Thickness gauge for galvanisation

General information

The durability of the CGI sheet and other items made of galvanized steel depends on the thickness of the zinc coating applied to prevent the steel from corrosion.

The thicker the zinc coating, the longer the service life of the CGI sheet. Service life of galvanized steel item is its duration of use until there is 5% corrosion on its surface.

Consequently, the zinc coating thickness is the most important inspection check to determine the quality of a CGI sheet and other items made of galvanized steel.

To check the thickness of the zinc coating on the hot-dip galvanized steel, it is not necessary to carry out destructive tests. There are magnetic instruments that measure the thickness of the zinc coating through magnetic particle inspection. These instruments are called coating thickness gauge or magnetometer. They are of 2 types:

• one measures the magnetic attraction between a permanently magnetized needle and the base steel or iron of the CGI sheet

• the other works on the principle of magnetic induction

These two types of instruments measure the zinc coating thickness in microns (µm) or mils.

The table below shows the advantages and disadvantages of two of them.

Selection:

As various coating thickness gauges exist, it is recommended to:

Make sure that the one you select can measure the zinc coating thickness on the thin base material that you have (iron or steel – ferrous base). Not every instrument measures zinc coating thickness on very thin material such as CGI sheets.

Choose an instrument which is accurate enough, easy to use in the field, and, if possible, which does not need to be calibrated for each use.

Basic values of zinc coating thickness for items in galvanized steel (hot-dip):

Conversion of zinc coating thickness into zinc coating mass:

Generally, the specifications given by the manufacturers or suppliers are in zinc coating mass (example: Z275: zinc coating mass = 275 g/m2 or 90 oz/ft2). It is therefore necessary to know how to convert the thickness (µm) into coating mass (g/m2) in order to verify that the measured thicknesses are consistent with the values required by the standard.

The density of the zinc is 7.14. Therefore, a thickness of 1 micron of zinc is equivalent to 7.14 g/m2.

To convert zinc coating thickness to zinc coating mass:

Zinc coating mass (g/m2) = thickness (µm) x zinc density (7.14)

To convert zinc coating mass to zinc coating thickness:

Zinc coating thickness (µm) = mass (g/m2) / zinc density (7.14)

Example: zinc coating thickness measured on one side of the CGI sheet is 20 µm. Then, the zinc coating mass is equal to: 20 x 7.14 = 135.66 g/m2 of zinc on one side.

Safety:

Always use gloves and hard shoes while manipulating the CGI sheet.

Specifications

Specifications of magnetic pull-off thickness gauge:

Specifications of electronic magnetic induction thickness gauge: