Coating processes – Heat decomposition of applied coating or base material – Base material decomposed or carbonized
Patent
1990-02-26
1991-12-03
Lusignan, Michael
Coating processes
Heat decomposition of applied coating or base material
Base material decomposed or carbonized
427380, 4273881, 427405, 427409, 427423, 428421, 428463, 428553, 428685, 428937, B05D 302
Patent
active
050699377
DESCRIPTION:
BRIEF SUMMARY
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 32 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 alu
REFERENCES:
patent: 4088046 (1978-05-01), Severinsson
patent: 4753848 (1988-06-01), Sugarman et al.
Lusignan Michael
Whitford Plastics Limited
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