Corrosion 7

[metman]

What is mils/yr and susceptibility to pitting corrosion in salt water C954 AL Bronze (UNS95400)?

 

[TVP]

I don't have specific data for that alloy, but the corrosion rate should be less than 2 mils/yr (50 microns/yr). Aluminum bronze has excellent resistance to salt water, including resistance to pitting.

 

[metman]

TVP - No wonder your'e at the top of the list. That was a quick reply. You are so right. My boss had my metals hdbk and when I got back found it was 1.9 in one test and 0.5 in another.

New question now that I have more info. I see that Al Bronze vs Aluminum in seawater has galvanic potential of +535 millivolts. If 6061 Al is anodized next to the Al Bronze in seawater, what is the galvanic potential?

 

[TVP]

metman,

Sorry, but I do not have any data on this potential. What I can tell you is that a properly deposited anodic coating that has been subsequently sealed, will provide an excellent barrier against galvanic corrosion. Hardcoat anodizing (thickness ~ 50 microns, vs. 2 microns for chromic acid process, and 10 microns for sulfuric acid process) will yield the best results.

The anodic coating is typically very porous, which is why subsequent sealing treatments increase the corrosion resistance so dramatically, especially in a galvanic environment. There are some good technical papers online regarding these processes at the following website:

http://www.metalast.com/index2.html

Some of the other members seem to have better access to mils/yr data, so maybe they can help. I haven't seen much data on galvanic couples when an inorganic coating is used on one of the metals.

 

[CoryPad]

I also do not have instant access to mils/year data, but can offer some insight into the galvanic potential for anodized material. At a microstructure level, anodizing creates an insulating ceramic layer on the material, so the new galvanic potential is a function of the breakdown voltage for the ceramic. The non-uniformity of the ceramic layer means you can't substitute the value for the bulk ceramic material, but it certainly gives you an indication of how much the couple can change. If this information is important for your application, you really should perform a test.

 

[metman]

TVP and CoryPad

You guys are fantastic. This info and clues to follow-up are right on target and very timely and helpful.

Thanks again

Leonard

 

[bilge]

Since I had the reference out to answer a later thread I might as well give you some numbers. The free corrrosion potential of aluminum alloys runs from -0.75 to -1.0 volts (sce). That of aluminum bronze, from -0.43 to -0.53 volts (sce).

 

[metman]

bilge
Thanks for your input. I suppose that the free potential numbers that you have provided would correlate with 0.535volt potential in seawater.

TVP replied to a thread by GRPatel on the metallurgy Forum 4/26 which is very helpful to my understanding and should be of interest to anyone following this thread.

Please correct me if I am wrong: If the anodised layer is of the thicker 50micron version it will be knit tightly enough that porosity will be minimal but organic sealing fills in the remaining porosity to enhance corrosion protection dramatically. Whereas if sealing is not done, porosity would allow localized pitting corrosion because of the resultant high current density produced by proportionally minute surface areas at pores.

So far I have not been asked to persue this investigation. If I do, I will very likely follow TVP's link to metalast.
Jesus is the WAY

 

Ref corrosion rate.
A word of warning - ALWAYS give your friendly corrosion eng the operating conditions eg is the salt water static or are there flow rates that you can quote. The difference in the answers will frighten you!! The corrosion data books have a habit of glossing over the test conditions on which the reults are quoted.

 

[metman]

volvo90
What- you admit to owning a volvo? Just joshing and since I am quarter Swede must show a mite of respect to the heritage.

Point well taken on operating conditions. This galvanic couple is on the nose gear of an amphibious float for airplanes which makes it difficult to quantify the conditions but since the gear is retracted for water landings, I doubt that there would be any direct impingement of seawater and therefore static results would apply. Yes I can imagine erosion/corrosion of seawater must be dramatic for most cases. Jesus is the WAY

 

Hi Metman,
My 1990 Volvo 740GLT is still running like a sewing m/c!!
And the bodywork is pristine - must be the way the Swedes applied the science of electrochemistry!!
Best Regards