scope:

General

ISO 8502-9 Range of Variance

The assessment and determination of surface contamination (by salts) prior to application of protective coatings is critical to their service life expectancy. Determination of the level of surface cleanliness is conducted by extraction of soluble salt contaminants following ISO 8502-6-The Bresle method, part of ISO 8502-9. The determination of the level of salt is performed by following ISO 8502-9- Field method for the conductometric determination of water-soluble salts. The field execution of this requirement involves using a syringe to inject deionized water into the Bresle patch, washing the substrate surface inside the patch, then extracting the test water for direct measurement of conductivity. Details of this procedure are provided in Appendix A (mandatory). Once a conductive value is determined by the conductivity meter, expressed as micro Siemens per centimeter (μS/cm). ISO 8502-9 provides a procedure to calculate the equivalent weight of the surface concentration as total surface density of the salts.

Step-by-step execution of the ISO 8502-6 and 8502-9 test requirements introduces a number of potential variances during the field evaluation. Some examples include the sensitivity and resolution of he conductivity meter, cycle time of the test water inside the Bresle patch, human errors in measuring and injecting the test water, and general operator experience. It is known that the accumulation of these individual process variances can add up to create an overall variance in the ISO protocol.

The range of variance in ISO 8502-6 and 8502-9 has been demonstrated by extensive laboratory tests conducted by a member of this task group.3 The precision of a single ISO 8502-9 test result was determined to be ± 8.2 mg/m2 in the range 30 to 80 mg/m2. The absolute variance, and not the relative or percent variance, was found to be constant in this range. See Appendix B (nonmandatory) for details about these tests and the data analysis. The test method defined in ISO 8502-9 allows certain execution parameters of the test (e.g., size of adhesive patch, volume of test solution, and time of dissolution of salts inside the adhesive patch: the dwell time) to vary. This standard adopted best practices and has set the parameters in Appendix A. With a dwell time at a minimum of 90 seconds, the variance from operator error was found reduced dramatically in case the operator timing is not perfect. Hence, a dwell time of 90 seconds was adopted in Appendix A. For the purpose of defining equivalence, the procedure in Appendix A shall be used to create the reference values to which the candidate method will be validated. See Appendix B for the test rationale.

Any equivalent tools, methods, or procedures then must show that they meet the same criteria, thereby providing the same weight measures, and be within the same range of variance as would be produced by following the procedure in ISO 8502-9.

The objective of this standard is to determine whether methods other than the Bresle patch application method are suitable alternatives for measuring salt contamination in the field. Although tests may be performed on flat, horizontal surfaces for ease of use, each method should also be capable of gathering measurements on vertical and overhead surfaces. When such measurements cannot be made on vertical or overhead surfaces, this limitation must be noted in the validation report (Section 3).

Reasons for Salt Measurement

It is well known that applying protective coatings over salt contamination may cause degradation of the coating/substrate interface. This may lead to corrosion of the substrate and detachment of the coating. These effects reduce the expected service life of the coating.

Different salt ions influence the rate of corrosion under coatings differently. However, most common salts promote corrosion and have a negative effect on the service life of coatings.

Salts on a surface under coatings create an osmotic cell, drawing water into and through the protective coating. The osmotic pressure generated can cause blisters to form.

The general relationship between the amount of salt contamination under the coating film and consequent deterioration of the performance can be considered to be linear.

Although unobtainable, a zero measure of salt contamination may be considered the only "safe" level before application of coating films. Therefore, the maximum quantity of salt contamination allowed in a specification should be based on a holistic risk/cost analysis.