METHOD FOR WELDING FERRITIC STAINLESS STEEL TO CARBON STEEL

Abstract

A method of welding a ferritic stainless steel part to a carbon steel part is described. The method comprises arc welding (e.g. GTAW welding) the ferritic stainless steel part to the carbon steel part using a duplex stainless steel filler metal. Welded article made in this way is useful for industrial electrolyzers and particularly for corrosion resistant cathode and carrier plate assemblies in a sodium chlorate electrolyzer.

Claims

1. A method for welding a ferritic stainless steel part to a carbon steel part comprising: arc welding the ferritic stainless steel part to the carbon steel part using a filler metal wherein the filler metal is a duplex stainless steel.

2. The method of claim 1 comprising GTAW welding the ferritic stainless steel part to the carbon steel part.

3. The method of claim 1 wherein the ferritic stainless steel is a 430, 432, 434, 436, 439, 441, 442, 444, 445, or 446 grade or a doped grade of ferritic stainless steel.

4. The method of claim 3 wherein the ferritic stainless steel is a 444 or 445 grade of ferritic stainless steel.

5. The method of claim 1 wherein the ferritic stainless steel comprises less than about 0.03% by weight carbon.

6. The method of claim 5 wherein the ferritic stainless steel comprises less than about 0.005% by weight carbon.

7. The method of claim 1 wherein the carbon steel is an A-516 or A-285 grade of carbon steel.

8. The method of claim 2 comprising: GTAW welding the ferritic stainless steel part to the carbon steel part with a heat input in the range from about 0.4 to 1.5 kJ/mm.

9. The method of claim 8 comprising: GTAW welding the ferritic stainless steel part to the carbon steel part with a heat input in the range from about 0.7 to 1.2 kJ/mm.

10. The method of claim 1 wherein the duplex stainless steel is 2507/P100, 2594, or 2209 grade of duplex stainless steel.

11. The method of claim 10 wherein the duplex stainless steel is 2507/P100 or 2594 grade of duplex stainless steel.

12. A welded article comprising a ferritic stainless steel part welded to a carbon steel part with a filler metal wherein the filler metal is a duplex stainless steel.

13. The welded article of claim 12 wherein the ferritic stainless steel part is a cathode for an industrial electrolyzer.

14. The welded article of claim 12 wherein the cathode comprises an electrolysis enhancing coating.

15. The welded article of claim 13 wherein the carbon steel part is a carrier plate for the cathode of the industrial electrolyzer.

16. The welded article of claim 12 wherein the ferritic stainless steel is a 430, 432, 436, 444, or 445 grade of ferritic stainless steel.

17. The welded article of claim 12 wherein the carbon steel is an A-516 or A-285 grade of carbon steel.

18. The welded article of claim 12 wherein the duplex stainless steel is 2507/P100, 2594, or 2209 grade of duplex stainless steel.

19. An industrial electrolyzer comprising the welded article of claim 12.

20. The industrial electrolyzer of claim 19 wherein the welded article is a cathode and carrier plate assembly.

21. The industrial electrolyzer of claim 20 wherein the industrial electrolyzer is a sodium chlorate electrolyzer.

Description

DETAILED DESCRIPTION

[0015] Unless the context requires otherwise, throughout this specification and claims, the words comprise, comprising and the like are to be construed in an open, inclusive sense. The words a, an, and the like are to be considered as meaning at least one and not limited to just one.

[0016] In addition, the following definitions are intended. In a numerical context, the word about is to be construed as meaning plus or minus 10%.

[0017] Stainless steel refers to a steel alloy with a minimum of 10.5% chromium content by mass. Ferritic stainless steels are distinguished by the primary alloying element being chromium (ranging from about 10.5 to 30 wt %). Duplex stainless steels are also known as ferritic-austenitic stainless steels, and contain greater than 21 wt % chromium and from about 1.4 to 8 wt % nickel. Duplex stainless steel has better weld corrosion resistance than austenitic stainless steel.

[0018] Surface roughness R.sub.q refers to the mean square of roughness as determined according to standards JIS2001 or ISO1997 and are what were used in the Examples below.

