Stripping Chrome-Plated Parts

Q. We have to strip, repair and rechrome parts made from LDX 2101 duplex stainless steel. We have been using 10% hydrochloric acid for more than a decade to strip plated stainless parts. Unfortunately, it seems to seriously affect the 2101 alloy. We have etching, erosion and severe mass loss and are losing the functional dimensions. In the photo (left), you can see that all 300 series stainless parts are unaffected and the 2101 is rough and gray. The surface is eroded so much you can feel a step between the weld seam and the 2101 surface.

This screen is made with 2101, slotted, plated, and with Type 316L stainless rings welded with Type 309 LMO filler. We know the LDX 2101 contains 1.5% nickel and 21% Cr compared with the Type 316, which is 11% Ni and 17% Cr. Is this happening because the nickel is too low and the chlorine is now too harsh against the chrome and other component into the 2101?

The LDX 2101 and other duplex stainless materials are becoming more and more popular ,and we would like to find a solution to our problem. What would you recommend to strip the chrome without any effect on the duplex stainless surface? Thank you.

 

A. The LDX 2101 is a duplex stainless steel, meaning it consists of a mix of both austenitic and ferritic microstructure. For general reference, this is similar to saying it is a mix of 300 and 400 series stainless steels. The material is chosen for specific reasons, with the primary one probably being price. The material’s reduced nickel levels compared with the austenitic stainless steels, as you cited above, make it a less expensive alternative to 300 series materials in some applications.

The mixed microstructure can also yield some benefits over 300 series stainless steels. For instance, the 300 series materials can perform well in most environments, but suffer from pitting or stress corrosion in the presence of chlorides. In this case the molybdenum-containing Type 316 is superior to the Type 304 stainless, which contains no molybdenum. In the case of the duplex stainless steels, the chemistry is such that they do not passivate, at least not to the degree that the austenitic stainless steels do, so their mode of attack is different.

In general, duplex stainless steels perform better in the case of localized corrosion phenomenon such as pitting and stress corrosion. However, as you are experiencing, the duplex stainless materials may have less resistance for general surface corrosion than the austenitic grades. In general, I would expect that you would have noticed this difference most in switching from Type 316 stainless to 2101, and not as severely had you switched from Type 304 to 2101. Although there are a lot of factors (temperature, solution, strength, etc.), the general corrosion resistance ranking from best to worst would be 316, 2101 and 304. Again, this is very general and dependent on many factors, so I don’t want to start a debate on this.

In summary, I would not think this is unexpected. I believe there are two potential alternatives you can pursue. One would be to change your stripping solution from hydrochloric acid to sulfuric acid. This should improve the situation, depending on strength and temperature, but still will may not be better than Type 316 for general corrosion resistance. Also, I am not sure how good a stripper it is. The other alternative would be to convince your customer to switch to a molybdenum-containing duplex stainless such as 2205 (~3–3.5% Mo). However, this could negate some of your cost savings with the added molybdenum and increase nickel content (up to about 5.6%). At that point, it may just be easier to go back to the Type 316 stainless.