Galvanic corrosion in a steel mold base
Q: We have a mold with severe corrosion around the water line holes in the aluminum cavity blocks. There is an o’ring that seals the water channel in the cavity block to the steel mold base. Is it possible to experience galvanic corrosion when using aluminum cavity blocks in a steel mold base?
A: Yes you will definitely experience galvanic corrosion and this will accelerate as the temperature increases. After reading about your problem, it occurred to me that you may be able to significantly reduce the problem by using an automotive coolant for your water circulation system. There are three basic types of coolant for car engines available: IAT (Inorganic Additive Technology), OAT (Organic Additive Technology) and HOAT (Hybrid Organic Additive Technology). The IAT coolants contain silicates that form a protective barrier on everything in the system, even rubber hoses. The newer OAT coolants work differently. Aluminum and ferrous metals form a surface-layer of corrosion in the presence of moisture accelerated when there is air. OAT coolants act on this layer forming a thin surface coating that protects against further corrosion. With either type of inhibitor, there must be enough present in solution to re-establish the barrier as needed. Silicates plate out on metal parts or form minute deposits, so the amount of silicates in a coolant solution may drop. OAT development has produced coolants that can effectively protect against corrosion without using silicates. The HOAT coolants use both silicate and organic acid corrosion inhibitors and allow them to be effective with only low silicate content.
I assumed that it is plain tap water and serves to heat and possibly cool the mold. I introduced the possibility of using an automotive coolant and here it would be a heating fluid. Certainly pH would be important and I assume should be kept as near to pH7 as possible since aluminum oxides can exhibit amphoteric properties. There are various inhibitors that can be used in these situations, but misuse or lack of monitoring can even accelerate the corrosion.
I don’t know how many molds you have, but you might want to try all three types. Depending on various factors, the IAT may cause deposits and affect the overall performance, so I would be more inclined to try the OAT or the HOAT coolant. The use of such coolants in your circulating system may be an inexpensive way to fix your problem.