Thermo-Chemical formation of Chrome Oxide coatings on Steels 2

[nistala]

Can any one please tell how to produce thin coatings of Chromium Oxide on steels by thermo-chemical treatment of steel surfaces. I read that a slurry of Chromic Acid and metal silicates is painted on the steel surface and heated at 700 to 900 degrees celsius to form adherent films of a ceramic coating. This is claimed to be highly resistant to wear.
many thanks

 

[metengr]

I believe what you are referring to is a surface treatment called "chromizing". The internet contains plenty of information on this process. Chromizing does not produce a coating on the steel, it is a treatment that alters the surface of the steel.

The surface of the steel is prepared and coated with a chromium/silicon compound. After this operation, the part is heated (around 850 deg C min) and held for a specific time period at this temperature to allow for diffusion of the chromium/silicon atoms into the steel surface. This increases the chromium and silicon chemical content to a specified depth into the steel, which provides for increased resistance to corrosion in certain environments. Chromizing can also increase resistance to sliding abrasion in certain process applications.

I have seen chromizing used on boiler tubing to increase corrosion resistance on water-touched surfaces, and also increase resistance to oxidation on steam-touched surfaces of tubing or piping.

The only drawback that I have read about this process is local decarburization at the boundary between the diffused chromium layer and carbon steel. Chromium has great affinity for carbon to produce chromium carbides. However, the use of silicon in conjunction with the chromium seems to have reduced this problem.

 

[nistala]

Thank you very much Sir, for your quick response.
I read that this process is slightly different from diffusion Chromising, Aluminising and related pack cementation processes.
It does produce Chromium Oxide coating on the surface, which is distinct from the chromium enriched diffusion layer.
K-Tech and BodyCote claim that this oxide coating is dense, pore free and wear resistant

http://www.bodycote.com

Bodycote K-Tech ceramics - a unique range of thermochemically formed ceramic coatings for the prevention of wear and corrosion in a wide variety of severe industrial applications including complex geometric shapes and internal bores.

http://www.actisuk.com

 

[metengr]

Coatings are fine for targeted applications. I am of the opinion that coatings have significant risk of spalling or exfoliating in service.

 

[unclesyd]

We used Chromizing and Alphatizing on mechanical components for our process for many years.
We Chromized mainly 4140, A2, and D2 machined components that were subject to an average 18 day heating cycle to 285°C then a short exposure to 535°C then cleaned in an alkaline ultrasonic bath followed with a HNO3 dip. The parts were very durable and I don't recall any being removed from service due to failure of the Chromizing. Some of these components were in service for 30 years. Chromized parts were gradually replaced, mainly design changes, with PH SS components.
The Alphatizing was an earlier CrO process and I believe it was a pack process.

The only CrO coating process that we found viable for our service was “Polymet” at the time from Lambertiville Ceramics. Though we never got the straight scoop it was touted as reactive process but later it was found to be arc process.

We tried every process on planet, both coatings and diffusion, with generally poor return with the exception of the Al2O3.
In an industrial irony the whole time we had an in house process to diffuse Cr up to 70% + at the surface of most metals to impart wondrous properties and no money was provided for development after several short pilot runs.