Does anyone knows where to go to find the pH range where different coatings can resist, say ceramics, carbides, etc...? Or the different chemicals they can resist?
Thanks.

yapwx Wrote:Does anyone knows where to go to find the pH range where different coatings can resist, say ceramics, carbides, etc...? Or the different chemicals they can resist?
Thanks.

Several of the larger players Praxair, Sulzer have generated information over the years. However, this information tends to be proprietary to each company. What I can confirm is that Chrome Oxide is inert to most acids with the exception of Concentrated Hydrochloric Acid (37%) - the coating can become weight exhibited.
Surprisingly, the HVOF Tungsten Carbide Cobalt coatings are the next best with good resistence to most acids (Aluminium Suplhate, Halides, Hydrochloric acid, Sulphuric acid, Nitric acid, Ammonium Hydroxide)Tungsten Carbide Nickel Chrome and Tungsten Carbide Cobalt Chrome also providing good corrosion resistance especially in sour gas and chloride saturated environments. I would not recommend Tungsten Carbide Cobalt for chloride duty.

regards

Stewart

Hi Stewart

to the Surface Engineering Forum and thanks for your input.

Hi Yapwx

Metco of old produced a "Handbook of Coating Recommendations" and in this, was a fairly extensive section on chemical corrosion resistance. This is the only type of coating specific chemical corrosion info I have come across covering many coatings/chemicals. But, I think this information should only be used as a rough guide or starting point.

There are many generic chemical corrosion resistance charts around, but these tend to cover bulk cast or wrought materials. Again useful as a starting point, but coatings can be very different considering porosity, metastable phases and oxide inclusions etc. which can make them respond very differently to a comparable wrought material.

Corrosion tables also do not consider the effect of substrates. Majority of coatings require sealing, so sealer corrosion resistance also needs consideration.

Thanks for your help, we have decided to do some testing.

(08-14-2008 05:55 AM)yapwx Wrote:  Does anyone knows where to go to find the pH range where different coatings can resist, say ceramics, carbides, etc...? Or the different chemicals they can resist?
Thanks.

look at Special Metals website or Carpenter alloys websites. Both have a good amount of information. Special metals has a whole book.

Hi Intel55

Thanks for the pointers, some very good information there

Very often this type of information is the only thing we have to estimate the likely outcome of a coating in certain environments without actually testing. I like to stress again though, that information on wrought and cast alloys should be used very carefully if applying that to thermal spray coatings formed from the same feed stock. Coatings differ in metallurgy and very often in chemistry. For example: customer specifies 316 stainless steel coating believing that this austenitic stainless steel is non-magnetic and provides well documented corrosion resistance in certain environments. Coater sprays job to specification, possibly being unaware to its final use. Customer then complains coating is magnetic and or is having corrosion problems. Wrought 316 is simply not the same as a coating sprayed from 316 wire or powder.