Gravimeter determination of Hot Dip Galvanization Ferrous Coating

As far as we commonly find, it could be rather tricky to determine the actual mass of certain coating on ferrous materials. Since the coating may hold the same chemical properties as the ferrous material does.

Now, let me introduce the methods of Gravimeter determination which were recommended by ISO. With this method, we would be able to weight the mass of the hot dip galvanized coating easily. Still, please pay attention to the conditions to be met before weighting. First, this method is applicable to those samples with relatively light/medium weight, we should turn for other weighting method when it comes to heavy samples. Second, we should make sure the shape of samples are easy to determine, otherwise we should turn for other methods of weighting.

General Principle

The hot dip galvanized coating on a surface of known area is dissolved in inhibited acid and the resultant loss in mass is determined by weighing the sample before and after the coating is dissolved.

Stripping Solution

Dissolve 3,5 g of hexamethylenetetramine in 500 ml of concentrated hydrochloric acid (ρ ≥ 1,18 g/ml). Dilute this solution to 1 000 ml with distilled water.

Procedure

Where necessary, the sample shall be degreased with an organic solvent that does not attack the hot dip galvanized coating and then dried. Before stripping, the sample shall be weighed to an accuracy better than 1 % of the presumed coating mass. The quantity of solution shall be measured so that at least 10 ml of solution are available for each square centimetre of the surface of the sample. The sample shall be completely immersed in the solution at room temperature and left until the coating has completely dissolved. The end of the dissolution process can be recognized by the cessation of the originally brisk evolution of hydrogen. The sample shall then be rinsed in running water and, if necessary, brushed to remove any loose substance which may be adhering to the surface, dipped in alcohol, quickly dried and again weighed to the accuracy indicated in the previous paragraph. After weighing, the surface area A of the exposed surface shall be determined to an accuracy of 1 %.

Method of Calculation

Calculate the mass per unit area Pa , of the hot dip galvanized coating, expressed in grams per square metre, from Formula (1): Pa =  × 10⁶

where

m1 is the mass, in grams, of the sample before stripping;

m2 is the mass, in grams, of the sample after stripping;

A Is the area, in square millimeter, of the exposed surface of the sample.

(Additionally:With steel wire, it is often advantageous to calculate the mass per unit area, P_A, of a hot dip galvanized coating, expressed in grams per square metre, using the following formula: P_A=   D  

where D is the diameter, in millimetre, of the wire after stripping and the density of steel is taken as 7,84 g/cm3. In this way, it is not necessary to know the length of the wire.)

Reproducibility

The reproducibility (different observers, different apparatus and operating conditions) is about ±5 % of the mean value.

Test Report

The test report shall contain the following information:

a) a reference to this document, i.e. ISO 1460;

b) type and dimensions of the sample;

c) the method of calculating the surface area for shaped samples;

d) the mass per unit area of coating, in grams per square metre or, if agreed between the interested

parties, the thickness of the coating, in micrometer;

NOTE

The approximate thickness, d, of the hot dip galvanized coating, in micrometer, can be calculated

from the formula  d=            which assumes that the density of the coating is 7.2 g/cm3.

e) any deviations from the procedure;

f) any unusual features observed;

g) the date of the test.

Thanks for your watching, have a nice day.