Aluminum Contamination

 Q. We run an eight-stage cleaner before our electrocoat tank. Tanks 1 and 2 are alkaline spray cleaning, 3 and 4 are rinses, 5 is iron phosphate and 6–8 are rinses. A couple of weeks ago, a significant amount of parts got knocked off in tank 2 (about 500–600 lb of cold-rolled steel and maybe 200 lb of aluminum). When the pile was discovered and removed, no action was taken to search the tank.

Since then we have lost the ability to clean, even though all our chemistry numbers remain in spec. Last Thursday and Friday, both wash tanks were drained and recharged and the finish looked good the rest of the day Friday and Saturday until about noon. Now we are once again unable to completely clean the parts.

If some of the aluminum made it down into the tank, would it react with the cleaner and make it unable to clean? I ask because it was obvious that the cleaner was dissolving the aluminum parts that fell into the spray area and I’m sure that a serious effort to remove all of the aluminum from the tank itself was not made. J.R.

 

A. I was going to ask the question, but then saw in your last paragraph the comment about the aluminum dissolving in the tank. This is helpful in discussing the problem.

If you are running both mild steel and aluminum through the same tank, it is likely that the aluminum dissolved, and you confirmed that you are running a high-pH cleaner. It does not appear that it is inhibited with anything like sodium metasilicate (since the aluminum is dissolving). As a result, the free caustic in the cleaner will continue to consume the aluminum until it is entirely dissolved. If you are performing a typical acid-base titration to control the tank, it will be misleading since the dissolved aluminum produces aluminum hydroxide which will also have an alkaline pH. As a result, the titration will continue to show the tank to be in spec; however, in reality it is severely depleted of any free caustic to effectively clean the parts.

It sounds like you already tried this, but I would recommend dumping and recharging the cleaning tanks. However, prior to the recharge, the tanks should be thoroughly inspected to insure all remaining parts are removed. Additionally, the aluminum hydroxide could generate a significant amount of sludge in the tank that should be removed prior to refilling and recharging with fresh chemical. Once you have eliminated the remaining aluminum from the system, you should expect effective cleaning to return.