Corrosion of threaded stainless steel screws on Aluminium plates 2

[sayee1]

Its a funny thing, referring to the galvanic series, Aluminium alloys are more anodic to stainless steels(passive or active) in marine environment. We have practical experience however of the preferrential corrosion of stainless steel screws on aluminium plates. The plates are not immersed in water but are in marine environement(typically used for offshore platforms). Any explanations, my guess though not educated is that the threads are preventing the flow of fluid/gas nearby the threads, causing lack of oxygen or supply of fresh oxygen and thus leading to crevice corrosion. The funny part is that in this case Al which is anodic is unaffetcted. Any refernces, technical literature, backup for the above surmises and theories on the above most welcome. Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
End of all knowledge is the attainment of immortality!

 

[Watermelon]

Hello.

I am not a corrosion engineer, so what follows may be nonsense. But I hope it is useful to you.

On sail boats, contact between stainless screws and aluminum plates is very common. It is a hot topic on many sailing sites. And many sailors are fanatical about making sure every nut and bolt is absolutely perfect. I would recommend you surf the net for such sites to gather information.

Typically some kind of sealant is used to isolate the two dis-similar metals. There are several brands on the market. I won't mention trade names here. Some remain soft so that the screw can be easily removed if needs be. Others will harden so that removal of the screw is next to impossible.

 

[kenvlach]

sayeeprasadr,
Your results are not surprising, and your opinion of the mechanism is basically correct. Both aluminum and Cr-alloys depend upon a very thin oxide film to resist corrosion. When the threaded connection is made, this passivating film is easily damaged (this is why both aluminum and SS readily self-gall, because metal-to-metal contact is made at a relatively low torque). From thermodynamics, as reflected in the galvanic series, aluminum has a stronger affinity for oxygen than chromium (especially with a_Cr ~ 0.18), so with a limited oxygen supply, the aluminum can re-passivate whereas the SS cannot. [and it’s a safe bet that a marine grade of aluminum plate was used]

Watermelon made a good suggestion about sailors. The Boatowner’s Guide to Corrosion, by E. Collier (2001), specifically mentions some lubricant/galvanic isolators for SS screws in aluminum: CRC Corrosion Inhibitor, Boeshield T-7, Corrosion Block, Corrosion X and LanoCote.

 

[Metalguy]

Kenvlach,

Good point, that's what is almost certainly happening.

Let's also remember that oxygen-related corrosion in general ocurrs where the oxygen concentration is lowest, and the rate is high when there is a big difference in oxygen levels.

 

[sayee1]

Thanx,
Kenvlach and Metalguy, a star for your explanations which helped me immensely. Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
End of all knowledge is the attainment of immortality!

 

[sayee1]

One more question, if it is not galvanic corrosion, what type of corrosion can it be classified as? crevice? uniform? Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
End of all knowledge is the attainment of immortality!

 

[Paskenell]

http://www.mcnallyinstitute.com/04-html/4-1.html

This article should answer your question

David

 

[kenvlach]

sayeeprasadr,
The corrosion is almost certainly a combination of crevice corrosion and stress corrosion cracking in the unaerated chloride environment. The stress is residual stress from the initial manufacturing; nearly all SS screws are made by cold heading and thread rolling. This gives the manufacturer a high yield strength (> 100 ksi) for advertising, but trashes the corrosion resistance of the SS (best when annealed).

An article was recently published on this type of reduced corrosion resistance for cold-worked SS: ”Effect of Cold-Working in the Passive Behavior of 304 Stainless Steel in Sulfate Media,” A. Barbucci, G. Cerisola and P. L. Cabot, J. Electrochem. Soc., Vol. 149, pp. B534-B542, December 2002.
“The pitting susceptibility also increased with cold-work, this effect being more evident as the chloride concentration increased...the increased susceptibility to pitting corrosion of the work-hardened samples were explained by the formation of a much more defective oxide during its anodic oxidation, with easy paths that enhanced sulfate ingress. The growth of such a much more imperfect oxide was related to the formation of defects in the grains and of more defective interfaces in the bulk of the material, both resulting from the accumulation of internal stresses during cold-rolling.”
from the abstract at

http://ojps.aip.org/getabs/servlet/...0014900001200B534000001&idtype=cvips&gifs=Yes