THERMAL SPRAYING OF STAINLESS STEEL

Abstract: High-chromium (18-35 % w/w) and optionally high nickel (8-15 % w/w) stainless steel particles of say 25-50 microns transverse diameter are flamesprayed on a metal surface preferably roughened at 15-20 microns peak-to-valley. Even aluminium can be used as the metal. The surface so produced is a corrosion-free roughened layer upon which can be sprayed or painted a solution or dispersion of fluoropolymer with 120 DEG -150 DEG drying and 200 DEG -450 DEG at 30 min curing or sintering to give a long-lasting layer upon an article such as a tool.

Description: "THERMAL SPRAYING OF STAINLESS STEEL" This .invention relates to the thermal spraying of stainless steel, especially as applied as an intermediate stage in the provision of a non-stick coating layer over a substrate metal. it is well known to spray on to a roughened substrate surface a fluoropolymer dispersion, to remove the dispersant or solvent, and to cure or sinter so as to give a coating layer. In order to improve the adhesion of this layer to the substrate it is also well known first to flame-spray metal droplets or particles on the substrate so as to give a rough surface for the fluorocarbon polymer adhesion. An example of such flame sprayed metal is stainless steel, which is a general name for alloys of iron, chromium, (typically about 18%) nickel (typically about 8%) and minor additives such as titanium, manganese, silicon, etc. However, it is known in the art that problems of coating breakdown can arise and it is recognised generally that many of these have their roots in corrosion phenomena as between the substrate and the flame-sprayed metal. For example, it is accepted that aluminium is not a very suitable substrate metal for such techniques, and this is believed to be because of the risk of "white rust" corrosion, that is to say formation of aluminium oxides within the coating layers. The present invention has originated from a consideration of this problem and involves utilisation of a form of stainless steel with an unusually high chromium level and optionally with increased nickel level. On an aluminium substrate this runs completely contrary to previous expectations, and the use of additional chromium is counter-indicated not only by cost but also by the expectation of increased possibility of white rust (corrosion due to aluminium salts), due to increased chromium content. However, and as discussed in detail below, we believe we have identified a different corrosion problem as being more significant although hitherto not appreciated, and we can alleviate that problem to obtain improved coatings, any risk of "white rust" formation being more than offset by curing this other problem having its origin in "red rust" (corrosion due to ferrous salts) formation. In one aspect the invention consists in a method of preparing a roughened surface for subsequent coating with a fluoro polymer in a liquid medium, in which method the surface is thermally sprayed (e.g. flame sprayed or sprayed on using an electric arc or plasma gun) with a stainless steel alloy the chromium content of which is increased over that of standard stainless steel. Preferably the chromium content of the stainless steel, being greater than the normal 18% of standard stainless steel, ranges up to 35%. Most preferably it lies within the range of 25 to 35% by weight. Preferably also there is some small adjustment in the nickel content of the stainless steel, which can be up to 15%, that is to say rather greater than the normal 8% figure for standard stainless steel. A preferred composition of stainless steel for carrying out the method of the invention is therefore 25 to 35% by weight of chromium. 8 to 15% by weight of nickel, 0.1% maximum by weight of carbon, 2% maximum by weight of manganese, and 0.4% maximum by weight of silicon, with the remainder being iron and incidental impurities. The method as defined above is typically carried out to provide a rough surface with protruding particles. These particles can occupy, for maximum roughness, somewhere from 50 to 85% of the total area of the substrate to be eventually coated. The roughness of the particles themselves is preferably from 25 to 50 microns transverse dimension, most preferably by way of example microns Typically, the substrate surface itself has previously been roughened before being thermally sprayed. Such previous roughening can typically have the dimensions of from 15 to 20 microns peak-to-valley distance. In the method of the invention as defined above a wide range of substrate metals can be used, but it is a. particular advantage of the invention that even the more difficult substrate metals such as aluminium may be utilised. The invention also extends to the metal coated substrate produced by the above method. In particular, however, the invention further extends to the above method combined with a subsequent coating stage with fluorocarbon polymer, for example coating the fluorocarbon polymer from liquid media (dispersant or solvent vehicle) and subsequent drying (e.g. at 120°C to 150°C) and curing or sintering stages (e.g. at 200 - 450 β C, for up to 30 minutes) which together achieve a non-stick and durable, corrosion resistant, layer. The invention of course further extends to an article having a surface comprising such a fluorocarbon layer secured to an intermediate corrosion-resistant layer as described above. The article in question can be a tool (secateurs, shears, fork). The above invention, using an excess of chromium in the thermally-sprayed alloy, is not predictable from prior art, especially in the context of an aluminium substrate where the use of excess chromium might be expected to give even more aluminium oxides. However, although we do not wish to be governed by any theory as to the operation of the method, it appears likely that the phenomenon of red rust, previously unexpected in stainless steel is in fact present, and although not always immediately noticeable, persists over a long period. It would appear to have its origin in the drastic thermal spraying conditions, apparently leading to at least superficial loss of chromium in at least some of the metal particles probably dependent upon their individual history as they pass differently through different temperature zones. The following observations appear to support the above hypothesis. 1. Glass panels metal sprayed with traditional stainless steel grades of wire produced red corrosion products in neutral salt fogs (ASTM B4117). static 5% sodium chloride solutions and static 10% acetic acid solutions within a 25 hour exposure duration. 2. Aluminium plaques (of both cast LM6 and wrought 1200 grades) metal sprayed with traditional stainless steel grades of wire produced both red and white corrosion products in boiling 10% acetic acid solution, in some cases after only 20 minutes exposure. 3. Electron icroprobe analysis of the metal deposited on to panels showed a cored structure, consisting of a very thin surface layer containing approximately 14% chromium and in some cases the surprising situation of a bulk central region with virtually no chromium present. The other alloy elements were evenly distributed. The non-staining properties of stainless steel are due to the formation of a tenacious and resistant oxide film based on chromium. It is generally regarded that chromium levels below 14-15% are insufficient to impart protection and the structure of the deposited layer is therefore such that the iron oxide film formed is extremely thin. Any abrasion is likely to break the film to expose the central core. Insufficient chromium is present to heal the break in the film and corrosion proceeds unabated. 4. Aluminium vessels which had been metal sprayed using traditional stainless steel wire were used to boil a 10% acetic acid solution for 3 hours. In all cases a red discolouration was found, allied sometimes with areas of white corrosion products. In an effort to identify the red product a pan was left standing for 30 minutes in a 10% oxalic acid solution. After this time the acid was discoloured and the pan restored to its original metallic appearance. Polarographic analysis of 2 the acid solution indicated the pressure of Fe + and Fe ions. Another similarly treated pan was filled with deionized water and was still discoloured even after a 2 hour boil. 5. Panels of typical cookware compositions such as cast or wrought aluminium, stainless steel or cast iron when coated with the specified wire in the prescribed manner have not shown any tendency to for either red or white corrosion products in typical cooking environments such as boiling salted water-, hot detergents, salty steam and boiling acetic acid solutions after a minimum three hour exposure.