[0019] The method of the invention provides for quality welds between a ferritic stainless steel part and a carbon steel part. The welds have a desirable microstructure and composition and exhibit good corrosion resistance for use in industrial electrolyzer applications, and especially in sodium chlorate electrolyzers.

[0020] The method may be employed for all grades of ferritic stainless steels and carbon steels. As disclosed in WO2013/159219, exemplary ferritic stainless steels for use in improved sodium chlorate electrolyzer cathodes include 430, 432, 436, 444, 445, 446 and other grades and also certain extra low interstitial types comprising a stabilizing dopant such as Cu, Mo, N, Nb, Sn, Ti, V, and/or W. Such ferritic stainless steels typically have low carbon and can be less than about 0.03% by weight carbon, and for certain types less than about 0.005% by weight carbon. Exemplary carbon steels for use in improved sodium chlorate electrolyzer cathodes include those with carbon content less than about 0.16% and include, for instance, ASTM A-516 or A-285 grades. Other possible types include A-612 and A-537. (These steels are preferable classified as low carbon equivalent types in accordance with ASTM A20. A preferred carbon equivalent is <0.38 wt % and more preferably <0.36 wt % based on the actual certified chemical composition.)

[0021] In the method, the ferritic stainless steel and carbon steel parts are arc welded together (e.g. using GTAW, GMAW, MIG/MAG, SMAW, or other welding method) using a duplex stainless steel filler metal (including lean, regular, super, and hyper types). Exemplary duplex stainless steel grades include 2507/P100, 2594, and/or 2209 grades. For use in improved sodium chlorate electrolyzer cathodes, improved corrosion resistance may be obtained using 2507/P100 and/or 2594 grades of duplex stainless steel as per designation AWS A519, EN 12076, or related equivalent.

[0022] Successful welds can be obtained using GTAW welding techniques. To minimize heat related changes in the welded article, intermittent or stitch welding is generally employed where possible. For cathode and carrier plate assemblies in electrolyzers, the manual welds are typically intermittent fillet type welds. In great part, standard recommended GTAW welding conditions may be used. (The welds have a certain length and spacing between them and along the total length of perpendicular contact between electrode and carrier plate.) For instance, exemplary welding conditions include: current range of 75 to 150 Amps; voltage range of 9 to 11 Volts; welding filler wire diameter of about 2 to 3.2 mm; and 2% thoriated tungsten electrode (about 2 to 3.2 mm). Further, standard shielding gas conditions may be employed, for instance pure argon (or mixed gas with nitrogen) shield gas purge rate of 6 to 12 L/min and pure argon backing gas of 5 to 30 L/min, but for better corrosion resistance shielding gas with about 2% nitrogen may be considered. Use of a copper chill bar comprising a groove for the weld and a hole for a backing shield purge may also be considered. An optional trailing shield of argon (e.g. 8-30 L/min) and purge cup sizes from #6 to #12 may also be used.

[0023] Standard total heat inputs may also be provided to effect the weld (e.g. 1.5 kJ/mm) but total heat inputs from about 0.4 to 1.2 kJ/mm are recommended. Also, travel speeds for the GTAW welding electrode that are in the typical range generate good weld results (e.g. 0.8 to 3.4 mm/sec). The quality of the welds also depends on controlling the interpass temperature, which is preferably less than about 100 C. whenever two passes are required.

[0024] The weld quality can be verified by point counting (a very precise standardized methodASTM E562) or by checking the ferrite content (e.g. about 42 to 50%), for instance using a FERITSCOPE FMP30. The weld joint quality, before and after corrosion testing or service, can also be evaluated by mechanical properties including hardness distribution, tensile/bend tests, and Erichsen values. Analyzing the chemical composition can be used to check for undesirable impurities (abnormal levels) of C, S, P, O, and N. Also, by examining the cross-sections of weld microstructures, defects like incomplete penetration, slag inclusions, pores, and grain size damage can be detected. Yet other methods (non-destructive) include penetrant testing, radiographic testing, and ultrasonic testing which look for defects such as pores and cracks.

