Heat Treating Aluminum and Steel

[tabrann]

This has been a topic of much debate... I work in an aircraft repair facility where sometimes we manufacture aluminum parts. These parts are then treated in various ways for temper, aging, annealing, etc. There is a concern though regarding steel and the fact that steal can and does liberate carbon during heating, dependent on the type of steel. In our case, it has come to my attention that we are using a very large steel grate and skid rails that are all made of mild steel and have begun to rust as well. What I need to know is how significant this is to our aluminum solution heat treating process? I have only heard that carbon deposits will cause the aluminum alloy to corrode faster, but I have heard nothing about the effects of direct contact with iron oxide. To my knowledge, neither of these conditions have affected hardness or conductivity testing, but that is not to say that corrosion has not been accelerated. If anyone has any reference material on this I would greatly appreciated it. We currently operate under AMS standards. I have tried to search the AMS database for this condition with no luck, but if anyone knows of an AMS document that does contain this condition, that would be best. Thank you all for your assistance.

Thomas A. Brannock

 

[SMF1964]

What temperatures are you dealing with? I would expect that the loss of carbon from the steel is not an issue with the aluminum ageing temperatures (too low). What sort of cleaning process is used once the parts are out of the furnace?

 

[tabrann]

SMF1964,
The temperature does not exceed 1000°F. I'm not sure what cleaning you're in reference to; part or furnace? As far as I know there is sufficient cleaning of parts before and after heat treat, but of the furnace, I believe they hardly ever clean it. Not that they would know how to. For uniformity, air is ducted through side panels that have tabs opened to various degrees. Getting behind said panels to clean might prove very difficult. I have heard that the real source of contamination occurs if you use the same quench tank for both materials. What is your take on that?

 

[Nadimuthu]

The answer to this will come very clearly from an industrial chemist.If there are iron oxide or carbon deposits on the aluminium that is being heat treated, and there is a difference in potential, a galvanic cell will be formed and this will lead to the corrosion phenomenon.
Contact your firms chemists with this information and they will lead you quickly to the solution.
If this is the cause, then you can try to prevent iron oxide, carbon deposition of the aluminium plates.

 

[tabrann]

Nadimuthu,

Thank you very much for your explanation of the effect. My company does not employ chemists. As a repair facility they only care about structural engineering and every attempt to get information in other disciplines has proven that they do not have any knowledge of any other field. But your explanation is exactly what I was looking for. I have limited knowledge of chemistry, but a bit on thermocouples and can apply that to what you mean by potential. I know that the same heat applied to different metals does not result in the same potential, so how much of a difference could cause the galvanic effect? I'm only making the assumption that while all materials are still solid, the potential differences would be minor and most likely incapable of galvanizing. Also, I was reviewing melting points for all materials and it would appear to me that theoretically, carbon cannot leave steel until it nears the melting point of iron which is nearly 2800 degrees. So in the case of carbon contamination, I believe it is a farce, only applicable at temps well above that for solution heat treating of aluminum. However, the iron oxide is very real. The aluminum does reach temperatures that suggest the aluminum is nearing an aqueous state in which it could absorb contaminants. And since aluminum that is near it's melting point would have more potential than iron oxide that is nowhere near it's melting point, then I could believe that the galvanic effect is possible. Now the question is how significant is it? The max surface area of the aliminum that might touch the steel grate beneath would be 1/3 of one side. Maybe it would be wise to suggest they keep the grate cleaned of all rust to prevent this. It would be a hassle, but probably worth it if the risk of corrosion is high. What do you think about that?

 

[Nadimuthu]

Galvanic potential in millivolts or so is enough to cause corrosion.
If you are unable to get a chemist to solve the problem which will be long term, you are remove the impurities by cleaning as you suggest.
Good luck.