How can I calculate the corrosion rate of dissimilar joint?

[jomyut]

I welded the lap joint of Al/Fe and use it in sea environment and I found the corrosion was taken place at the crevice area of Al/Fe lap joint. I read some books and they explain that it was the crevice corrosion in the lap joint but most of the books have explained only in case of the lap joint of similar metal. No book has explained for dissimilar lap joint so far. I wonder, what kind of corrosion in this case? Are there crevice or galvanic corrosion? Most of the formula for calculation the corrosion rate of metal are available for single metal, are there any formula that can calculate the corrosion rate of the dissimilar joint?

 

[Metalguy]

You should be seeing the Al side corroding away as it protects the Fe. Corrosion rates are hard to predict-depends on many factors.

*IF* you cannot change this bi-metal connection or protect it with a good epoxy, etc., you might try protecting it with sacrificial zinc anodes.

 

[EdStainless]

The reason that you can't do calculations for this is that the corrosion is not uniform and kinetics get in the way. There are a lot of rate controling steps that you simply can't estimate the values of.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[mshimko]

Generally I concure with edstainlkes in that there are many factors involved, including (but not limited to) pipe diameter, salinity, presence of other dissimilar metals, etc.

I also recal, though, several years ago we required the detailed designer to estimate the galvanic corrosion rates due to incorporating large segments of noble metal (we wanted SOMETHING to base our maintenance intervals on). Lacking field data, we accepted an analysis using a methodology published by...I believe "Ashlie" we the investigator's name, and involved the use of hyperbolic trig functions.

That was over 10 years ago and my memory's sketchy on details, if I have a chance to dig up my archives and find any details I'll provide later.