[0025] And, as illustrated in the following Examples, the corrosion resistance can be determined using accelerated methods. For good weldability and also to reduce the need for post-weld cleaning, all joint surfaces and the adjoining ones must be thoroughly cleaned before welding. Dirt, oil, and grease can be removed using an organic solvent (e.g. acetone) or commercial cleaning agent (e.g. Avesta Cleaner). Post-weld cleaning may also be used to achieve fully satisfactory corrosion resistance. This can be done mechanically (e.g. grinding, brushing, polishing, or blasting) and/or chemically (e.g. pickling). Post-weld heat treatment (e.g. from 900 to 1150 C. with quenching or rapid cooling) may be considered to improve the weld quality. The carrier plate can be pre-heated (e.g. 65 to 95 C.) with a gas torch prior to welding or the weld wire can be heated by applied power (e.g. 12V, 60 A) during the live feed to the weld joint.

[0026] As mentioned above, the welding method of the invention is particularly useful for preparing cathode and carrier plate assemblies for sodium chlorate electrolyzers using the improved ferritic stainless steel based cathodes disclosed in WO2013/159219. These and other materials, including Ru oxide and other experimental coated materials, were unexpectedly found to be improved cathode materials if their surfaces had been appropriately modified. Overvoltages similar to or better than that obtained with carbon steel could be obtained, without an unacceptable loss of corrosion resistance, if the surfaces were roughened an appropriate amount. These improved surface modified, low nickel content stainless steel cathodes can replace present conventional carbon steel cathodes while advantageously providing better durability, cost and performance. However, stainless steel cathode options cannot be welded to conventional carbon steel carrier plates like present conventional cathode materials can (e.g. Stahrmet cathodes can be welded to carbon steel carrier plates using carbon steel filler wire and GMAW). Instead, other welding techniques must be employed or alternatively different materials must be used for the carrier plates. The present method provides for much improved results.

[0027] The following Examples have been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.

Examples

[0028] Welded article samples were prepared using a variety of steel combinations and filler materials. These samples were then evaluated for their corrosion resistance and other characteristics for use in sodium chlorate electrolyzer applications.

[0029] The following steel materials were used in this testing: [0030] ASTM A-516-55 grade of carbon steel (denoted CS) [0031] 430 grade ferritic stainless steel with a composition of 0.02% C, 0.33% Si, 0.40% Mn, 0.027% P, 0.001% S, 16.04% Cr, 0.21% Cu, 0.032% Mo, 0.53% Ni by weight, the remainder being Fe (denoted 430) [0032] 432 grade ferritic stainless steel with a composition of 0.004% C, 0.07% Si, 0.08% Mn, 0.022% P, 0.001% S, 17.20% Cr, 0% Ni, 0.18% Ti, 0.01% N, 0.48% Mo, 0.02% Cu, by weight, the remainder being Fe (denoted 432) [0033] 444 grade ferritic stainless steel with a composition of 0.015% C, 0.50% Si, 0.50% Mn, 0.040% P, 0.030% S, 18.00-20.00% Cr, 1.75-2.25 Mo, 0.015% N, 8(C+N)Nb, 0.20 V, by weight, the remainder being Fe (denoted 444) [0034] 445 grade ferritic stainless steel with a composition of 0.010% C, 1.00% Si, 1.00% Mn, 0.040% P, 0.007% S, 0.60 Ni, 22.00-23.00% Cr, 1.50-2.50 Mo, 0.020% N, 16(C+N)Nb+Ti, by weight, the remainder being Fe (denoted 445) [0035] a doped high purity grade of ferritic stainless steel with a composition of 0.004% C, 0.06% Si, 0.10% Mn, 0.023% P, 0.001% S, 17.32% Cr, 0.21% Sn, 0.19% Nb+Ti combined, 0.011% N, by weight, the remainder being Fe (denoted Doped) [0036] LDX2101 grade of lean duplex stainless steel with a composition of 0.021% C, 0.67% Si, 5.01% Mn, 0.022% P, 0.001% S, 21.3% Cr, 1.6% Ni, 0.218% N, 0.28% Mo, 0.29% Cu, by weight, the remainder being Fe (denoted 2101) [0037] 2507/P100 grade of duplex stainless steel welding rod (as filler material) with a typical composition of 0.012% C, 0.34% Si, 0.3% Mn, 0.014% P, 0.001% S, 25.0% Cr, 9.4% Ni, 0.234% N, 3.91% Mo, 0.01% Nb+Ta combined, 0.08% Cu, by weight, the remainder being Fe (denoted 2507) [0038] 2209 grade of duplex stainless steel welding rod (as filler material) with a typical composition of 0.01% C, 0.4% Si, 1.6% Mn, 0.014% P, 0.017% S, 22.8% Cr, 8.7% Ni, 0.16% N, 3.1% Mo, by weight, the remainder being Fe (denoted 2209) [0039] 2594 grade of duplex stainless steel welding rod (as filler material) with a typical composition of 0.012% C, 0.41% Si, 0.39% Mn, 0.016% P, 0.0008% S, 25.09% Cr, 9.27% Ni, 0.24% N, 3.9% Mo, 0.085% Cu, 0.01 W by weight, the remainder being Fe (denoted 2594) [0040] ER309L grade of austenitic stainless steel welding rod (as filler material) with a composition of 0.016% C, 0.34% Si, 1.98% Mn, 0.019% P, 0.001% S, 23.0% Cr, 13.9% Ni, 0.062% N, 0.25% Mo, 0.10% Cu, 0.10% Co, 0.008% Al, 0.07% V, by weight, the remainder being Fe (denoted 309L). [0041] ER309L Mo grade of austenitic stainless steel welding rod (as filler material) with a composition of 0.008% C, 0.33% Si, 1.43% Mn, 0.019% P, 0.002% S, 21.4% Cr, 14.95% Ni, 0.061% N, 2.59% Mo, 0.08% Cu, 0.045% Co, 0.004% Al, by weight, the remainder being Fe (denoted 309L Mo).

