Practical Guidelines for the Inspection and Repair of Hot Dip Galvanized Coatings

Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA

Thickness Testing

There are several methods to determine the thickness of the zinc coating on a hot dip galvanized article. The size, shape and number of pieces to be tested, will most likely dictate the methods of testing. The specified test methods are either destructive or non-destructive and are detailed in ISO 1461. Identical methods are detailed in EN 10240. The most practical tests are the nondestructive type, such as gauges utilising the electromagnetic principle.

1. Electromagnetic Testing Method.

Instruments, which rely on electromagnetic principles are probably the most widely used devices to determine the thickness of hot dip galvanized coatings. Electromagnetic instruments measure coating thickness in specifically-identified local areas of the article. The average thickness (and therefore mass) of the coating is calculated from measurements taken at a suitably large number of points distributed over a reference area on the surface of the article. There are a number of electromagnetic gauges which can be used to give accurate measurements of the zinc coating thickness. These gauges give reliable thickness readings provided they are properly calibrated and the manufacturer's instructions are observed. The most commonly used type of gauge is a portable electronic instrument. The gauge uses a temperature-compensated magnetic transducer to measure the magnetic flux changes that occur when the probe (a magnet) is separated from a ferrous metal substrate by a non-ferrous coating such as hot dip galvanizing. The output signal from the probe is proportional to the distance of separation and, therefore, to coating thickness. The probe signal is amplified and indicated on a meter calibrated to show coating thickness. These battery powered instruments have typical accuracies up to 5% and have the advantage of not requiring recalibration with probe orientation. Similar electromagnetic instruments with even greater accuracies are available for laboratory use.

The specification requires that the thickness gauges shall (in terms of ISO 2178:1982) be-

• Capable of minimizing errors in reading caused by the magnetic permeability, dimensions, surface finish and curvature of the article being tested;
  

• Capable of measuring the thickness of the coating being tested within an accuracy of 10% or 1.5m whichever is better^A, and
  
  ^A When curved surfaces are dealt with. particular care should be taken to ensure that the thickness gauge is capable or obtaining readings to an acceptable level of accuracy.
  

• Calibrated by taking readings at zero (or near zero) thickness and at thicknesses of at least 100m, on suitable nonferrous shims placed on acceptable standard pieces of metal of composition, thickness and shape similar to those of the articles under test.

TABLE 4.

To avoid possible errors in the use of magnetic instruments, certain precautions should be taken:

• Follow the manufacturer's instructions. The instrument should be frequently recalibrated against non-magnetic film standards of known thickness.

• The base steel should be backed up with similar material if thinner than the critical thickness for the magnetic gauge or the instrument should be recalibrated on a substrate of similar thickness.

• Readings should not be taken near an edge, hole, or inside corner.

• Readings should not be taken on curved surfaces without additional calibration of the instrument.

• The test surface should be free from surface contaminants such as dirt, grease, and corrosion products.

• Test points should be taken in each reference area to avoid obvious peaks or irregularities in the coating.

• A sufficient number of readings should be taken to obtain a true average.

2. Testing threaded articles by fitting mating parts.

The zinc coating on external threads shall be free from lumps and shall not have been subjected to a cutting, rolling or finishing operation that could damage the zinc coating. The zinc coating of an external standard metric thread that has not been undercut shall be such as to enable the threaded part to fit an oversized tapped nut in accordance with the allowances given in table 5.

TABLE 5.

On bolts greater than M24, undercutting of bolt threads is frequently preferred to oversizing of nut threads. The allowance should be increased to 0,4mm.

Threaded articles shall fit their mating parts and, in the case of assemblies that contain both externally and internally threaded articles, it shall be possible to screw mating parts together by hand.

3. The Chemical Stripping (Gravimetric) Test (to EN ISO 1460:1992).

This method is applied where material is inspected after hot dip galvanizing. Since this is a destructive test method, it is generally not suitable for the inspection of large or heavy items unless smaller or representative specimens can be substituted for them (see Test Sampling). The test specimen is cleaned with naphtha or other suitable organic solvent, rinsed with alcohol, dried and weighed. It is then stripped of the zinc coating by immersion in a solution containing 3.5g of hexamethylenetetramine and 500ml concentrated hydrochloric acid in 1 litre of water. The stripping of the coating is complete when evolution of gas ceases. After washing and drying, the specimen is weighed; the differences in mass before and after stripping divided by the surface area of the test specimen gives the mass of coating per unit area. In the case of threaded articles, such as bolts and screws, the determination is made on an unthreaded portion of the article.

The stripping test gives an accurate average coating mass of the zinc coating. However, it does not provide any information on how evenly the coating is distributed.

To test compliance with EN 10240, two test pieces shall be taken from the tube to be tested. These should be between 30 and 600cm' in surface area, with length between 50 and 150mm taken at least 600mm from the tube end.

4. Metallographic Examination.

Where the hot dip galvanized coating composition and thickness are of interest, microscopic examination is a reliable tool. This test is a requirement for the testing of compliance of Coating A1 to EN 10240. This very accurate method uses a small polished and etched cross-sectioned of the hot dip galvanized component to provide information about the relative thickness of the alloy and the free zinc layers which comprise the hot dip galvanized coating.

The following procedures should be adhered to -

• Water should not be used as a lubricant at any stage during the polishing procedure due to staining or mild corrosion of the galvanized layer.
  

• The enchant should be 2% (max) Nital, i.e. (2ml concentrated HNO₃ in 100 ml of 95% ethanol or methanol).

Important disadvantages of this technique are that -

• Specimens cut from the hot dip galvanized article are required,
  

• Coating thickness measured only applies to a very limited area, it does not indicate the variation in coating distribution on the article and,
  

• It is necessary to examine a number of specimens to determine the average coating thickness on the hot dip galvanized article.