How to Clean Parts Properly Before Powder Coating

Q: We are having cleaning problems and hope you can help us figure out why we aren’t cleaning parts adequately. We are using a highly caustic spray cleaner that has performed very well since the line started six years ago. Our vendor says that nothing has changed with the cleaner formula. We are cleaning steel parts that have mill oil. Our steel supplier says that our coils have the same mill oil we have been successfully removing in the past. Our cleaner vendor took a bath sample to see if they could find a reason why our paint adhesion ratings have fallen from a consistent 5B to 3B ratings. The analytical test results were described as normal for a solution that is six months old.

A: Thank you for your question, and the time you had to talk when I called you with some follow-up questions. You indicated there were no craters or pin holes observed in the cured powder paint. These observations are significant because they indicate the reduction in crosshatch adhesion performance is not related to insufficient organic soil removal. If the mill oil was not removed, you would be observing pin holes and/or craters in the cured powder paint.

Thank you for sending me a copy of the cleaner bath analysis. Based on the trend analysis, it appears you are controlling the solution very well. The solution also appears to be old based upon the pH and carbonates. The safety data sheet (SDS) for your spray cleaner shows a pH of 12.5 under Section 9, Physical Properties. The bath appears to be old based upon the pH buffering down into the low 10s. The carbonates and total-to-free alkalinity ratio (TA/FA) are just two ways to manipulate the significance of these titrations to judge bath life. Personally, I have found little value in these values except for situations where the soils and the washer chemistry are controlled and consistent. Even with consistent chemistry, the ratio or difference between total alkalinity and free alkalinity do not exclusively determine the usefulness of the alkaline cleaner. Many will state that when the TA/FA becomes three times the initial alkalinity ratio of the cleaner, then it is time to dump the tank. This is usually not true, particularly with free-rinsing cleaner chemistry. Usually, these baths buffer out quickly in a spray system. It appears that your oil splitting system is working well with your cleaner based on the absence of oil in the analysis.

 

LWR #	Ranges	12270	12141	12004	11860	11694	11553
Date Sampled		11/28/18	11/14/18	10/31/18	10/17/18	10/03/18	9/19/18
pH	10.0 - 12.5	10.2	10.2	10.2	10.2	10.1	10.3
Free Alkalinity, ml	8.0 - 10.5	8.3	8.7	9.0	8.8	9.0	8.6
Total Alkalinity, ml	<45	38.5	38.8	38.9	36.9	38.6	37.9
TA/FA ratio	<5	4.6	4.5	4.3	4.2	4.3	4.4
Carbonates (TA-FA)	report	30.2	30.1	29.9	28.1	29.6	29.3
Concentration, % v/v	3.0 - 4.0	3.2	3.3	3.4	3.3	3.4	3.3
Conductivity, mS/cm	report	42.0	40.6	42.0	40.5	42.4	41.3
% Brix	report	5.3	5.6	5.5	5.2	5.4	5.3
% Oil, Babcock	< 0.5	0.0	0.0	0.0	0.0	0.0	0.0
Brix oil	report	5.3	5.6	5.5	5.2	5.4	5.3
Surfactant titration, mls	>2.8 ml	8.9	9.6	8.7	8.8	8.9	10.1

 

You mentioned the parts were cleaned water-break-free exiting the cleaner rinse and the end of the washer. This indicates the organic soil has been removed. The absence of pin holes and craters also verifies this as well. Your adhesion issue is probably related to insufficient inorganic soil removal. Considering this is something that you are recently experiencing, I would speculate there are a couple of possibilities for you to investigate. Your paint adhesion issue may be related to physics as well as chemistry.

I understand that you dump and muck out your cleaner tank once a year, but have never descaled the cleaner stage. You should investigate your solution delivery system. There are reaction products formed in your cleaner stage. Some of these reaction products are not soluble and become what most call sludge. Sludge ultimately forms scale which builds on internal surfaces of the plumbing, effectively reducing flow. While your conveyor speed may maintain 60 seconds of spray time, the reduced internal diameter (ID) is reducing the flow or amount of chemical that impinges and flows across the work surfaces. You may be maintaining your spray pressure by manipulating the pump bypass valve, but you aren’t maintaining the solution flow.

There may be a chemical component to the paint adhesion issue as well. The reason I think this is because this is the first year you have experienced adhesion issues, while everything is reported to be the same. Solution control procedures don’t generally measure each and every cleaner formulation constituent. The bath analysis didn’t show any evaluation of sequestrate in the cleaner. Considering you are using a caustic cleaner for cleaning steel and the product claims on the technical data sheet, you must have either a sequestrate or chelate in the cleaner formulation. The major intent of these types of ingredients is to remove inorganic soil. Considering you are only processing steel parts, the inorganic soil is in the form of iron salts. Many chelates or sequestrates used for removing iron salts are most effective over a pH of 12. The buffering of your cleaner solution is yet another reduction in your cleaning efficiency. You should discuss this with your cleaner vendor. It is likely you need to address both the physics and chemistry of your prepaint treatment line with your vendor to address your crosshatch adhesion issue. While you buy chemistry to clean steel, it is the physics that maximizes the efficiency of your operation.

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