This standard is covering all aspects related to the determination of contaminants that are required to removed before coating application. It is therefore titled: “Preparation of steel substrates before application of paints and related products – Tests for the assessment of surface cleanliness”
It is a well known standard that serves as starting point for many coating applicators. You need to realize that the requirements for cleanliness are different per product and application. The method described in this standard is applicable to all metallic surfaces, whether they have been blasted or not. The quality of paint and other coatings applied to steel is considerably dependent on the condition of the steel surface before painting. The following are some important factors that have been identified as affecting performance:
a) presence of rust and mill scale;
b) presence of surface contaminants, including salts, dust, oils and greases;
c) surface profile.
The ISO 8502 standard describes 9 different methods
ISO 8502-1: This part of the standard is dealing with the removal of loosely adherent contaminants by solvent cleaning. After cleaning, the solvents must be removed completely by drying prior to further surface preparation or coating.
ISO 8502-2: Laboratory determination of chloride on cleaned surfaces. This method is used to determine the amount of chloride on the surface prior to coating. The test involves the use of a solution of silver nitrate and potassium chromate.
ISO 8502-3: Assessment of dust on steel surfaces prepared for painting (pressure-sensitive tape method). This method is used to determine the amount of dust on the surface prior to coating. The test involves the use of a pressure-sensitive tape, which is applied to the surface and then removed. The tape is then placed on a black background and examined under a microscope.
ISO 8502-4: Guidance on the estimation of the probability of condensation prior to paint application. This method is used to determine the likelihood of condensation occurring on the surface prior to coating. The test involves the use of a wetted finger or glass rod, which is placed on the surface for 30 seconds. The temperature and humidity of the surrounding air are then recorded.
ISO 8502-5: This part of the standard is dealing with the Measurement of chloride on steel surfaces prepared for painting (ion detection tube method). This method is used to determine the amount of chloride on the surface prior to coating. The test involves the use of an ion selective electrode, which is inserted into a tube containing a sample of the surface being tested. The reading on the electrode is then recorded.
ISO 8502-6: This part of the standard is dealing with the Extraction of soluble contaminants for analysis — The Bresle method. This method is used to determine the amount of soluble contaminants on the surface prior to coating. The test involves the use of a Bresle patch, which is placed on the surface and then extracted with distilled water. The extract is then analyzed for chloride, sulfate, and phosphate content. The methods described in this standard are suitable for use on all types of steel substrates. The choice of method will depend on the type of contaminant to be removed and the amount of time and resources available.
ISO 8502-9: The ninth part is dealing with the Field method for the conductometric determination of water-soluble salts. This method is used to determine the amount of water-soluble salts on the surface prior to coating. The test involves the use of a conductivity meter, which is used to measure the conductivity of a sample of the surface being tested. The reading on the meter is then recorded.
ISO 8502-11: Field method for the turbidimetric determination of water-soluble sulfate. This method is used to determine the amount of water-soluble sulfate on the surface prior to coating. The test involves the use of a turbidimeter, which is used to measure the turbidity of a sample of the surface being tested. The reading on the turbidimeter is then recorded.
ISO 8502-12: Field method for the titrimetric determination of water-soluble ferrous ions. This method is used to determine the amount of water-soluble ferrous ions on the surface prior to coating. The test involves the use of a titration kit, which is used to titrate a sample of the surface being tested. The reading on the titration kit is then recorded.
Applying the ISO 8502, things you need to keep in mind
All methods described in the standard are qualitative methods. This means that they only give an indication of the cleanliness of the surface, but they do not provide a quantitative measure. The reason for this is that the amount of contamination that can be tolerated before it affects the performance of the coating is depending on too many factors to be able to give a general limit.
The standard does not cover all methods that can be used for surface preparation. It only covers those methods that are commonly used and that have been found to be suitable for the assessment of surface.
To see the relationship to other coating related standards: The ISO 8501, ISO 8502 and ISO 8503 standards series provide methods for assessing the cleanliness of steel substrates while the ISO 8504 series provides guidance on available preparatory methods that indicate each method’s capability in attaining a specified level of cleanliness. The International Standards for architectural steel do not provide any guidance on the protective coating systems to be used on the steel surface. They don’t even contain advice regarding particular conditions and how good surface quality affects the choice of protective coating to use and its performance. These sorts of recommendations may be found in nationally accepted standards and codes of practice.
Users of these International Standards must make sure that the specified qualities are compatible with both the environmental conditions where steel will be exposed and the protective coating system to be used, as well as within capability of cleaning procedure. ALso, it is important that norms and standards are considered to be the the minimum requirement. This does not mean that there are no better or smarter solutions available in the market. Nor can the creation of such standards be separated from vested interests in the industry. So it is important to keep a critical mind of your own when applying such standards. Especially now that we are in a period of major societal changes and changes in our climate. The current standards, for example, do not set requirements regarding micro-organisms, while it is now widely known that MIC-related micro-organisms will play an increasing role due to global warming and the presence of more organic pollutants. Read, for example, our article on the problems in Belgium
What is clean?
ISO 8502 is divided into different parts, and this part specifically described a method to assess chloride-containing salts that are readily soluble in water and present on steel surfaces. Even if the rust on steel substrates is of grade C or D (as specified in ISO 8501-1), it can still be contaminated by corrosion products and other pollutants. Most of these contaminants are invisible to the eye. You can find it at the website from ISO. It is very important to keep this in mind if you use this standard.
Many poluting chemicals are almost colorless and are concentrated at the bottom of the rust pits. If they aren’t removed before painting, chemical reactions can lead to large accumulations of rust that harm the bond between the substrate and the applied protective coating. Even if salt is readily soluble in water, it may be difficult to remove it totally from the surface using this method since dissolved salt serves as a lubricant for preventing adhesion failure.
Introducing new threats
The techniques mentioned in the ISO8502 do not, determine the total amount of chloride on the surface. It only provides an indication of the surface’s cleanliness level. The washing time should be extended to remove a greater percentage of the salt. In this respect it is also important to use wash water that is free of chemical or biological contaminants. Realize here that tap water is often not free of micro-organisms, but that they are usually present in reasonable amounts. Read more about corrosion by micro-organisms in clean water sources.
