatment and Immersion Treatment Protocols)

[0194] There are two methods which can be used to treat the metal substrate. These two examples do not necessarily limit the present application to these two methods but may include other methods that produce the abovementioned advantages.

[0195] The first method (immersion treatment) starts off by soaking a metal substrate in the formulation at a solution/metal surface ratio of approximately 2.1 ml/cm.sup.2. The immersion was carried out under ambient condition (25° C. and 1 atm). Some heat and gas are released in the first 15 minutes. The temperature of the solution increased to approximately 70 to 80° C. within this period of time and then decreases back to ambient temperature. The total immersion time lasted 1.5 hours.

[0196] In the second method (flowing treatment), the setup in FIG. 3a is used. The present formulation is pumped through a metal pipe at a targeted flow rate and the formulation is circulated through this system for a number of times. This can be driven by a gear pump. Taking a SS321 tube (with internal diameter of 4.6 mm), the volume of formulation used is based on 6 ml per cm length of the tube. In this instance, the flow rate is set at 550 ml/min and a total purging time of 30 minutes was needed. A large amount of heat and gas is typically produced in the first 5 cycles and the pressure stays typically over 1 atm but below 5 atm during the treatment.

[0197] Oil Thermal Degradation Test

[0198] A thermal degradation test was carried out to examine whether there is a formation of carbon-based particles due to prolonged heating instead of solely surface effect. As shown in FIG. 10a, the leftmost vial shows an almost clear oil solution. As the oil is heated for a longer period (from leftmost to rightmost vial), the oil changes to a darker color indicating more carbon-based particles, which can deposit on the surface of a metal substrate, are formed. The aerospace lubricant used in the present is from Honeywell oil.

[0199] FIG. 10b shows the optical microscopic image of the used oil sample of the leftmost vial which indicates minor formation of carbon-based deposits. FIG. 10c shows the optical microscopic image of the heated oil that has been contacted with a treated tube. It can be seen that there are some carbon-based deposits in the oil extracted, particularly one agglomerated carbon-based particle. This indicates the particles are not deposited on the tube but remains in the oil.

[0200] FIG. 10d is an optical microscopic image of the heated oil from the crude tube. This image shows some carbon-based deposits forming that have not yet agglomerated. This means that most of the agglomerated particles have been deposited on the tube hence leaving behind the smaller particles in the oil extracted.

[0201] The heating temperature and period for these tests remained the same throughout for FIG. 10b to FIG. 10d.

[0202] Analysis Via Raman Spectroscopy For Evidence of Carbon-Based Deposits on SS321 Tube

[0203] An analysis was carried out via Raman spectroscopy to determine the amount of carbon-based deposits present on an untreated tube. The analysis was carried out in three ways. One acts as the control which simply uses a pristine SS321 tube where no oil is passed through, another is a stationary oil test in which the oil is simply sealed in the tube and the third is where oil is continuously flowed through the tube to determine if shearing has any effect on the deposition. The Raman spectrum obtained is shown in FIG. 14.

[0204] Observably, some carbons were present due to the original composition SS321. It can be observed that continuous flowing oil does help to reduce carbon deposits but the stationary oil left in the tube causes the most carbon-based deposits.

Example 2

Impact of Formulation on the Microstructure of the Internal Surface of a SS321 Tube

[0205] The SS321 surface, in particular the interior wall of SS321 tube (¼″), has been used as the typical substrate to investigate chemical polishing effect. As observed from the SEM in FIG. 8a, there are numerous structure defects on the internal surface of SS321 tube. They were left from the cold drawing process used to manufacture metal tubes. The surface composition analysis (XPS) revealed that the surface is covered by a thick oxide layer. Contrary to this, the surfaces obtained after polishing using LHC solution (FIG. 8b and FIG. 8c) displayed different microstructures. A dense topography with irregularly oriented domains that assemble as spherulites (labelled by dash circle) and each domain contains a ridge was observed under the SEM. With the use of a chelating agent (e.g. etidronic acid), individual domains are converted to thin stripes assembling in parallel in a spherulite. This microstructure ostensibly has more ridges due to the protection role of the chelating agent as proposed above. On the other hand, with the use of a stabilizer (e.g. NDSA), a dense topography with a mountain-valley contour was developed (FIG. 8d).

[0206] As this is a mild polishing system, only less than 10% of the mass of substrate (SS321 tube) is removed over an immersion period ranging from 1 to 4 hours at ambient conditions.

