Corrosion of Stainless in Marine Environment

[Adamgnt]

Hello all,

I have frequented this site often this week searching for my answers before starting a thread. But everyone does such an awesome job wih quality replies I thought I'd just dive in.

My issue is some pitting corrosion I am experience with a 316 stainless jaw that holds onto a composite clevis pin in a loaded marine environment. In the two examples I have, the system had been under a 40 lb load for 6 months underwater in coastal New England. The corrosion is only evident at the area where the jaw is experiencing it's load and the rest of the framework has resisted. Out of the possible choices I cannot determine if it is due to crevice corrosion, (the water may be stagnant around the contact area?) S.C.C. (I was under the impression this only occured at higher loads and warmer temperatures?), or the fact that the motion of the jaw slightly rotating due to currents is maybe rubbing off the passivation layer? Also, there are already sacrificial zinc anodes present elsewhere on the device due to the presence of aluminum and stainless.

Some suggestions have been made to coat the whole system with some coating but I am not convinced it will help. One opinion states that if the exposed surface area of the cathode (the untouched steel) is reduced, then there will be less electron availabilty to drive the corrosion of the anode (the pitted area next to the pin). I can see the logic but I am curious to hear what this community has to offer for insight. Wide open to anything that anyone would like to add!

Adam

[Gordon]

Hi Adam

to the Surface Engineering Forum.

Quote:Out of the possible choices I cannot determine if it is due to crevice corrosion, (the water may be stagnant around the contact area?) S.C.C. (I was under the impression this only occured at higher loads and warmer temperatures?), or the fact that the motion of the jaw slightly rotating due to currents is maybe rubbing off the passivation layer?

I think all three modes, possibly in combination. S.C.C. (Stress Corrosion Cracking) or chloride stress cracking is certainly made worse by increasing load and or temperature (also by conditions favouring crevice corrosion and wear) but I'm not sure if there is any cut off point where S.C.C. can't occur.

Quote:Also, there are already sacrificial zinc anodes present elsewhere on the device due to the presence of aluminum and stainless.

I think this should help reduce the problems. May be more are needed, or placed more locally to the effected area.

Quote:Some suggestions have been made to coat the whole system with some coating but I am not convinced it will help. One opinion states that if the exposed surface area of the cathode (the untouched steel) is reduced, then there will be less electron availabilty to drive the corrosion of the anode (the pitted area next to the pin). I can see the logic but I am curious to hear what this community has to offer for insight. Wide open to anything that anyone would like to add!

Well in theory Coating the whole system in a coating that isolates it from environment and electrical conduction will help, BUT only if compete and durable. Any breakdown in the coating may just make things a whole lot worse. Coating system with a sacrificial zinc or may be aluminium could be another way (caution - not sure on effects regarding S.C.C). Make you think may be better off making structure from cheaper carbon steel and coating with zinc or aluminium.

What is the composite material in contact with jaws? Is this complicating the problem?

Coating the contact area of jaw with coating like Hastelloy material. Again you need to be careful to protect the coating edge/stainless steel interface as this will become another potential site for problems. An additional overlap coating of zinc/aluminium over this interface area may solve that problem.

As you are probably well aware - a very complex issue

[Grettir]

Hello,

If you suffer from pitting corrosion, you have cleary a to low PRE-N value of your stainless steel. AISI 316 is a poor material to use in marine/subsea environments.
SCC cracking strongly related to the type of material as well. The higher temperature the higher alloyed steel you will need to prevent this.
A duplex steel will help for the pitting problem as it has a much higher PRE-N value. It also has much better resistance against sCC with higher temperatures. Crevice corrosion is possible if the electric contact between the 2 parts is not changed but remains the same. Larger clearances can avoid for example crevice corrosion.
But pitting corrosion is mainly related to the PRE-N value of the steel.

[Gordon]

Hi Grettir

to the Surface Engineering Forum.

Thanks for your input

To expand a little on pitting resistance equivalent numbers (PREN)

http://www.bssa.org.uk/topics.php?article=111

[vittorio]

(09-21-2009 12:18 PM)Adamgnt Wrote:  Hello all,

I have frequented this site often this week searching for my answers before starting a thread. But everyone does such an awesome job wih quality replies I thought I'd just dive in.

My issue is some pitting corrosion I am experience with a 316 stainless jaw that holds onto a composite clevis pin in a loaded marine environment. In the two examples I have, the system had been under a 40 lb load for 6 months underwater in coastal New England. The corrosion is only evident at the area where the jaw is experiencing it's load and the rest of the framework has resisted. Out of the possible choices I cannot determine if it is due to crevice corrosion, (the water may be stagnant around the contact area?) S.C.C. (I was under the impression this only occured at higher loads and warmer temperatures?), or the fact that the motion of the jaw slightly rotating due to currents is maybe rubbing off the passivation layer? Also, there are already sacrificial zinc anodes present elsewhere on the device due to the presence of aluminum and stainless.

Some suggestions have been made to coat the whole system with some coating but I am not convinced it will help. One opinion states that if the exposed surface area of the cathode (the untouched steel) is reduced, then there will be less electron availabilty to drive the corrosion of the anode (the pitted area next to the pin). I can see the logic but I am curious to hear what this community has to offer for insight. Wide open to anything that anyone would like to add!

Adam

as a general rule stainless steel 316 is not a good choice for seawater exposure expecially in stagnant conditions.
coating could be a solution but the best thing will be to use a more resistant materials in seawater like superduplex.

Stainless Steel Corrosion Water Seawater

hope this help

Vittorio

corrosion & rust control

[pulaunias]

I remember that SS is sensitive to chloride, which can prevent the self-healing of the passivating layer, once it breaks at a microscopic scale (it alwasy does).

If the part being corroded is in contact with something, a reason may be the stagnant fluid. The theory is said to be that, once SS corrodes a tiny bit, the corroded product (metal ions) is acidic; in stagnant conditions, these acidic products accumulate and, in turn, acclerate the corrosion, thus forming a self-acclerating vicious cycle. Moreover, the stagnant seawater is somewhat different from the bulk seawater in, for example, concentration of dissolved oxygen; this difference creates a Galvanic cell and also accerate the corrosion at the "hidden" part in contact with the stagnant seawater.

Titanium can resist seawater for a long long time, but pretty expensive and also weaker compared with some steels. Not sure if it is a good candidate here or not.