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!

 

[Metalguy]

Passivated SS will cause faster corrosion of a less noble metal in a corroding situation.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[EdStainless]

No, the two ranges have nothing to do with passivation.
They have to do with the state of the SS.
Is it still passive, not corroding, or is it active and experiencing corrosion.

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Plymouth Tube

 

[WillCole]

Metalguy - So the sword does cut both ways...

Is this effect short lived? i.e. In the example I gave with 410 stainless and 1010 carbon steel would the corrosion of the 1010 effectively contaminate the 410 depassivating it?

Or is this a function of compounds that make up the surface of the passivated 410 acting as the noble agent?

I am just having trouble understanding the mechanism in this particular instance. Thank you for the guidance.

 

[unclesyd]

The rate on 1010 CS will be higher that the rate on 410 SS as both will corrode in any corrosive enviroment. Under certain conditions, not a passivization, 410 SS will get an oxide coating that will protect it from atmospheric corrosion. This oxide layer isn't very stable as it can be altered by many environments.
410 SS doesn't passivate very well using Nitric or Citric Acid and I would question if there is any at all.
If you look at components made from 410 SS like the cheap SS knives they will start corroding almost out of the box.

 

[WillCole]

unclesyd - I understand that 410 isn't all that corrosion resistant. I guess, I was/am having trouble getting my around why passivated stainless causes more corrosion than active stainless when coupled to 1010 carbon steel.

I understand from the galvanic chart that the passivated stainless is more noble. However the passivated steel is still the same stainless just with a oxide compound over the surface. I just haven't figured out that accelerates corrosion in the coupled material. The only thing I have been able to guess is that the oxide layer is acting as a more noble metal. But in that case I would think that as soon as the passivated 410 started to corrode at all it would behave the same way - like active

Obviously, there is something I am not getting. Thanks for bearing with me.

 

[WillCole]

I might have finally gotten something through my head here...

I found this on a vendor site:

"Passivation is the chemical process that removes free irons from the surface of your stainless steel. This creates a chromium rich surface"

I think my issue was that I was failing to realized that by creating a 'chromium rich surface' that I am effectively bringing a higher chromium surface into contact with the less noble metal. The oxide layer isn't much of a player - as unclesyd pointed out.

Am I back on track now?

 

[EdStainless]

First, passivation is what the quote said, this allows alloys to achieve their optimal corrosion resistance.

If you have 410 and CS bonded in a moderately corrosive environment the CS will actively corrode, thereby acting as a sacrificial anode and keeping the 410 in its passive non-corroding state.

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Plymouth Tube

 

[Metalguy]

>"Passivation is the chemical process that removes free irons from the surface of your stainless steel. This creates a chromium rich surface"<

This statement isn't really correct. Pass. will remove free iron/steel, but it doesn't create a Cr-rich surface. What it does is make a thicker surface of chromium oxide, and it is the oxide that protects it from corrosion.

Look at Ti and Al. Both are very reactive metals, but the surface oxides which form on contact with air/oxygen provide the corrosion resistance. This resistance depends on both chemical properties AND the mechanical properties of the OXIDE.

Study repassivation properties in corrosive, stressed conditions. Enough to make grown men cry!

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[moltenmetal]

Correct me if I'm wrong, but I thought it was even possible for active and passive sections to be present on the same piece of stainless steel, creating a galvanic corrosion cell between the two sections themselves. The example I recall was the stainless steel shaft of a pneumatic cylinder used in a piece of water treatment equipment. The active sections were generated by removal of the passive layer by abrasion. What you ended up with was a shaft which looked like it had been chewed by squirrels, i.e. small but continuous areas (rather than pits) which had been corroded away to some depth, leaving other areas looking shiny and untouched.

 

[Metalguy]

It is, and is very common. It is the surface oxide layer which provides protection, and if that oxide is removed and for various reasons cannot reform, the base metal (substrate) can corrode in environments which would be harmless/very low corr. rates to the oxide.

Take a piece of 304 SS and passivate it. Immerse it in a weak solution of sulfuric acid (reducing envirn.-room temp, no corrosion. Now scratch the surface while it's under the acid solution, and it will start corroding where scratched (no protective oxide there).

Now pour in more acid, but this time use nitric (oxidizing). The scratches will repassivate, and the corrosion stops.

This subject, repassivation properties/kinetics, has cost the PWR owners/ratepayers billions of $$$ in the US, and lucky me was able to work over in Italy for 6 great years because of it.

Ciao!

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[unclesyd]

Here is one of many good papers from Imagineering that cover passivisation of metals. It covers all the points mentioned above.

One point not mentioned is that passivisation isn't the remedy for all corrosion problems. It will only delay the onset of red rust in a corrosive enviroment. It the metal, SS, isn't exposed to a reducing enviroment it will passivate it self in a very short time due to thickness of the oxide layer. Loner times normally will not increase this thickness due to the fact that the initial oxide layer is protecting the metal. A good percentage of the time this oxide layer is only one molecule thick.

