Inconsistent Results from Passivation

Is improper heat treating the more likely culprit?


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Q. I have passivated the same part, made from 416 SS, in my Nitric/Sodium Dichromate bath a couple of times. On one occasion the parts were attacked and subsequently became scrap. The parts—a few days before and a few days after—were all OK. I document the percentages Nitric Acid on a weekly basis and everything is within specifications (ASTM A-380 Code G). Why would the one batch get attacked and not the others? After having the material analyzed the only major differences in the chemical composition is 0.3% Manganese higher in the attacked parts and 0.25% higher in Copper as well. Could this be the problem? Is improper heat treating the more likely culprit? Any help would be greatly appreciated. A.C.

 

A. I would look at the temperature control from the company performing your heat treating first. The 416 grade of stainless steel is in the martensitic stainless steel family. This is similar to the ferritic grade in that it uses chromium to provide corrosion resistance, however, the carbon content is higher than the ferritic grades that allow the martensitic grades to be heat treated. Within this category, the 416 is considered to have relatively low carbon content, but can still suffer from embrittlement and loss of corrosion resistance if improperly heat treated.

Between about 700–1,000°F, the martensite transforms into ferrite and iron carbide which will also tend to tie up some of the chromium making the stainless steel significantly less corrosion resistant. This would explain the attack you see in the acidic passivation bath. Instead of forming chromium oxide and dissolving impurities, the solution will attack all the areas of the grains where the chromium is depleted and will no longer be stainless steel in those locations. Also, if heat treatment occurs in this region, the material will also show a loss in ductility and toughness.

I am guessing you are sending your parts out for tempering or annealing which is the next temperature range higher than the problem range I mentioned above. It could be that the company performing your heat treating is feeling the energy pinch like everybody else and operating their furnace on the lower end of their specification range to save money. By doing that, some of the parts may lag others and never reach the correct temperature. This would be the worst possible point to let that happen, too, since the loss in corrosion resistance will grow in this temperature region (700–1,000°F) and tend to peak at about 950°F. You would want your parts to get to about 1,400–1,500°F to make sure they fully transition out of this range. You will have to work with your heat treating service to understand how they process the parts including handling, racking, furnace temperature profile and finally validating that your parts receive enough heat in that load. Generally, the coldest spot in a heat treat furnace is near the bottom and by the door, especially if shielded from the heating elements. You will need to have the coldest part temperature profiled in order to insure that this is not happening. Correcting this will likely solve your passivating problems. 

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