why alloy steel use zinc coating while stainless steel use copper coating?

[yudhichen]

Dear all,

anybody can tell me why we have to choose zinc coating on alloy steel (eg: 4130) and copper coating for stainless steel (eg: 410)?
i'm not so familiar with coating. though most of literature mentions this typical coating for corrosion inhibitor purpose,
but so far i havent had seen any literature that mention 'why' zinc & 'why' copper on particular steel.
is there any of this fact obviously mention on some ASTM standard?

thank you in advance

yudhi

 

[tbuelna]

I've never seen cooper plate specified for corrosion protection on stainless steels. I have seen zinc plating often specified as corrosion protection on carbon and alloy steels, but while zinc plating is cheap, it also does not provide the best level of corrosion protection.

What is the particular application you are asking about?

 

[yudhichen]

well....
the one that I'm asking about especially in thread.
in my line work as i mostly deal with oil field casing hanger with thread,
i saw 'copper' is for stainless steel, while 'zinc' is for alloy steel.
(i mostly deal with 410SS and alloy steel 4130).
then, it made me curious why the pair is always:
zinc - alloy steel
copper - stainless steel
aint there any other coating for alloy steel instead of zinc.

so far i didn't get solid answer why it has to be like that from scientific perspective.
or is it just only budget issue, zinc is cheapest and works.

any idea folks?

thanks anyway for replying, appreciate that.

 

[TomDOT]

Are you talking hot-dip galvanizing for the zinc? Properly applied hot dip is not only good corrosion protection for steel, it's tougher and more abrasion resistant than most other potential coatings, particularly the iron/zinc alloy layers.

 

[btrueblood]

I've not seen copper plating for stainless steel - presumably this is to avoid galling of the fasteners? I have seen copper-containing anti-sieze (anti galling lubricant) used, but we prefer (have had better luck with) MoS2-containing lubes for stainless fasteners.

 

[CoryPad]

The basic reason to use zinc plating on steel is that it is relatively low in cost and offers adequate performance. There are a multitude of coating options for alloy steel other than zinc, too many to detail here.

For your application, the use of a copper coating on stainless steel appears to be to prevent galling. The reason zinc is not used is that there would be galvanic corrosion between the zinc and stainless steel.

It seems like you need to get some training on coatings for metals. Try NACE or similar:

http://www.nace.org/cstm/education/Program.aspx?id=f017dc9e-4853-dd11-889d-0017a446694e

 

[yudhichen]

all...thanx for the response.

i agree...looks like copper coat on stainless steel was used for anti galling lubricant.

as CoryPad said about galvanic corrosion, i started to dig more information about it.
looks like i found the reason why we have to use zinc coat on alloy steel.

if i look at galvanic corrosion chart, supposed to be SS still less prone to corrosion than zinc
if we consider zinc coat on stainless steel.... am i correct?