ALLOY SURFACE ACTIVATION BY IMMERSION IN AQUEOUS ACID SOLUTION

Abstract

A process for surface activation or depassivation of an article, in particular an alloy, by immersion of the alloy in an aqueous acid solution. The surface activation methods of the present invention can be performed during a relatively short period of time and achieve reductions in production costs and provide environmental friendliness as compared to prior art processes. In a further embodiment, after surface activation, the article is immersed in a second liquid that prevents re-formation of a passivating oxide layer on the surface of the article. In a further embodiment the surface-activated alloys are subjected to surface engineering by a process that infuses carbon or nitrogen through the surface at a temperature sufficiently low to suppress precipitation of carbides or nitrides.

Claims

1. A surface activated article-containing composition, comprising: an article having at least one portion of its surface activated; and a liquid covering the at least one portion for temporarily preventing the formation of a passivating layer on the at least one activated portion, the liquid comprising one or more of an aqueous acid, a fatty acid, an alcohol, water, and oil.

2. The composition according to claim 1, wherein the liquid comprises the aqueous acid.

3. The composition according to claim 1, wherein the liquid comprises one or more of the alcohol, the fatty acid, the oil, and the water.

4. The composition according to claim 2, wherein the article is immersed in the liquid.

5. The composition according to claim 3, wherein the article is immersed in the liquid.

6. The composition according to claim 1, wherein the article comprises one or more of stainless steel, a nickel-base alloy, a cobalt-base alloy, and a titanium-base alloy.

7. The composition according to claim 6, wherein the article comprises stainless steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention will be better understood and other features and advantages will become apparent by reading the detailed description of the invention, taken together with the drawings, wherein:

[0023] FIG. 1 is a temperature-time diagram of a prior-art process.

[0024] FIG. 2 is an image showing the wall of a nuclear-fuel cladding tube made from AISI-316L austenitic stainless steel. The case (hardened, corrosion-resistant carbon-rich layer at the alloy surface) is seen as featureless bright bands at the inner (left) and outer (right) side of the tube. This specimen was low-temperature-carburized without HCl gas. Instead, the surface was activated by immersing the tube in aqueous HCl solution.

[0025] FIG. 3 is an image showing a polished and etched cross-section of an AISI-316L coupon (sheet-metal specimen), treated in the same way as the tube specimen.

[0026] FIG. 4 illustrates X-ray diffractograms from AISI-316L specimens (noise-reduced by lowpass-filtering). Subscript CSS: Specimen surface-activated by immersion in liquid HCl solution (invention) and low-temperature-carburized for five hours. Subscript AR: As-received, non-carburized reference sample. The two peaks on the left represent the spacing of {111} lattice planes, whereas the two peaks on the right represent the spacings {200} lattice planes in the low-temperature-carburized (CSS) and as-received reference (AR) specimen, respectively. Compared to the corresponding AR peaks, the CSS peaks are shifted to lower diffraction angles. This indicates that the spacing of these lattice planes has increased after carburization. This, in turn, indicates a high concentration of dissolved carbon atoms, which expand the spacings between (and lattice plane spacings of) the (substitutional) metal atoms because they reside in interstitial sites, i.e. between the metal atoms.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention provides methods for surface activation of alloys by immersion in an aqueous acid solution. The concentration of the acid can be varied in order to produce desired surface activation. Various aqueous acids can be utilized in the practice of the present invention including, but not limited to, hydrochloric acid, hydrofluoric acid, hydrobromic acid and sulfuric acid. The concentration of the acid, the immersion time, and the temperature of the acid need to be adjusted for completely removing the passivating layer from the alloy surface while, at the same time, minimizing damage to the alloy part, e.g. by removal of alloy material below the passivating layer or pitting. The suitable range of acid concentrations corresponds to the pH range from +4 to 1. The suitable range of etching time is between 1 s and 10 ks. The suitable range of etching temperature is between 220 K (50 C.) and 380 K (100 C.). The acid may contain wetting agents and/or components for buffering the pH value or controlling viscosity.

[0028] Likewise, many different articles or alloys can be subjected to the surface activation process of the present invention including, but not limited to, the following: (i) Stainless steels, such as austenitic stainless steels, martensitic stainless steels, precipitation-hardened stainless steels, duplex stainless steels. (ii) Nickel-base alloys. (iii) Cobalt-base alloys. (iv) Titanium-base alloys. Various other parameters, such as processing temperature, processing time, etc. can be varied. Room-temperature processing and the option of processing outside of the CSS processing furnace reduces the need for additional equipment or devices to maintain the chosen temperature of the aqueous solution in a processing vessel.

