Coal [carbon] dust combined with electrolyte [moisture] will destroy [unprotected] high strength aluminum alloys. What You will see is generalized corrosion attack [primarily pitting and exfoliation]... which could easily proceed to SCC, presuming part is thick and heavily machined exposing susceptible end-grain boundaries.
Even a thick anodize [not recommended without shot-peening] and multi-layered coatings [isolation] will not hold-up long in an abraasive environment. Note: overaging parts to -T73 temper, shot-peening, anodizing, painting and sealing will reduce susceptibility to exfoliation and SCC initiation... but once the finishes" are breached, You will still see rampant corrosion on bare metal [especially in the coal-mine].
Thank you for your reply.
Is it the sulphate salts produced by pyrite oxidation that destroy the alloy?
Will I be looking for sulfur in the crack zone under the SEM?
Is it possible to predict how quickly SSC will begin in my 2.5" thick machined part under heavy load?
There has to active corrosion (loss of surface passivation) in order for SCC to occur. If there is stress and any signs of active corrosion, then SCC can occur at any time.
In coal mines that I have seen there have been enough different sulfur compounds to kill almost any alloy.
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Corrosion never sleeps, but it can be managed.
Mercury will also induce SCC in aluminum alloys - technically this might be categorized as Liquid Metal Embrittlement. I know there's a big push to quantify mercury emissions in coal plants, maybe the levels are sufficient in the coal mining industry as well?
I appreciate all the responses.
Since I sent out my first question, I have learned that coal mining can indeed create the right environment to bring on SCC in the 7075 aluminum alloy.
There are 10 tie rods on this machine, pinned in tension and statically loaded to 40% yield for 2 hrs / day. After 3 years in the coal mine, none of them are showing pitting or exfoliation. A crack showed up in one of the tie rods a year ago starting at the outer surface of the tie rod and migrating toward the pin hole, typical of tear out type of failure. Examination revealed Cl, Ca, Si and trace S on the fracture surface and intergranular cracking consistent with SCC. All other tie rods were x-rayed and there were no cracks. Now another tie rod has cracked, similar to the first but in a different location on the machine suggesting that it is not an overload caused by misalignment.
The part is a critical structural member so I am inclined to replace the aluminum tie rods with steel. What is troubling is that we have many machines of the same design that have been operating in similar environments for up to 15 years with no evidence of cracking. Is it just a matter of time?
Time, stress, and environment (contaminant concentration) are the major variables. You can try coating the parts to reduce failure probability. Even a simple polymer coating (epoxy, urethane, etc. based paint) could improve the condition and be much cheaper than making new steel parts.
Regards,
Cory
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