SITA Messtechnik - Optimizing Metal Cleaning Processes

Optimizing Metal Cleaning Processes

Best instrument for this application:

Application:

Metal Cleaning Baths

Benefits:

1. By using only the required amount of cleaner, costs can be minimized.
2. The cleaning results are more stable, thus a quality assurance can be set up.
3. If an unnecessary cleaner is no longer used, standing times of the cleaning bath and the rinse water can be reached.

Physical Background

To reach the desired cleaning effect a certain amount of cleaner must be added to the cleaning bath. During the process the cleaner is washed out of the bath in order to keep up its cleaning ability. It is important to build up a "washing power reserve". This is normally done by adding so much cleaner to the bath that its concentration exceeds that which is necessary. This causes unneeded costs, not only because more cleaner is used. The excess cleaner is then washed into the rinse water where it reduces the standing time of the rinse bath. Also, the concentration of cleaner is not monitored, so it changes during the process. Thus, the cleaning result of the bath is not stable making it difficult to set up a quality assurance system.

To solve this problem it is important to know the actual concentration of cleaner in the bath. Determining the concentration of cleaner can be done by measuring the surface tension. The cleaner consists of surfactants, which reduce the surface tension of the bath. Common methods (Du-Novy-ring, Wilhelmy, contact angle) only allow one to determine the concentration of cleaner up to the CMC (Critical Mycell Concentration). But, in most cases, the desired cleaner dosage exceeds the CMC, hence these methods are not useful to determine the actual concentration of cleaner in the bath. SITA instruments measure dynamic surface tension, which also makes it possible to determine the concentration of cleaner above the CMC. Due to the capability of SITA devices to allow a mobile, online measurement, the actual state of the cleaning bath can be monitored easily in real time. That allows the operator to keep the characteristics of the cleaning bath constant by adding additional cleaner when the surface tension goes below a defined limit.

Dynamic Surface Tension Measurement using SITA Devices

SITA devices work on the basis of the maximum bubble pressure method. Because surfactant solutions have a dynamic surface tension, the surface tension measured depends on the bubble lifetime. Different bubble lifetimes will result in different surface tension readings. To determine the surfactant concentration in the solution the surface tension values of differently concentrated solutions should ideally be scattered over the whole measuring range. SITA surface tension meters allow one to adjust the bubble lifetime from 30ms to 10s, which is sufficient to obtain a wide split of surface tension readings in differently concentrated solutions. To determine the bath status the following procedure is recommended:

1. Prepare samples from the cleaning bath which have different concentrations of cleaner solutions. Ideally, the samples should include an under-dosed, an over-dosed and a correctly dosed bath sample.
2. Measure the surface tension of each sample using the whole frequency range of the surface tension meter (fig 1).
3. Determine the appropriate bubble frequency in order to achieve a wide split of the surface tensions measured.
4. Record a reference curve using the bath samples and the ideal bubble frequency found in step 3 (fig 2).
5. Determine the actual bath state by comparing the surface tension of the sample with the reference curve.
6. The actual bath status is displayed to the operator. If the surface tension reaches a limit set by the operator, additional cleaner must be added.

diagram 1

Surface Tension in Different Concentrations of Cleaning Baths

diagram 2

Surface Tension of a Nickel Bath
at Different Concentrations of Wetting Agents

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