Comparative Example 1

[0207] The present formulation serves as improved chemical polishing solution that enhances the coke-resistant capability of a treated surface, particularly a metal surface, by restructuring the surface topography or morphology. The present formulation may render the surface a spherulite contour, in which each grain may exhibit a ladder-like topography on a micron scale when scrutinized using the scanning electron microscopy (SEM).

[0208] In addition, the surface composition, primarily the atomic ratio between Fe and Cr of a stainless steel substrate treated with the present formulation, undergoes little change after polishing. This can be determined according to a X-ray photoelectron spectroscopic analysis (XPS).

[0209] The technical benefits of the present formulation may be determined through a test of encapsulating an aerospace lubricant (e.g. aerospace grade Mobil Jet Oil II provided by Honeywell) in an enclosed metal tube (e.g. SS321 metal tube) at a temperature 20 to 40° C. above its boiling point (e.g. 300 to 320° C.) for a certain period of time to determine the coking extent. Other temperatures falling outside this range may also be used depending on the fluid used for testing. Through such a test, when the treated and untreated surfaces (crude) are compared, the surface treated with the present formulation manifested almost no deposition of coke particles up to 60 hours. The topographical changes after treating the surface with the present formulation or from chemical polishing using the present formulation can be observed from the internal of the SS321 pipe and such changes may be effective against coke deposition.

[0210] In contrast, the crude surface that has not been treated with the present formulation suffers significant coke deposition after a shorter period of 12 hours testing.

[0211] The above results can likely be attributed to a surface treated with the present formulation, which undergoes restructuring due to the combined use of the various components that formed the present formulation. The treated or restructured surface becomes capable of stimulating a thin surface eddy fluid field at the testing temperature of 300 to 320° C. This is possibly due to the morphology dependent turbulent heat flux that is likely to be present because of the restructured surface. As temperature increases, the eddy fluidities may increase thereby causing turbulent flow and this helps to reduce surface decomposition of molecules present in a hydrocarbon stream due to the agitation caused by the turbulence.

Comparative Example 2

[0212] Two control samples were used as benchmark. They were etched using conventionally existing chemical polishing formulation, SVA and SHP, respectively. Their formulations could be found in Table 2. The polished surfaces by these two polishing solutions display similar microstructures, composed of narrow rectangular stripes but they do not exhibit a clear association pattern in spherulite shape.

TABLE-US-00003 TABLE 2 Composition of conventional chemical polishing formulations Chemicals SVA SHP Hydrochloric acid (37%) 100 ml 100 mL Nitric acid (65%)  10 mL 0 Hydrogen perioxide (20%) 0  15 mL Water 100 mL 100 mL

[0213] The modified topographies of the surface of a SS321 tube treated with these conventional formulations are shown in FIG. 9a and FIG. 9b.

[0214] In addition to the surface microstructure, the surface roughness is also a crucial parameter characterize the impact of chemical polishing. Table 3 shows the comparison of the three typical surfaces whose surface morphologies are shown in FIGS. 8a, 8b and 9b. As compared to the crude inner surface, the one polished with the LHC solution which was developed herein, offers a lower average roughness though it possesses a higher peak roughness that is attributed to the presence of ridges. On the other hand, the one treated with SHP formulation promotes surface roughness in all aspects.

TABLE-US-00004 TABLE 3 Analysis of surface roughness of the selected inner surface of SS321 tube Inner Maximum Root Mean surface of Average Average Square Peak Valley the SS321 roughness Roughness Roughness Roughness Roughness Roughness tube (R.sub.a) (Max R.sub.a) (R.sub.q) (R.sub.p) (R.sub.v) (R.sub.t) Crude 0.6439 0.2379 0.8173 1.5655 2.0570 3.6224 Polished 0.5235 0.3472 0.6438 1.6982 1.1864 2.8846 by LHC By SHP 1.4541 0.3849 1.6176 2.3779 3.2701 5.6479

Example 3

Examining the Coking-Resistance of the Different Surfaces

[0215] To test the anti-coking capability of the restructured surface by chemical polishing, an oil sample was sealed in the tube and subjected to heating at a designated temperature for a period of time (FIG. 6).

[0216] This design simulates the real situation where lubricant circulation in pipes is stagnant and exposed to the heat soak released from the turbine during shut down. The heat soak can reach as high as 450° F. (232° C.) or 300° C. and can last from about 6 to 12 hours. Two typical protocols have been designed as follow.

