The key aspects to inspection of galvanized steel are to check the fabrication of the component prior to galvanizing and then, after galvanizing, to visually inspect the product for defects and measure the coating thickness.
Visual inspection is the simplest and most important means of assessing the quality of galvanized coatings. A useful characteristic of the galvanizing process is that if the coating is continuous and has a satisfactory appearance it will be sound and adherent, with a zinc coating mass meeting the requirements of the Standard, as discussed in more detail under Coating Thickness.
The galvanizer will inspect the finished article for appearance and coating thickness against the requirements in the standard or contract. Unless specifically requested, the achieved coating thickness will not usually be recorded by the galvanizer. If inspection of galvanized work is critical to the project, this should be carried out by an agreed party at the galvanizer’s works. A list of independently accredited inspectors is available here. For critical projects, the GAA can provide inspection services (at a cost) and many of the inspectors listed may also provide an independent inspection service. Contact us for more information.
INSPECTION OF WORK BEFORE DISPATCH TO THE GALVANIZER
Fabricated assemblies, castings and other components for galvanizing should be inspected before despatch to the galvanizer to ensure that the articles are suitable for galvanizing. This will avoid costly rectification and delays at the galvanizing plant.
Size and shape. Check that work is suitably sized and dimensioned for the handling and galvanizing facilities of the selected galvanizer. It may be too late to make changes to the design, but it is costly to despatch work that the galvanizer cannot process.
Structural steel. Check that the steel used is suitable for galvanizing and test certificates are available if required. Check that bending, punching and shearing have been carried out in conformity with the recommendations in the embrittlement section.
Satisfactory galvanizing. Observance of the points listed below will ensure optimum galvanized product quality and minimise extra costs or delays:
Check that the galvanizer is aware of any ‘fit for purpose’ requirements and that these have been considered in the fabrication – ensure appropriate hanging points have been provided, if required
Check that closed vessels and hollow structures are vented for safety and satisfactory galvanizing.
Check that any structural sections and welded frames are vented to eliminate air pockets and allow free drainage across all surfaces to avoid zinc traps.
Check that all overlapping surfaces are sufficiently vented
Check that welding slag and spatter have been removed.
Check any critical thermally cut edges have been ground back.
Check that any assemblies likely to suffer from dimensional instability (distortion) at temperatures encountered in the molten zinc bath have been appropriately braced.
Check that appropriate temporary or permanent markings are provided.
A galvanized coating should be continuous and free from gross surface imperfections and inclusions. While the heavy zinc coating on general galvanized articles should be continuous it cannot be compared with the smooth surface of continuously galvanized sheet steel or wire since these are produced by processes which permit close control of coating thickness and appearance.
Differences in the lustre and colour of galvanized coatings do not affect the corrosion resistance of the coating and the presence or absence of spangle has no effect on coating performance. Uniform or patchy matt grey galvanized coatings give equal or better life than bright or spangled coatings.
Magnetic gauges provide reliable and fast non-destructive measurement of the galvanized coating thickness and are the preferred method for compliance testing. If required, a mass calculation can be made from the coating thickness reading.
The gauges provide coating thickness readings over very small areas and several readings should be taken and averaged. Uniformity as well as actual thickness can thus be easily checked. Accurate readings cannot be obtained near edges of work and obvious peaks or irregularities in the coating should be avoided. Surface curvature, surface area and steel thickness all affect readings in a predictable manner and allowances must be made.
The most common instruments used in Australia are the Elcometer 456 and the Positector 6000. Each of these instruments include inbuilt statistical measuring, have a wide range of probes to suit different surface conditions and are Bluetooth compatible with apps for accurate data recording. More information is available from the GAA.
Guidance on the use of these instruments is given in AS 2331.1.3 “Methods of test for metallic and related coatings Method 1.3: Local thickness tests – Magnetic method”, and AS 2331.1.4, “Methods of test for metallic and related coatings, method 1.4: Local thickness tests – Magnetic induction and eddy current methods”.