[0029] Once the desired surface activation of the alloy has been achieved, the part or article can be then subjected to CSS processing (carburization, nitridation, or a combination thereofnitro-carburization) in order to case-harden at least one portion of the part.

[0030] In a further embodiment, after the activation or depassivation step, the article is or at least portions of the article are contacted with, preferably immersed, in a liquid that prevents or significantly retards the formation of an oxide layer, such as chromium-rich oxide, on at least one surface of the article. The article or alloy can remain immersed or otherwise coated with the liquid on desired surfaces thereof until the article can be subjected to CSS processing. Suitable liquids include, but are not limited to, alcohol (such as but not limited to ethanol), water, oil, or fatty acids (such as but not limited to a mixture of iso-octadecanoic acid, iso-tridecanoic acid, and 2-butyl octanoic acid).

[0031] In one important aspect of the present invention, the post-passivating solution is a liquid that has a suitable boiling point that allows the solution or residuals thereof to evaporate upon heating in the carburization process. Suitable boiling points range from about 50 to about 500 C., and preferably from about 300 to about 450 C. Immersion of the article can be maintained for convenience and/or handling purposes until the article is ready to be subjected to the carburization process or any other desired processing step. In another aspect, the post-passivating liquid tends to wet the alloy surface. For improving this behavior, it may contain suitable wetting agents.

[0032] It is also noted that surface activation or depassivation can be performed utilizing other techniques for activating stainless steel and other metal articles prior to the process for preventing formation of the oxide layer by immersion in or coating with the post-passivation liquid. Examples include contacting the workpiece with a hydrogen halide gas such as HCl or HF at elevated temperature (e.g. 260 to 450 C.), contact with a strong base, electroplating with iron, contact with liquid sodium and contact with a molten salt bath including sodium cyanide. These techniques are described, for example, in U.S. Pat. Nos. 6,093,303; 5,792,282; EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364). See also Stickles et al., Heat Treating, pp 312, 314, Volume 4, ASM Handbook, copyright 1991, ASM International as well as U.S. Pat. Nos. 4,975,147, and 5,372,655, the disclosures of which are also incorporated herein by reference.

[0033] Various Commercial Advantages Provided by the Invention are as follows:

[0034] Immersing alloy articles or parts into aqueous acid, for example, HCl, solution before they are loaded into the gas furnace for infusion of interstitial solute will avoid the corrosive damage that is caused by the application of HCl gas in the conventional process.

[0035] Under the aspects of safety and environmental pollution and sustainability, usage of aqueous acid, such as HCl solution close to room temperature (18 to 50 C.), is much less problematic than using HCl gas at high temperature.

[0036] Surface activation by immersing in aqueous acid solution, for example HCl solution, can be performed within minutes, i.e. much faster than the 4 hours currently needed for activation by HCl gas (plus heating/cooling for an intermediate 2 h step for initial exposure to carburizing gas, see FIG. 1).

[0037] The above factors imply large reductions in production costs, improvements in safety, and environmental friendliness.

[0038] The processes of the present invention can be utilized with generally any article that comprises an iron-, nickel-, cobalt-, or titanium-base alloy containing alloying elements (e.g. chromium, manganese, titanium, aluminum) making the material capable of forming a hardened surface layer or case by diffusing high concentrations of carbon, nitrogen, or other interstitial solute atoms into the surface of the material without formation of precipitates. The invention is particularly applicable to case hardening of steels, especially steels containing from about 5 to about 50 weight percent nickel and about 10 to about 50 weight percent chromium. In one embodiment a metal alloy contains 10 to 40 weight percent nickel and 10 to 35 weight percent chromium. Also preferred are stainless steels, especially the AISI 300 series steels, superaustenitic stainless steels, precipitation hardened stainless steels, martensitic stainless steels, duplex stainless steels, and Ni-base and Co-base alloys. Of special interests are the AISI-316, 316L, 317, 317L and 304 stainless steels, alloy 600, alloy C-276 and alloy 20 Cb, to name a few non-limiting examples.

[0039] The present invention is also applicable to articles of any shape. Examples include pump components, gears, valves, spray nozzles, mixers, surgical instruments, medical implants, watch cases, bearings, connectors, fasteners, electronic filters, shafts for electronic equipment, splines, ferrules and the like.

[0040] Moreover, the present invention can be employed to case harden all the surfaces of the workpiece or only some (portion) of these surfaces, as desired.

