Testing and inspection of hot dip galvanized coatings.
The requirements of the various standards for coatings concern:
- Thickness or mass per unit area
- Appearance
Thickness measurements are made using the following techniques:
- Electromagnetic thickness testing gauge. This should be capable of measuring to within 10% or 1.5µm whichever is the lower. In addition the instrument must be capable of minimizing errors in reading caused by the magnetic permeability, dimensions, surface finish and curvature of the article being tested.
- Gravimetric techniques whereby, the coating is dissolved in inhibited hydrochloric acid and the mass lost converted to uniform thickness of coating.
- Metallographic examination of a representative sample of the article. With this method thickness measurements can be combined with identification of the various alloy layers comprising the galvanized surface.
Methods 2) and 3) are destructive methods requiring the removal of a sample piece for examination. In some instances this is not practical and, therefore, method 1) is preferred.
Appearance
It is always wise for the customer to discuss the intended use of the object with the galvanizer. If the customer has special demands on the uniformity and appearance of the coating, these should be discussed between the various contracting parties as early as the tendering stage. Visual inspection should take into account the following:
- The surface should be relatively smooth and free of visible faults such as blisters, barbs, zinc ash, fluxing agent residue and uncoated areas. Lumps, drops and thick runs are not permitted where they can affect the use of the hot dip galvanized object. The coated object shall be clean and undamaged.
- The presence of darker grey areas, a cell like pattern of dark grey lines between lighter grey areas, a certain amount of uneven cover and/ or wet storage stain do not qualify for reasons for reclamation.
- Unsatisfactory pre-cleaning will inevitably show up in the form of black spots in the coating, easily visible to the naked eye. Small uncoated spots cannot always be avoided. Whether or not these need to be remedied depends on their size and shape, and on the use of the object.
- When judging the evenness of the zinc layer, it should be understood that hot dip galvanizing involves dipping in molten metal. The molten metal must then run off the surfaces of the object as it is withdrawn from the bath. This means that the appearance of the zinc layer cannot be compared with an electro-plated object, or with mechanically wiped objects such as sheet, wire or tubing. Small irregularities that do not affect function or seriously impair appearance, should not be ground. Filling or grinding often does more harm than good. Small accumulations, such as oxides from the bath surface, generally even out after one or two years of exposure.
- Certain irregularities such as "curtain formations" thickenings and small amounts of oxides from the bath surface cannot always be avoided. Their acceptability depends on the size and shape of the object, its management in the zinc bath and its intended use.