If you don't mind the color, thicker oxide layers can be forced to form on the SS.

As the paper points out there is judgement call on when and how to passivate a metal.

Even tough Nitric Acid can passivate SS it will also can cause a high rate of corrosion. You will see data on many corrodents, which should only be used as a giude as a lot of them are done with reagent grade material and don't reflect the real world.

http://www.imagineering-inc.com/white_paper/passivation.pdf

 

[EdStainless]

After SS has been formed and fabricated you should always passivate. Removing the free iron from the surface will reduce the chances of local breakdown of the the passive film.

The surface Cr enrichment is very shallow, 10-50A is all.
But that is the layer that makes SS corrosion resistant.
If the oxide layer gets too thick it can actually reduce corrosion resistance.

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Plymouth Tube

 

[WillCole]

Metalguy - I started to realize I wasn't exactly on the right track when I noticed that Active 410 is actually listed as more noble than chromium plated.

If the Cr-rich surface was the real player then the plating would be more noble.

So I am still stuck a bit on "why" passivated 410 causes greater corrosion than active 410 when coupled 1010. I get that the more noble metal will accelerate corrosion in the less noble. But it seems to me that passivated 410 hasn't really become more noble, but rather is just wrapped in an oxide layer.

So I guess that brings me back to the idea that the oxide itself is acting as the more noble material...

Why do I feel like I am chasing my own tail?

unclesyd - I read the pdf and it was helpful. Especially the following line: "Even among the metal finishing community, there is some disagreement about the theory behind the process of chemical passivation."

At least I am in good company.

 

[EdStainless]

Forget the passivation, that is not part of this. This step just assures that the material will have an optimum condition. Good clean well processed material will not improve with passivation, typical material will.

You are talking about basic galvanic corrosion.
The surface Cr oxide layer exists on all SS, it is the very heart of what makes SS corrosion resistant. With different alloys this layer will resist breakdown differently. In 25% Cr 6%Mo it takes high chlorides and low pH as well as elevated temperature.

When you have one material and the passive film fails in one location that area becomes the anode and the rest of that part the cathode, hence you get localized corrosion.
When you have two material coupled and they have significantly different corrosion potentials (and the environment isn't so severe that they both will corrode rapidly)then the less corrosion resistant one (CS) becomes the anode and it will protect the more noble (410) by corroding itself.

If for some reason part of the 410 starts actively corroding then there is less of a corrosion current causing galvanic corrosion of the CS.

A pure Cr surface does not form a very robust Cr oxide layer. The mixed Cr Mo oxide layer that forms in Cr-Ni-Mo alloys is the most resistant to breakdown in many different environments.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[SJones]

why do I feel like I am chasing my own tail

You feel this way because you do not have a sufficient grounding in electrochemistry and because you are confusing the surface treatment procedure of passivation with the electrochemical classification of a material, based on polarisation behaviour, as 'passive'.

Steve Jones
Materials & Corrosion Engineer

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

 

[WillCole]

edstainless - Actually I am specifically talking about the impact of passivated versus non-passivated stainless on other materials. Somehow I must be stating my confusion poorly or misusing terms.

Let me try this another way...

If we go with this chart:

http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm

In the coupling of #43- Stainless steel 17-7PH (active) and #56 Red Brass the 17-7PH is more anodic.

Where as in the coupling of #56 Red Brass and #69 Stainless steel 17-7PH (passive) the Red Brass is more anodic.

I took/am taking the meaning of 'passive' in the chart to mean 'passivated.' This suggests to me that which metal will be sacrificed switches. I am having trouble understanding how passivizing the stainless (a surface treatment) could entirely reverse the galvanic process as the stainless under the surface doesn't change state.

I can understand how passivating stainless would make it more tolerant of coupling with a more noble metal. That seems intuitive to me as you have added a barrier between the base metals.

The part I am confused about is how (by what mechanism) the passivizing stainless negatively effects the (in this case) red brass.

I have to say I am impressed with folks willingness to help me get my head around this. I hope I am at least providing entrainment value. Thanks again.

 

[WillCole]

Just to be clear. I do understand that passive in the chart does not literally mean 'passivated.' I am just assuming that passivated stainless is passive at least until the oxide layer is breached.

 

[SJones]

May I suggest that you spend time reviewing Chapter 8 of Uhlig's Corrosion Handbook, or the appropriate section of some similarly august tome, as it is difficult to write an answer out in the small text box available to us?!!!

Steve Jones
Materials & Corrosion Engineer

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

 

[unclesyd]

I' going to pass on SJones's recommendation of using uhlig's book as it is hard to find and to purchase it is expensive.
Here are three papers that go into the galvanic corrosion process.
If these don't answer your question please post.

http://www.atlassteels.com.au/documents/TN7-0506-Galvanic Corrosion.pdf

http://www.npl.co.uk/upload/pdf/bimetallic_20071105114556.pdf

http://www.goes-r.gov/procurement/flight_documents/MIL-STD-889 B.pdf