Claims: Claims . 1. A method of preparing a roughened surface tor subseqent coating with a fluoropolymer in a liquid medium, in which method a surface is thermally sprayed with a stainless steel alLoy the chromium content of which is greater than that of standard stainless steel. 2. ft method as claimed in claim 1 in which the stainless steel contains from 18 to 35% by weight of chromium. 3. A method as claimed in claim 2 in which the stainless steel contains from 25 to 35% by weight of chromium. 4. A method as claimed in claim 1, 2 or 3 in which the stainless steel contains from 8 to 15% by weight of nickel. 5. A method as claimed in claim 1 in which the stainless steeL comprises 25 to 35% by weight of chromium, 8 to 15% by weight of nickel, 0.1% maximum weight of carbon, 2% maximum by weight of manganese and 0.4% by weight of silicon, the remainder being iron and incidental impurities . 6. A method as claimed in any of claims 1 to 5 carried out to coυer from 50 to 85% by area of the underlying surface . 7. A method as claimed in any one preceding claim in which the particles themseiυes range from 25 to 50 microns transverse dimension, 8. A method as claimed in any one preceding claim in which the underlying surface is itself roughened. 9. A method as claimed in claim 8 in which the peak-to-υalley distance of the underlying roughened surface is from 15 to 20 microns. 10. A method as claimed in any one preceding claim in which the underlying surface is an aluminium surface. 11. A method as claimed in claim 1 and substantially as herein specifically described. 12. A metal-coated substrate produced by the method as claimed in any one preceding claim. 13. A method of coating a substrate with a fluorocarbon layer in which the metal coated substrate as claimed in claim 11 has a fluorocarbon polymer coated thereon from a liquid medium, and is dried and cured or sintered. 14. A method as claimed in claim 13 in which drying is carried out at 120 -150°C and curing or sintering is carried out at 200-450°C tor up to 30 minutes. 15. An article having thereon a fluorocarbon surface coated by the method as claimed in claim 13 or 14. 16. An article as claimed in claim 15 in the form of a tool.

Inventors: WALL NICHOLAS JOHN, WALL, NICHOLAS, JOHN
Applicants: WHITFORD PLASTICS, WHITFORD PLASTICS LIMITED
Application year: 1988
Main classification: C23C4
IPC codes: A47J36, B05D5, C23C4, C23C28