[0041] Supporting Experimental Data

[0042] Results of Surface Analysis by XPS (X-ray Photoelectron Spectrometry)

[0043] While the chromium atoms in an alloy, here AISI-316L, are in a neutral state of charge, the chromium atoms that participate in the surface oxide are positively charged ions. XPS is a technique that analyzes the topmost few atom layers of a specimen, and its energy resolution is sufficient to discriminate between photoelectrons emitted from chromium atoms in these different states. Therefore, XPS spectra can be analyzed to reveal what fraction of a surface is metallic, i.e. not (yet) covered by oxide. A suitable parameter for the metallic fraction of the surface is the ratio Rmet of integrated spectral intensity from chromium ions over the integrated spectral intensity from chromium in any charge state (ionized plus neutral). Such analysis was performed on as-received specimens (for reference) and specimens that were (i) etched in aqueous HCL solution for 0.6 ks (10 min), (ii) rinsed in either ethanol or water for 0.3 ks (5 min), and (iii) exposed to air for three different amounts of time. The resulting R.sub.met values, compiled in Table I, indicate that following surface activation with aqueous HCl solution by rinsing with ethanolcompared to rinsing in watersignificantly retards oxidation.

TABLE-US-00001 Air Rinse 0.12 ks 18 ks 36 ks Ethanol 0.3 ks 70% 17% 17% Water 0.3 ks 17% 8% 12% As-received 4%

[0044] In more than one decade of research in this field, we have established a variety of methods to verify successful CSS processing (carburization, nitridation, or a combination thereofnitro-caburization).

[0045] Optical Metallography

[0046] After CSS processing, the case (hard shell) generated by the high concentration of interstitial atoms in solid solution can be observed by polishing a cross-section and exposing it to a chemical etchant that attacks the non-infused core of the alloy but not the (more corrosion resistant) interstitial-atom-rich layer near the surface. FIG. 2 shows an example of a low-temperature-carburized nuclear-fuel cladding tube of AISI-316L austenitic stainless steel. The case (hardened, corrosion-resistant carbon-rich layer at the alloy surface) is seen as featureless bright bands at the inner (left) and outer (right) surface of the tube. This specimen was low-temperature-carburized without HCl gas. Instead, the surface was activated by immersing the tube in aqueous HCl solution. This specimen was low-temperature-carburized for only 5 h. Nevertheless, the micrograph reveals a case thickness of about 10 m. This result was reproduced with an AISI-316L coupon (sheet metal specimen), shown in FIG. 3.

[0047] One prior art process requires about 20 h and accomplishes a case depth of about 20 m. Considering the known square-root-of-time law for the diffusion depth, which we have confirmed to apply in other studies, a fourfold increased processing time should double the case depth. This implies that with the new surface activation process we invented, we can accomplish the same case depth as the conventional process after a comparable CSS processing time (while significantly reducing the time needed for surface activation).

[0048] X-Ray Diffractometry

[0049] Large fractions of interstitial atoms dissolved in a metal matrix lead to a measurable expansion of the distances between the metal atoms. This expansion of interatomic spacings can be measured with the help of XRD (X-ray diffractometry). In X-ray diffractograms recorded in the Bragg-Brentano (-2) setting, the spacings of crystal lattice planes manifest themselves by reflections of the primary X-ray beam that are emitted from the specimen if the primary beam hits these planes under a characteristic angle, which fulfills the Bragg condition

=2d.Math.Sin[],

where is the wavelength of the X-rays, d is the spacing of the lattice planes, and is the reflection angle. According to this equation, the expansion of a given plane spacings d will cause the corresponding reflection to occur at a smaller angle . This corresponds to a shift of the corresponding peak in the X-ray diffractogram towards smaller angles, i.e. to the left.

[0050] FIG. 4 illustrates X-ray diffractograms from AISI-316L specimens (noise-reduced by lowpass-filtering). Subscript CSS: Specimen surface-activated by immersion in liquid HCl solution (invention) and low-temperature-carburized for five hours. Subscript AR: As-received, non-carburized reference sample. The two peaks on the left represent the spacing of {111} lattice planes, whereas the two peaks on the right represent the spacings {200} lattice planes in the low-temperature-carburized (CSS) and as-received reference (AR) specimen, respectively. Compared to the corresponding AR peaks, the CSS peaks are shifted to lower diffraction angles. This indicates that the spacing of these lattice planes has increased after carburization. This, in turn, indicates a high concentration of dissolved carbon atoms, which expand the spacings between (and lattice plane spacings of) the (substitutional) metal atoms because they reside in interstitial sites, i.e. between the metal atoms.

[0051] In earlier work we established a quantitative correlation between peak shift and carbon concentration. Evaluating the average of the peak shifts observed in FIG. 4 with the coefficient described in indicates a carbon concentration of about 8 at % at the alloy surface. Again, considering the processing time of only 5 h in this example, this result compares favorably with the prior art process, which accomplishes surface concentrations of 12 to 15 at % after processing for 20 h.

[0052] In accordance with the patent statutes, the best mode and preferred embodiment have been set forth; the scope of the invention is not limited thereto.