Rinsing Difficulties

Q. I own a coatings application job shop and we struggle with some rinsing problems. We perform chromate conversion, and iron and zinc phosphate, all to the appropriate military specs. We have good rinsing techniques, use deionized water (DI) and have three rinses after the process baths. The specific issue we face is salts being left behind in the blind and small threaded holes after parts are rinsed and dried. Some parts have dozens or even a hundred or more holes in them.

We have been told by a pretreatment chemical supplier that it may be worth looking at adding a rinse aid to our rinses. How helpful do you think that actually could be? Will it make that much difference or have that much of an impact on surface tension that the water will, in fact, rinse out more readily or help to rinse the various diluted chemicals still trapped in those holes? M.P.

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A. From your description, it seems you are doing all the right things from the standpoint of system design. Three rinses and DI water following your process tanks is admirable. With that much forethought with the design, I assume that you have also employed counterflow rinse techniques to minimize water usage. In short, this involves introduction of the cleanest water into the final rinse tank, then counter flowing into the previous two tanks.

You can expect a rinse aid, wetting agent or surfactant (all essentially the same) to provide a significant reduction in surface tension of the water. A very low concentration of a surfactant (on the order of ~0.001 percent) can cut the surface tension in half or more. This can make a significant difference in the ability of the rinse water to penetrate those blind holes and recesses you are having trouble with. It will 1) displace the vestiges of the conversion coating and 2) enable the rinsewater to be displaced by progressively cleaner and cleaner rinse water.

It sounds like you have invested a significant amount in the design of your system and paid particular attention to proper rinsing, particularly with the multiple tanks. The addition of a rinse aid or surfactant can effectively enable you to take advantage of all three progressively cleaner rinse tanks. Without it, those blind holes are receiving what is essentially a single rinse, since there is no turnover or dilution due to the high surface tension of the water.