Chloride pitting in hard chrome plating?

[JasonLouie]

Someone told me that hard chrome plating is susceptible to pitting from chlorides. Is this true? I thought hard chrome plating was pretty corrosion resistant. We have heavy hydrocarbon stream with up to 5000 ppm chlorides at 210-250F. The pump rotors are plated with 0.010" and we are worried what the chloride pit depth would be.

 

[DedalusEnviro]

It is not entirely clear to me what you mean.

You allude to a "heavy hydrocarbon stream". Do you mean organic chlorides in it, such as perchloroethylene? In that case, you should be fine.

If you mean water with dissolved chloride ions, that is something else again.



Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[JasonLouie]

The stream contains oil and water and the chlorides would be in the form of sodium/calcium/magnesium chlorides.

 

[swall]

What can happen is that chlorides penetrate the microscopic cracks in the hard chrome deposit and cause the underlying substrate to corrode.

 

[DedalusEnviro]

If it is a good hard chrome job, I doubt that there are any pits or cracks that would penetrate all the way down to the substrate.

Is the aqueous phase very acidic? Acidic media with high levels of chlorides are truly, the Olympics of corrosion resistance.

I've looked into this, a little, and haven't found any real hard information. My sense is that if the pH is above, say, 4 - 5, that you should be alright.

Any stray currents, or intermetallic galvanic potentials generated could cause trouble in a hurry, however.

Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[swall]

Dave, I would have to disagree with you. It is well established that hard chrome deposits can experience corrosion of the steel substrate unless crack free chrome is specified or a nickel underlayment is used prior to hard chrome.

 

[DedalusEnviro]

I retract what I said.

I did a little looking up, and apparently the substrate in hard chromium plating is not invariably nickel plated, as it must be in decorative nickel plating. Also, some degree of micro porosity is often considered desirable, for lubricant retention. Though, if I were making pump rotors, I think I'd specify a nickel underlayer.

Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[TVP]

JasonLouie,

Steel that has been plated with hard chromium does not possess outstanding corrosion resistance in chloride containing environments. The best quality hard chromium will resist ~ 100 hours of continuous salt spray before the appearance of pits. This is difficult to achieve on complicated parts with changes in section, crevices, corners, etc. I would expect that pump rotors will experience pitting on the edges in your environment.

 

[JasonLouie]

Thank you everyone for replying to my post. I've received very good advice and information.