Practical Guidelines for the Inspection and Repair of Hot Dip Galvanized Coatings
Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA
Adherence of the Coating
Adherence is concerned with the practical conditions of transportation, erection and service. The hot dip galvanized coating should be sufficiently adherent to withstand handling consistent with the nature and the thickness of the coating in normal use of the article, without peeling or flaking. Bending or forming, other than mild straightening after hot dip galvanizing, is not considered to be normal handling.
When certain grades of steel or very heavy steel sections are hot dip galvanized, coatings may occur, which are thicker than usual. The galvanizer has limited control over the development of thicker coatings. Thick coatings are a function of the chemical composition of the steel or the longer immersion time required for massive items. Heavy hot dip galvanized coatings, usually greater than 250mm thick, may be more brittle than a typical coating. Consequently, application and interpretation of the standard adherence tests must take this into account. The requirements for transportation, handling and erection should be evaluated against the additional corrosion protection afforded by the thicker coating.
Testing Adherence
Testing adherence is not necessarily a true measure of the adhesive strength of the metallurgical bonding of the hot dip galvanized coating to the base steel. It serves, however, as an indicator of the adherence properties of the coating.
Paring Test. This simple but effective test is conducted by cutting or prying the hot dip galvanized coating with a sharp knife. Considerable pressure is exerted in a manner tending to remove a portion of the coating. Adherence is considered satisfactory when it is possible to remove only small particles of the coating.
It should not be possible to peel any portion of the coating in the form of a layer so as to expose the underlying iron or steel in advance of the knife point. Although not mentioned in ISO 1461, this test has shown practical significance as a test for adherence.
TABLE 6.
| DEGREE OF FLATTENING FOR TESTING COATING ADHERENCE FOR TUBES | |
| Tube type | Distance between platens |
| Square | 75% of side |
| Rectangular tube | 75% of shorter side |
| Round ≤ 21.3mm | 85% of outside diameter |
| Round > 21.3 ≤ 48.3mm | 80% of outside diameter |
| Round > 48.3 ≤ 76.1mm | 75% of outside diameter |
| Round > 76.1 ≤ 114.3mm | 70% of outside diameter |
| Round > 114.3mm | 65% of outside diameter |
For compliance with EN 10240, the most popular test is cold flattening in accordance with EN 10233. Test pieces not less than 40mm in length are flattened between parallel flat platens as shown in table 6. No cracking or flaking of the coating shall occur on the surface away from the cut surface.