What's Causing Increased Usage of Alkaline Spray Cleaner?

Q: We have a five-stage iron phosphate line to prepare our parts for powder coating.  Recently, the consumption of our alkaline cleaner in Stage 1 has almost doubled. We track our usage daily by marking and dating the cleaner level on the translucent container. Our vendor says the cleaner was within its quality control specifications. Why are we seeing increased usage?

A: Let’s assume the cleaner usage question is not related to increased production and that the cleaner concentration is maintained within a defined range. Most alkaline cleaners are maintained by a free-alkalinity titration, which can be related to product concentration. While dramatic changes don’t happen often, they do happen, and the question could be reworded to ask what situations reduce free alkalinity.

There are several situations related to solution loss or chemical neutralization to investigate when usage changes dramatically. Solution loss can occur in a variety of ways; most are obvious and can be ruled in or ruled out by examining the overflow weir and the drain trough to see if they contain water or if they are wet. If the cleaner tank is overflowing, you probably have a bad water fill valve or level control system. You will also want to see if an adjacent stage is spraying over into the cleaner stage. Spray headers have been known to crack and can be the cause of overflowing tanks.

Conservatively and simply stated, solution drag-out can represent 50 percent of chemical usage. When you observe abnormal chemical usage, you need to determine if you are losing solution by investigating whether the line speed has increased. Has the proportion of parts more conducive to carrying over solution increased? To a degree, increases in these situations can be remediated by changing the positioning of the drain zone ramp. (You should also verify that the drain zone ramp has not been compromised or moved.) If you have established a counter-flow system to reduce chemical consumption, then you also should verify that system is functioning. Has anyone shut a valve? Counter-flowing rinse tanks can save water and product, so if you are using this strategy and usage goes up, you should verify its functionality.

Physical solution loss also can be the result of errant spray. You should maintain a nozzle positioning strategy for process efficiency, but also so that the operators know where and how the nozzles should be directed. Sometimes it is difficult to see errant spray. To determine whether it is an issue, take samples of the cleaner and the adjacent rinse water. Turn off water valves, turn the pumps on without running parts, and run them for an hour or two before retaking samples and analyzing. Observe the water level in each tank. If the Stage 1 cleaner level goes down and the Stage 2 rinse maintains an overflow, then you have errant spray. The samples can be used to determine the magnitude of the errant spray and chemical loss. To determine the magnitude of loss, re-establish the solution level of the cleaner and resample Stage 1 to determine concentration loss. One last place to look for physical solution loss is on the roof of your facility. Washer exhaust systems can get out of balance. If you see whitish residue around the exhaust on your roof, then you need to investigate the washer exhaust system.

Increased alkaline cleaner usage can also be related to chemical neutralization; one form of neutralization is referred to as carbonation. Spray cleaner systems are more prone to carbonic acid neutralization than immersion systems, because spraying a solution dramatically increases the surface area to which carbonic acid can be absorbed from the atmosphere. This normally happens, but in this context, we are looking at increases in airflow or amount of air scrubbed. Of course, we know there is carbon dioxide in our atmosphere. The exact chemical species absorbed is carbonic acid. As it is absorbed, it reduces the cleaner’s pH, or rather the pH buffer of the solution. A pH buffer is simply a mixture of a weak acid and its conjugate base, or a weak based and its conjugate acid. For alkaline cleaners the base is usually a hydroxide. So as the bath absorbs carbonic acid, the pH and free alkalinity titration decrease. This occurrence is more pronounced with potassium hydroxide than sodium hydroxide, as potassium carbonate has a pH of about 9.0 and sodium hydroxide has a pH of about 12.5. The significance is that the former is within the range that phenolphthalein changes pH, and the latter is well above the pink/clear endpoint.

All of this dissertation leads to my next point: Carbonation happens all the time. So then, if you are experiencing high usage, has the amount of air passing through the cleaner stage been increased? Although not a dramatic change, as nozzle orifice wears, it becomes more jagged and will break the solution up into even more surface area. I doubt this situation would cause an abrupt change, but it is a selling point for replacing plastic nozzles. Another source of carbonic acid is from burner exhaust. Depending on your washer’s design and heating system, this may or may not be possible. However, I have seen this a few times in my career. The issue is easily detected with a carbon monoxide meter.

Lubricant changes can produce increases in product usage. If the lubricant chemistry includes partially saponified or unsaponified fatty acids or esters, then they will get totally saponified in your alkaline cleaner tank.

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Ken Kaluzny is the technical director at Coral Chemical. Visit coral.com.