[0042] Welded article samples were then prepared using various combinations of the above. In all cases, a representative single cathode and carrier plate assembly was prepared. The carrier plate was always slotted carbon steel plate (CS) with dimensions 821.5 cm. The cathode was a stainless steel plate (as indicated) with dimensions of 820.2 cm or 820.3 cm. All cathode plates were sandblasted prior to welding with 120 grit, AlO.sub.2 powder on both sides such that the average surface roughness, R.sub.q, was in the range of 1.6 to 2.8 um (as determined using a Mitutoyo Surftest SJ210). Cathodes were initially fitted into a carrier plate slot and stitch welded in two places using a filler material and a welding procedure as indicated below. In all cases, the weld and heat affected zone of the stainless electrode was brushed with a stainless wire brush on both the front and back sides of the weld afterwards. Normally this was done with warm surfaces (e.g. 35 to 60 C.) and being careful not to pick up iron from the carrier plate.

[0043] After preparation, the samples were subjected to an accelerated corrosion test in which individual samples were exposed to corrosive, circulating hypo-containing electrolyte from a pilot scale chlorate reactor. (The hypo-containing electrolyte comprised an approximate 4 g/L solution of HClO and NaClO, which circulated at a flow rate of 60 L/h, at about 70-80 C., and was obtained from the reactor operating at a current density of 4 kA/m.sup.2.) The samples were exposed to the electrolyte for a period of 3-6 hours per cycle and then visually examined for signs of corrosion (specifically in the area of the weld itself and the area of the rest of the electrode).

[0044] In a first set of tests, various ferritic and duplex stainless steels with dimensions of 820.2 cm were used as cathode materials. Samples were welded together using two different filler materials and the following welding procedures: [0045] Two GTAW welds were made in each case using 2.4 diameter wire filler material; the first ran from the centre of the sample to an end and the second ran from the other end back to the centre [0046] The range of welding voltages was from 9.3 to 10.2 V [0047] The welding current was 85 A [0048] Varied total heat inputs and travel speeds were used for each weld as indicated in Table 1 below

[0049] Table 1 below summarizes the sample combinations prepared and the results obtained from the corrosion testing in the first set of tests.

