This short article aims to gather information on the basic physico-chemical parameters studied during the operation of a bath for surface preparation prior to powder coating.
What is the purpose of such research?
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Maintaining constant chemical treatment parameters.
This is a very important issue, which ensures that we have a reproducible surface preparation. This is followed by the constancy of the quality parameters. Knowing that the chemistry works steadily - in case of problems with the coating - we can eliminate this factor and focus on other potential causes.
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Tracking changes in trends of controlled parameters.
Monitoring the status of the surface preparation bath will help catch any fluctuations in parameters that may be the cause or symptom of potential quality problems.
Which physico-chemical parameters should be measured and how often?
The parameters monitored depend on the type of product used and its supplier.
The most basic sizes are:
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Concentration of [%]
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pH
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temperature [°C]
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sometimes conductivity [uS] is still recommended
The frequency of measurements depends on the type of treatment being carried out and the quality requirements to be met by the finished, varnished component.
Ideally, concentration and pH measurements are carried out once per shift. In practice, with a stably running surface treatment line (aided additionally by automatic chemical dosing), these checks are less frequent and carried out 2 - 3 times a week.
It is recommended that measurements are taken each time before refilling. Knowing the current concentration and pH will allow you to calculate the required amount of fresh product concentrate per refill.
Below, we will move on to a description of the individual parameters and how they are measured. At the end you will find a short video showing the measurement of concentration by titration and the measurement of pH with a laboratory pH-meter.
Concentration
This is by far the most important parameter.
It tells you how much of the chemical concentrate is in solution. It is most often expressed as a percentage - yes, the analogy with alcohol is striking, although in this case more does not always mean better ;).
The concentration tells us whether our chemistry is strong enough to wash and apply the conversion coating. If the concentration is too low, we will not be able to degrease the parts properly. When the concentration is too high - we are in danger of applying too thick a conversion coating.
Concentration measurement
Most commonly, the concentration is measured using the titration method.
And what the heck is that?
In simple terms, we add another reagent drop by drop to the bath sample from an instrument called a burette. This is a graduated glass tube mounted on a plastic bottle.
To know when to end this titration after a drop, we add an indicator to the sample, which changes colour. If it's pink, for example, it means it's over.
The number of millilitres of reagent that was used to titrate the sample is multiplied by a factor (product-specific) and the result is the concentration value in %.
The correct concentration range will depend on the type of chemical and the washing application itself. However, for our products, for steel washing we need about 0.5 %, i.e. 5 litres per 1,000 litres of bath.
You can see how to carry out the titration in the video below.
pH
It tells us how acidic, neutral or alkaline (alkaline) our chemistry is.
pH 7 is an alkaline reaction. The pH of pure water is neutral and can be between 6.5 and 7.5.
We assess the pH level together with the concentration. If we have the concentration at the expected level, then the pH is usually in the acidic ranges, e.g. between 4.5-5.5.
pH measurement
Measurement of pH can be carried out using indicator strips or electronic devices called pH meters.
With indicator strips, it is important that they have the right measuring range.
A pH-meter is an electronic instrument that gives greater measurement accuracy. pH-meters can be divided according to their application:
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Laboratory pH meters - quite expensive, but with the best accuracy. They are also available in cheaper pocket versions (the size of a thick pen) with high measurement quality. Most commonly used with suitable electrodes adapted to measurements in acidic environments.
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Cheap pocket pH-meters - much lower accuracy, higher measurement error, takes a long time to stabilise the result, often no replaceable electrode,
It is important that the pH meter allows simultaneous measurement of pH and temperature. The instrument then corrects the measurement result itself according to the temperature of the bath under test. This is known as automatic temperature compensation. The pH meter should also be able to calibrate on buffer solutions (pH standards).
The pH measurement can be seen in the video below.
Temperature
Temperature, in addition to concentration, is directly responsible for the efficiency of the wash and the rate at which the conversion film formation reaction takes place.
Typical phosphate technologies require high temperatures - between 40 °C and 60 °C.
These types of products at temperatures below 37-38 °C form an intense foam. We then have a "foam party" at the paint shop 😉 A fun affair, albeit not always desirable ;).
Too low a temperature - bad - unwashed details, foam.
Too high - also bad - conversion coating too thick, powder on the surface.
Unlike these preparations, Kairos products wash effectively at 25-27 degrees Celsius - there is no need to heat the bath to such high ranges as above.
Temperature measurement
We won't talk about it in detail - I think everyone knows what it's about 😉
A little bonus at the end - downloadable sheet for recording the measurement results.
And below is a video showing how concentration is measured by titration and pH is measured using a pH meter.
