Water Analysis Report and Staining of Aluminum

We have noticed a light staining on our aluminum machined parts after we wash in warm water. What, if anything, should I look for in the analysis that could cause stains?


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Q. We have noticed a light staining on our aluminum machined parts after we wash in warm water. We have had the water analyzed but I can’t interpret the results as it relates to our stain problem. What if anything should I look for in the analysis that could cause stains? E.D.

 

A. There are several things that could contribute to the staining of your aluminum parts after washing. I am not sure if you had the rinse water analyzed or your incoming water, but I would suggest checking both so you can have some idea about the possible source of some of the contaminants.

It is possible that the staining could be coming from your water supply itself. Hard water contains a high level of dissolved minerals, particularly calcium and magnesium. The calcium and magnesium will combine with carbonate in the water and create either a calcium or magnesium carbonate precipitate. This precipitate could be the cause of your staining problems. The Water Quality Association provides the following guidelines for classifying water hardness:

You are more likely to have hard water if you are on a private well than a municipal water supply that gets its water from a lake or river. Even if you have been on the same source of water for some time, it is possible that something has changed with it. So you can see it is important to have the quality of your water checked so you can understand its starting conditions.
Even if your water shows up as being hard, that may not be the only source of your staining problems. That is why it is important to also take a sample of the rinse water to see what else may be building up in there. It may be possible that you are carrying over too much cleaner into the tank and not overflowing enough rinse water to keep the tanks clean. Parts should be well drained when they are removed from the cleaner tank, but will still contain cleaner residue (that is the reason you are rinsing them off). If your rinse tank is not kept clean, the level of dissolved cleaner residue will continue to build over time, eventually leaving stained parts because of the residue. You may even want to take two rinse tank samples for analysis to determine how quickly any residues build up over time. Not knowing what your particular cleaner is, I am generalizing as to what species to look for in the water analysis. You may be able to glean more information from the manufacturer’s MSDS. If residue and carryover from the cleaning tank were leading to the water staining, the rinse tank water analysis would likely show elevated anion levels of carbonate, phosphates, and borates. The primary cation would be
sodium.

To prevent this, it is important that the tank have some amount of overflow to the drain. You can minimize the overflow if you have two or three rinse tanks, preferably counterflowed. In this layout, you have the freshwater entering your last rinse tank, then flowing to the next to last, and then the first tank (if you have three rinse tanks). The counterflow arrangement is the most economical use of water overflow you can make. Rinsing is roughly a logarithmic relationship, so you will use approximately 1/50th to 1/100th the water you would use with a single overflowed rinse tank with good mixing in each. If you currently do not have any waste water, check with local and state authorities before starting to dump this water to the drain.

So it is possible that your staining problem could be coming from one or both sources. Based on the lab analysis you should have an idea if it is poor incoming water, poor rinse tank maintenance or both. If you are still unsure, you may want to do something simple like dump your rinse tank and see if the problem goes away or is lessened to any degree. If your incoming water is part of the problem, you will have to invest in some sort of water treatment, such as deionization or reverse osmosis in order to get it to the point where it will not stain your parts.

If rinse tank maintenance is part of the issue, you will need to increase the incoming water overflow along with good mixing. You can minimize water usage by adding one or two rinse tanks and counterflowing them. A final step to take to insure rinse water quality and minimize usage would be to install a conductivity controlled solenoid valve on the final rinse tank. Through trial and error, you can see what conductivity level provides you with stain-free parts. Then the valve will only allow water into the tank when it is needed rather than all the time, or only when somebody remembers to turn on the overflow valve.

CLASSIFICATION HARDNESS (ppm as equivalent CaCO3) GRAINS/GALLON  
Soft 0-17 0-1
Slightly Hard 17-60 1-3.5
Moderately Hard 60-120 3.5-7.0
Hard 120-180 7.0-10.5
Very Hard Greater than 180 Greater than 10.5

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