Active vs Passve Stainless Steel Galvanic Corrosion 2

[WillCole]

I have a question about stainless steel and galvanic corrosion.

Most galvanic series lists show two different values for stainless steel. One for the active metal and one for the metal after it has been passivated.

I understand that passivation will increase galvanic compatibility with more noble metals. This stands to reason as the passivated layer provides a buffer between the two raw metals as well as having removed various possible contaminants from the surface.

My question is does passivation stainless steel actually increase corrosion in a less noble metal?

i.e. Which will corrode faster 1010 steel coupled to active 410 or 1010 steel coupled to passivated 410.

My intuition is that the passivation can only help and the charts are simply reflecting the improved compatibility with more noble metals. I would be very grateful if someone more knowledgeable could shed some light on this issue for me.

Cheers!

 

[EdStainless]

Or Fontana (or Fontana and Green) is another good ref.

In the chart Active and Passive refer to the material in the non-cording passive condition or the actively corroding state.

Pasviation as a treatment is a cleaning process that is meant to help optimize the surface condition of a stainless steel.
All SS will naturally form a passive surface layer on it's own in the air.
The treatment removes impurities that interfere with that layer formation (such as iron which will corrode easily and create a break in the film) and assure a well developed surface passive layer.

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Plymouth Tube

 

[swall]

What I took away from reading Uhlig 2nd edition, chapter 5, is that the passive or active behavior in the galvanic series is mostly dependent on the aqueous environment the galvanic couple is exposed to. So, back to the original proposition, the intentional chemical passivation of stainless would not, in itself,render the stainless passive in a galvanic situation.

 

[WillCole]

unclesyd - I read through the articles posted and while they are very informative those don't really address the thing that is tripping me up.

I know that active stainless is more anodic than passive stainless, but I don't really get why, which is where my confusion appears to be rooted.

My current understanding:

In active stainless there are oxides present from the corrosion of the underlying material, but the oxide layer does not form an encompassing barrier.

In passive stainless the oxide layer does form an encompassing barrier.

In both cases there are oxides present, so the mere presence of the oxides isn't changing the nature of the interaction - unless they are different oxides.

In the active state the underlying metal is exposed and it is free to interact with anything it is coupled too.

In the passive state there is no underlying metal exposed so whatever you couple passive stainless too is really only interacting with the oxide layer.

This is where is gets fuzzy:

This suggests to me that the voltage potential of active stainless is the 'true' voltage potential of the stainless material and that voltage potential of passive stainless is really the voltage potential of the oxide coating.

And... I did just order the Uhlig's book as suggested by SJones. I looked at the Fontana and Green, but I noticed it was from 1967 so I went with the newer reference. I will read the book before I continue to bug you very patient gentlemen on this issue.

 

[EdStainless]

The true potential of the stainless in the passive one.
If you see the active state you selected the wrong material!

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Plymouth Tube

 

[unclesyd]

If you pickle and passivate or just passivate SS it will be (passive0 in any particular media initially. As posted above SS will passivate itself when exposed to air thereby becoming (passive). It is hard to keep SS (active) when trying run potentiometric studies. because of this.
If you look at the Mil. galvanic series charts you see that based on their positions in the table 304 (passive) should corrode 304 (active), but it doesn't happen there is not enough driving force. Normally when SS is active and couples with a more noble metal the SS will become (passive) due to the O2 at the anode. If either metal is corroded by the electrolyte all assumptions about (active0 or (passive) go out the window.

 

[SJones]

Chapter 9 deals with passivity!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04