Example 3a

Batch Test

[0217] The testing condition was kept at 300° C. for 60 hours. The oil used in the encapsulation comprises spent oil from the aviation industry and fresh lubricant oil (Mobil Jet Oil II) in the ratio of 1:9. The reason for the inclusion of spent oil was to reinforce the coking condition since the spent oil contains tiny coke and oxide particles that are seeds of larger coke grains and hence cause easier coke deposition. After the test, the tube was cut open and examined by scanning electron microscopy. From FIG. 11a to FIG. 11c, it can be observed that the crude surface is covered by coke and coke granules (FIG. 11a) while the LHC-polished surface remains basically clean (FIG. 11b). The top tube in FIG. 11c shows the crude tube covered with coke. Although the bottom tube in FIG. 11c shows the LHC-polished tube having lesser coking extent, nevertheless, coke is still deposited on the internal treated surface.

[0218] In addition, both the LHC (etidronic acid)-polished surface and SHP-polished surface (the control sample) were also tested by using the fresh lubricant as coking medium. After a 56 hours test, the former surface displays tiny coke particles (FIG. 12a), it can be concluded that the addition of chelating agent (etidronic acid) into LHC may enhance the anti-coking capability of the resulting surface. In addition, most of these coke grains were grown on the ridges.

[0219] On the contrary, the conventional chemical polishing formulation using only mineral acid and oxidant e.g. SHP solution, could not improve the coke resistance capability. A noticeable extent of coking took place on the SHP-polished surface after the 56 hours oil encapsulation test. Compared with FIG. 9b, the SHP-polished surface is heavily overlaid by coke (FIG. 12b).

[0220] Hence, lactic acid is a critical component since it provides metal ion sequestering role in deactivating highly reactive sites, which permits a more uniform etching so as not to entirely remove the grain boundary structure as validated by the surface roughness analysis (Table 3 and FIG. 8b to FIG. 8c).

Example 3b

Thermal Cycling Test

[0221] The testing setup was the same as above but the heating pattern was changed. The setup was heated from room temperature to 300° C. at a rate of 9° C./min and this temperature is maintained (dwell) for 12 hours before cooling naturally to ambient temperature.

[0222] Such a cycle is repeated after the setup is held at room temperature for 3 hours. Five consecutive cycles were conducted. This test aims to enhance the adsorption of viscous species generated from heat-driven intermolecular condensations of oil molecules on the metal surface during the cooling process in order to assess the anti-coking capability of a surface under conditions that favourably induces coking. Both crude and LHC-polished SS321 tubes were tested. The tubes were examined after five cycles of test, in which the oil sample's cumulative dwell period at 300° C. was 60 hours. The photos of their sectioned segments and the respective electron microscopic images display very different coking extents. A coke layer covered the entire surface of the crude sample as shown in the left tube of FIG. 13c while its accompanying surface SEM image is shown in FIG. 13a. Meanwhile, the polished surface was basically free of coking (see right tube of FIG. 13b and FIG. 13c).

[0223] With regards as to whether the combination of a chelating agent (such as an etidronic acid) or a stabilizer (such as NDSA) with the LHC formulation could improve the coking resistance in the oil encapsulation test, a modified cycling test has been carried out by extending each dwelling at 300° C. from the original 12 hours to 56 hours.

[0224] In addition, the used oil after each cycle was drained out and fresh oil topped up for the subsequent cycle. This test simulates the oil-change situation during the real maintenance of an aircraft, where despite the longer service interval of more than 56 hours, the difference can be rectified by using a higher heating temperature (e.g. 300° C.) compared to the average temperature used in a real situation.

[0225] The amounts of coke deposited in the four specimens through the 3-cycle test were evaluated by the increment of weight percentage, which are summarized in Table 4. Two conclusions can be drawn from this test as follow.

[0226] 1. It verifies the effectiveness of LHC (FIG. 11b) vs. LHC+Etidronic acid (FIG. 12a).

[0227] 2. The formulation of LHC+NDSA shows better performance than LHC alone in terms of reducing coke accumulation by about 36%, attributed to the impact of the mountain-and-valley microstructure as found in FIG. 8d.

TABLE-US-00005 TABLE 4 Comparison of the coke deposition in the 4 selected SS321 tubes through a 3-cycle coking test (sample tubes were washed with 2-propanol after the test and then dried. The percentages were obtained from the gravimetric analysis). LHC + Crude LHC LHC + NDSA Etidronic Accumulative percentage 0.708% 0.044% 0.028% 0.288% of weight gained after 3 consecutive cycles of oil- encapsulation heating

SUMMARY OF THE EXAMPLES

[0228] The two testing protocols disclosed above are designed to examine the anti-coking capability of the treated (chemically polished) inner surface of a SS321 tube. The composition of the chemical polishing formulation is vital to the anti-coking capability because it achieves two goals.