TABLE-US-00001 TABLE 1 Visual {Total heat appearance input; travel Visual of rest of speed} for appearance electrode Sam- each weld, in of weld after after ple Cathode Filler {kJ/mm; corrosion corrosion # steel material mm/s} testing testing C1 2101 2507 {0.6; 1.4} Very Very {0.6; 1.2} good good C2 Doped 309L {1.2; 0.7} Poor Poor Not available C3 430 309L {0.9; 1.0} Poor Poor {0.7; 1.1} C4 2101 309L {0.8; 1.0} Poor Good {0.7; 1.1} C5 430 309L Mo {0.8; 1.1} Fair Fair {0.9; 0.9} 1 Doped 2507 {0.6; 1.4} Good Poor {0.9; 1.0} 2 430 2507 {0.8; 1.1} Good Poor {0.5; 1.7} 3 430 2507 {0.9; 0.9} Good Poor {0.8; 1.1}

[0050] Comparative sample C1 comprised a duplex stainless steel cathode and duplex filler material. Both the weld and rest of the electrode showed minimal signs of corrosion after testing. Comparative samples C2 to C5 all used conventional 309L filler material in the welding though. Regardless of cathode steel material used (e.g. doped, 430, or 2101), the weld area showed substantial evidence of corrosion after testing. In addition, the rest of the electrodes in most of these samples showed obvious significant corrosion. The rest of the 2101 duplex electrode in comparative sample C4 showed minimal signs of corrosion though.

[0051] Inventive samples 1, 2, and 3 all showed good results at the weld area with minimal evidence of corrosion observed. However, there was obvious, substantial evidence of corrosion in the heat affected zone of the rest of the electrode area. While this is not a desirable result, it is expected that such corrosion might be successfully mitigated in an operating electrolyzer via use of cathodic protection or by using a higher alloyed grade of stainless steel for the electrode.

[0052] In addition, in some cases, an elemental analysis of the hypo-rich electrolyte was performed following corrosion testing to determine the type and amount of elements that had been leached from the samples. For instance, elemental analysis was performed on the electrolyte from tested comparative samples C3 and C4 which had both corroded severely at the weld. In both cases, unacceptable amounts of Ni had leached into the electrolyte. In an operating chlorate electrolyzer, such amounts in the reactor liquor could precipitate and cause the oxygen levels to rise well above normal operating levels and cause a degradation in performance.

[0053] On the other hand, elemental analysis was performed on the electrolyte from tested comparative sample C1 and also from inventive samples 1 and 2. The amounts of leached Ni were acceptable in all these cases, demonstrating weld stability of 2507 with all three types of cathode materials tested.

[0054] In a second set of tests, additional types of ferritic stainless steels with the same dimensions were used as cathodes and these were welded using duplex stainless steel filler materials. This time, the range of welding voltages was from 10.3 to 10.7 V. The welding current used was 88 A except for one indicated sample where the current was 140 A. Otherwise, the welding procedures were the same as before. And also as before, these welded cathode and carrier plate samples were then subjected to the same accelerated corrosion testing. Table 2 below summarizes the sample combinations prepared and the results obtained from the corrosion testing in the second set of tests. For some samples (as indicated), the ferrite content of the weld was determined non-destructively using a FERITSCOPE FMP30 to check ferrite content. Additionally, a determination of weld quality was made visually and using microscopy (10-200 magnification) for cross-sections (with and without etching) of microstructures.

TABLE-US-00002 TABLE 2 Visual {Total heat Visual appearance input; travel appearance of rest of speed} for Ferrite of weld electrode each weld, content after after Sample Cathode Filler in {kJ/mm; of weld Weld corrosion corrosion # steel material mm/s} (%) quality testing testing 4 Doped 2507 {0.6; 1.7} 42 High Good Poor {0.5; 1.8} 5 444 2507 {0.4; 2.2} 50 High Very Very {0.5; 2.0} good good 6 444 2209 {0.4; 2.0} 38 Medium Fair Very {0.5; 1.9} good 7 445 2209 {0.6; 1.5} 44 Medium Good Very {0.5; 1.9} good 8 432 2507 {0.5; 2.0} 60 High Good Poor {0.4; 2.3} 9 445 2507 {0.6; 1.7} 57 High Very Very {0.5; 1.9} good good 10 444 2507 {0.5; 1.8} 43 High Very Very {0.5; 1.8} good good 11* Doped 2594 {0.2; 6.8}* 55 High Good Poor {0.2; 6.8}* *Welding current was 140 A for both welds

[0055] As is evident from Table 2, superior welds were obtained in all cases for all the ferritic grade cathode materials tested when 2507 welding wire (filler) was used (e.g. ferrite content was always between 20 and 70%). However, test samples prepared with 2209 welding wire (which has lower Cr, Ni, and Mo but higher Mn than 2507) resulted in somewhat inferior welds to those made with 2507.

