6.2 Thickness
6.2.1 General
Coatings applied by hot dip galvanizing are designed to protect the iron and steel products against corrosion (see Annex E). The length of time of corrosion protection by such coatings is approximately proportional to the coating thickness (see ISO 14713-1 [7]).
6.2.2 Test methods
In case of dispute regarding the test method, the method of calculating the coating thickness shall be by the determination of the mean mass of hot dip galvanized coating per unit area using the gravimetric method in accordance with ISO 1460, and the nominal density of the coating, 7,2 g/cm3, shall be used for calculation purposes. Where less than 10 articles are involved, the purchaser shall not have to accept the gravimetric test if that would involve the destruction of articles and unacceptable remedial costs to the purchaser.
Tests (see Annex D) are most commonly carried out by one of the magnetic methods given in ISO 2808 and ISO 2178 (also specified in ISO 3882). Instruments for magnetic methods measure either the magnetic attraction between a permanent magnet and the base metal, as influenced by the presence of the coating, or the reluctance of the magnetic flux path passing through the coating and the base metal. Alternative methods include the gravimetric and the microscopic cross-section methods (see Annex D).
The test methods given in ISO 2808 and ISO 2178 (also reviewed in ISO 3882) are most appropriate within works and for routine quality control. Because the area on which each measurement is made in these methods is very small, individual figures may be lower than the values for the local or mean coating thickness. If a sufficient number of measurements is made within a reference area, effectively the same local thickness will be determined by magnetic as well as gravimetric methods.
6.2.3 Reference areas
The number and position of reference areas and their sizes for the magnetic or gravimetric test shall be chosen with regard to the shapes and sizes of the article(s) in order to obtain a result as representative as possible of mean coating thickness or mass per unit area, as applicable. On a long article in the control sample, the reference areas shall be cut approximately 100 mm from the edges and 100 mm from each end and the approximate centre, and shall comprise the whole cross-section of the article.