Detection of chlorides

ISO 8502-2 describes a laboratory test method for determination of chlorides on cleaned surfaces. The method utilises the principle of titration where Mercury ions react with free chloride ions.

At present there is no approved field test method available. Work is still being done to get a method for use on site.

Detection of all water-soluble salts.

A field test method must be very easy to use, both with respect to sampling, the analysis and the equipment to use. The most user-friendly method available today is to dissolve the salts being present at the surface and to measure the conductivity of the water sample. Conductivity is an indication of how well a liquid solution will conduct electricity and is measured in micro-Siemens (µS). The conductivity can through calculation be converted to a corresponding salt content on the surface. A test method for determining total amount of soluble salts is described by two standards:

· The sampling is given by ISO 8502-6, “Extraction soluble contaminants for analysis” the Bresle sampler.

· The analysis is described by ISO 8502-9, “Field method for soluble salts by conductometric measurement”.

Remember; by using this method all types of water-soluble salts, i.e. chlorides, sulphates, iron salts etc., will contribute to the final conductivity measured. Thus, the salt level calculated will be a conservative value as the chlorides are the salts most dangerous ones with respect to promoting osmotic blistering. Also remember to use distilled water for sampling.

Calculating from conductivity to salt content. (ISO 8502-9)

The surface density of soluble salts, S in mg/m^2, can be determined by a simple calculation, given by the equation:

S = 6 x (L sample - L zero)

S = Surface concentration of soluble salt, mg/m²

6 = A constant. The factor is 6 when the water sample volume is 15 ml.

L sample = Measured conductivity of water sample, µS

L zero = Measured conductivity in the distilled water before sampling.

Example: 6 x (8µS - 3µS) gives 30 mg salt/m²

Direct analysis of salt content (SCM 400).

An alternative to the above sampling and calculation is to use a small portable conductivity meter. The sampling is easy, using a filter paper wetted with distilled water. The filter paper is pressed onto the steel surface for to dissolve the soluble salts and then placed in the portable unit for measuring the conductivity. This is not an ISO standard, but one which is commonly used.

Figure 8 4.

The chloride concentration on a substrate may vary significantly with exposure condition and with method used for cleaning the surface.

Dust.

The presence of dust on a steel surface may first of all give adhesion problems and blisters for under water surfaces. A commonly used standard for quantifying the amount of dust present on a surface is given by ISO 8502-3 “Assessment of dust on steel surfaces prepared for painting”. This test is done by pressing a pressure-sensitive tape onto the surface. The tape is rubbed into the steel surface and then removed. The assessment is done by placing the tape against a white background, and the amount of dust present is compared with pictorial ratings for both quality and size.

Figure 8.5.

Дата добавления: 2015-10-29