[0056] After corrosion testing, samples 4 and 11 which employed the doped steel cathode again good results at the weld but poor results over the rest of the electrode. Again, this lower grade ferritic alloy showed severe pitting corrosion in the heat affected zones during the ACT, but the weld was not attacked. (Again, cathodic protection may be expected to mitigate the observed pitting corrosion.) Superior results when using 444 or 445 cathode materials and 2507 welding wire (e.g. samples 5, 9, and 10 showed good to very good corrosion resistance everywhere). Results (samples 6 and 7) when using 2209 welding wire were acceptable but not as good as when 2507 welding wire was used with the same cathodes.

[0057] In a third set of tests, an additional 444 ferritic stainless steel electrode of thicker dimension 820.3 cm was welded to certain of the samples previously prepared and tested according to Table 2. In all cases, a single extra 444 cathode was welded using 2507 welding wire on the opposite side of the carbon steel carrier plate. The range of welding voltages here was from 10.2 to 11.5 V. The welding current again was 88 A. Otherwise, the welding procedures were the same as before. These samples comprising two welded cathodes were then subjected again to accelerated corrosion testing. Table 3 below summarizes the sample information and the results obtained from the corrosion testing in this third set of tests. As in the second set of tests, a determination of weld quality was also made here.

TABLE-US-00003 TABLE 3 Visual appearance {Total heat Visual of rest of input; travel appearance each speed} for Ferrite of welds electrode Extra extra weld, content after after Sample cathode Filler in {kJ/mm; of weld Weld corrosion corrosion # steel material mm/s} (%) quality testing testing 5+ 444 2507 {0.8; 1.1} 58 High 1.sup.st: Good 1.sup.st: Good 2.sup.nd: Very 2.sup.nd: Very good good 6+ 444 2507 {0.8; 1.3} 59 High 1.sup.st: Fair 1.sup.st: Very 2.sup.nd: Very good good 2.sup.nd: Very good 8+ 444 2507 {1.2; 0.8} 55 High 1.sup.st: Good 1.sup.st: Very poor 2.sup.nd: Very 2.sup.nd: Very good good 10+ 444 2507 {0.7; 1.3} 58 Medium 1.sup.st: Good 1.sup.st: Very good 2.sup.nd: Good 2.sup.nd: Very good

[0058] Table 3 shows that superior welds were again obtained for most of the samples. Inventive sample 10+ however appeared only of medium quality as there were some minor pits in a few areas where there was either too little filler, a discontinuity in travel, or where there was an underlying tack weld that was not fully cooled or cleaned.

[0059] After corrosion testing, all the inventive samples 5+, 6+, 8+, and 10+ showed good to very good corrosion resistance results at all of the weld sites and very good results over the rest of all the 2.sup.nd electrodes.

[0060] These examples demonstrate the effectiveness of the welding method of the invention and the resistance to corrosion it provides. In particular, quite superior results were obtained here using 444 or 445 cathode electrodes which had been welded with 2507 duplex filler.

[0061] All of the above U.S. patents, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.

[0062] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. For instance, while the preceding description and examples were directed at sodium chlorate electrolysers, the invention might instead be useable for welded joints in other industrial electrochemical processing equipment in which water or an aqueous solution is electrolyzed, e.g. hydrogen electrolysis, desalination of seawater, or electrolysis of an aqueous solution of an acid or an alkali metal chloride. For instance, aqueous acidic solutions are electrolyzed in electrowinning, electrotinning and electrogalvanizing processes. Aqueous alkali metal chloride solutions are electrolyzed in the production of chlorine, alkali metal hydroxide, and alkali metal hypochlorite. The invention can also be used for other electrochemical applications, which may or may not employ impermeable ion exchange membrane separators and which require an active, low cost, chemically resistant cathode electrode material, e.g. the electrolysis of non-aqueous electrolytes and electrosynthesis, or possibly in certain batteries or fuel cells. Such modifications are to be considered within the purview and scope of the claims appended hereto.