Inspection of work before despatch to the galvanizer

Fabricated assemblies, castings and other components for galvanizing should be inspected before despatch to the galvanizer to ensure that the following points conform to design requirements detailed earlier. This may 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 which the galvanizer cannot process.

Structural steel. Check that bending, punching and shearing have been carried out in conformity with the recommendations under Embrittlement.

Satisfactory galvanizing Observance of the points listed below and described in more detail previously in this section will ensure optimum galvanized product quality and minimise extra costs or delays:

1. Check that closed vessels and hollow structures are vented for safety and satisfactory galvanizing.

2. Check that welding slags have been removed.

3. Check that assemblies comprising castings and steels of widely differing surface conditions have been abrasive blast cleaned to minimise differences in galvanized finish.

4. Check that castings are abrasive blast cleaned before despatch unless otherwise arranged. Check that large grey iron castings have been normalised.

5. Check that appropriate temporary or permanent markings are provided.

 

Inspection of galvanized products

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 of at least 600 g/m² on fabricated articles over 6mm thick, as discussed in more detail under Coating Thickness.

 

Appearance

A galvanized coating is normally smooth, continuous and free from gross surface imperfections and inclusions. While the heavy zinc coating on general galvanized articles should be smooth and 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 significantly affect corrosion resistance 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 normal bright or spangled coatings.

For more information about variations in appearance and their relationship to coating quality please contact us or your galvanizer

It is recommended that inspection of galvanized work should be carried out by a designated party at the galvanizer’s works in accordance with the following guidelines, and tested when necessary.

 

Non-destructive testing for coating thickness

Magnetic gauges provide simple nondestructive testing methods for coating thickness, which are reliable and more convenient than the physical tests given under the various national standards listed under Standards for galvanized products.

Most gauges described are compact and can be used very quickly. They give 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.  These magnetic gauges give reliable thickness readings although some require frequent recalibration against non-magnetic coatings of known thickness and the makers’ instructions are followed precisely.  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. 

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”.

  

PosiTector 6000

The PosiTector 6000 is an easy-to-use digital readout single-point coating measurement instrument which works on a magnetic field simulation principle. It needs no calibration and gives accurate results unaffected by shock, vibration, or temperature.

 

Elcometer 456 Coating Thickness Gauge

The Elcometer 456 Ferrous Model Coating Thickness Gauge can be used for measuring galvanizing thickness on steel. This operates on the Magnetic Induction Principle.  This simple to use single point gauge is easily calibrated to the range of coating thickness to be measured and provides an accuracy of +/- 1%. Statistics such as the number of readings, the average, highest, lowest and standard deviation can be displayed. These units are available as a basic non data logging unit up to a top data logger with Bluetooth Wireless download to software provided free of charge by the manufacturer.

 

The Inspector magnetic balance

The magnetic balance is based on the calibration of magnetic attraction to the steel beneath a coating. The same principle is used by pull-off type gauges, but the magnetic balance gives a stable reading and incorporates a counterbalanced magnet, allowing use in any position.

 

Pull-off type gauges

Simple pull-off magnetic thickness testing gauges such as the Tinsley Pencil Gauge and the Elcometer Pull-off Magnetic Gauge Model 157 are convenient and inexpensive, but require greater operator skill and in general do not provide the accuracy of the gauges described above.