SS corrosion

[Graham_Wright]

There have been various references on the forum to crevice corrosion on 316 SS.
In underwater situations (oxygen starved) do zinc anodes prevent or decrease this?

 

[ccscott49]

Zinc anodes have no effect on SS crevice corrosion. For or against. From what I have been informed. The zinc anodes are there to protect steel and brass etc.

 

[boatmike]

No simple answer, but they may help. I am a marine engineer not a metallurgist, but the way I have had it explained to me by others better qualified than I, is as follows. True crevice corrosion is theoretically a function of electrolytic action within the intermetallic structure of the material (as I understand it) and the best answer is to try to introduce a flow of seawater in the affected area to oxygenate it, or change the material to Monel or a Duplex form of stainless that is not affected in this way if that is not possible. Many times however the pitting seen in stainless is mostly caused by conventional electrolytic erosion not crevice corrosion and here anodes will help of course. Where it gets complicated (again as I am lead to believe) is where both conditions occur together the electrolytic erosion will often be concentrated at the same place that the crevice corrosion occurs. In other words the crevice corrosion makes that area more likely to be anodic than the rest of the shaft or whatever, In this case anodes will help on the one hand, and introducing a flow of seawater over the area in question will also help. Perhaps a better qualified metallurgist would confirm but I think thats right. One thing is certain though. You cant have too many anodes and they can't do any harm!

 

[eyehavit]

Well, I am a metallurgist (Dip. Met. WA.) but do not practice as one. Someone, an Englishman, not me, [although he absoloutlely has my full support], /forums/images/graemlins/smile.gif said. [sound of trumpets] "A metallurgist is a Jack of all trades and a Master of some".
As long as oxyygenated water is present, crevice corrosion cannot occur. ...................Therefore...............................
Sorry, but I I have been wanting to say this for years. ..............." Gentlemen, start your engines!!!!"..................................
If you want some serious technical answers, pm me.

 

[Birdseye]

Oxygen is required to maintain the oxide film that makes stainless stainless. All a zinc anode can do is to give something that will erode sacrificially in sea water, but it can only do that if there is an electrical connection. So if the oxide film is damaged or rubbed away, then the zinc anode properly connected will help minimise electrolytic corrosion. Nothing more.

 

[catmandoo]

Well to join the fray I thought crevice corrosion as a localised event regardless of anodes elsewhere . Eg a little battery local to the crack

I also remember that in dealing with Stainless steel pressure vesels that during hydrotest we had to use water with a low chloride content to avoid crevice corrosion

 

[boatmike]

Agreed. I think what you and birdseye have said is another way of looking at it.
I too have had pressure vessels tested and had to use de-ionised water. A little local battery is a good way of looking at it but as birdseye implies this does not need a crack (although a crack or scratches will make it worse) it only needs the oxide layer to break down locally and then conventional electrolysis can set in. This can occur often at the inner bearing of a flooded propshaft when the engine has not been run for a while and the trapped water becomes de-oxygenated.
Thats why eyehavit suggests running engines. Bit of a problem if your boat sits in a marina all week though.....