[0229] These goals are the removal of cracks and pits over the internal surface of the SS321 tube and the creation of a microstructure that could bring about turbulent heat flux that leads to a turbulent fluid field over the surface. This surface turbulent fluid motion effectively prevents the adsorption of any coke precursors and reduces the thermal decomposition rate of oil molecules by minimizing local temperature gradients or hotspots.

[0230] As shown, an exemplified formulation consisting of lactic acid, hydrogen peroxide, hydrochloric acid and a chelating/stabilizing agent (e.g. NDSA or etidronic acid) offers the distinctly superior anti-coking performance as demonstrated through the crude and control samples when compared to treated samples. The advantages are sustained due to the specific surface microstructure formed by using the present formulation and method. The microstructured surface contains irregularly oriented domains and spherulite contours in which each domain contains ridge. The spherulite contour has been demonstrated to be critical since it aids in producing a spiral fluid field which is a turbulent flow.

[0231] The present chemical polishing method, using the present formulation, has been derived to create a specific surface topography on the internal of a SS321 tube for enhancing the anti-coking capability. The resulting treated surface exhibits stronger coking resistance compared to the unpolished counterpart.

[0232] The coking resistance was evaluated by heating an aerospace lubricant sealed up in the tube under designated conditions that simulate the soak heat happening in the engine system of aircraft. By treating the internal of the SS321 tube with the present formulation, the topography of the internal wall is modified to become a rather smooth surface. It is reiterated that this microstructure stimulates a surface eddy fluid field that significantly reduces the thermal degradation of the oil and discourages the adsorption of coke particles to the surface due to its agitation effect.

[0233] Therefore, the present formulation and method developed is related to the concept of self-curing since no any external intercalation such as coating or addition of coking inhibitor is required.

[0234] The present formulation and method are not only cost efficient and scalable but the applicability of the present formulation and method do not suffer from any surface geometry or size limitation. They provide a remedy to the dilemma of coke choking in metal pipes encountered in transport, energy and chemical industries.

INDUSTRIAL APPLICABILITY

[0235] The formulation as defined herein is capable of mitigating carbon-based deposits on a metal or solid substrate through the combined use of an oxidizing agent, an etchant, a sequestering agent and a chelating agent. Together, these components are able to modify the topography of the surface of a metal substrate such as the inner surfaces of a metal pipe or tubing. The modified surface is capable of inducing turbulent heat flux and fluid flow which eliminates particulate deposits on a surface. This formulation is not constrained by the duration of any heating as it does not require heat input in order to attain the above effects or advantages. The sole use of corrosive acids for cleaning pipes may be avoided and hence unnecessary damages to a metal pipe may be circumvented.

[0236] Advantageously, the present method also provides the above effects and advantages. The present method further protects a solid surface, particularly a metal substrate, from undesired accumulation of carbon-based deposits. This method circumvents the aforementioned limitations.

[0237] The present method or formulation may be used to treat a metal substrate such as a pipe in order to attain the abovementioned advantages simply by contacting the formulation with the metal substrate. No application of coating is required.

[0238] The present formulation and method is advantageously capable of removing cracks and pits present on the internal surface of a metal pipe or tube. They are capable of creating a microstructure that may bring about turbulent heat flux which in turn leads to turbulent fluid flow over the surface of the metal pipe or metal substrate. This surface turbulent fluid flow effectively prevents the adsorption of carbon-based deposits such as coke and its precursors, thereby reducing the thermal decomposition rate of carbon-based deposits substance (e.g. hydrocarbon molecules) through the minimization of local temperature gradients or hot spots.

[0239] When the present formulation and method are used, hazardous gaseous oxidizing agents such as oxygen, ozone, chlorine, nitrogen dioxide, sulfur dioxide or sulfur trioxide may be avoided. Hence, the risk of an explosion when using the gaseous oxidizing agent, for instance oxygen, may be avoided through the use of the present formulation.

[0240] The present formulation and method are not only cost efficient and scalable for keeping a metal substrate free of carbon-based deposits but also do not suffer from any constraints imposed by the surface or geometry of the metal substrate or metal pipe to be treated.

[0241] Hence, the present formulation and method readily address the problem of pipes or tubes choking encountered in the transport, energy or chemical industries due to the carbon-based deposits. By using the present formulation and method, process downtime can be significantly reduced.

[0242] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.