Free iron is an impurity that will contaminate the surface of the stainless steel. In order to make the stainless steel last longer, the free iron needs to be removed from the stainless steel. This is because free iron acts as an anodic protection to the stainless steel. So the free iron will oxidize or rust making the stainless steel corrode faster.
Nusantara Technologies Laboratory Testing offers two types of Free Iron tests to be conducted on steel:
- Copper Sulphate Test
- Ferricyanide Test
The purpose of these tests is to detect the presence of free iron on the steel surface. The test is in accordance to ASTM A380 standard.
There are various possibilities that give contribution to the source of free iron contamination. One of them is when any iron or steel that comes in contact to the stainless steel will be a potential source of the contamination. For example, chains, tools and removed scale. The free iron may also contaminate the stainless steel when the steel was previously processed or used on other steel or iron parts. For instance, blasting, grinding and polishing abrasives. The iron parts that were previously used on may be from containers or the systems used to handle the abrasives, such as blasting cabinets. Iron or steel shot or grit used to remove scale or other materials from the stainless steel items would leave residue on the surface.
One of the hardest sources of iron to be avoided is from the atmosphere itself. Industrial areas have a surprising amount of iron in the air. This iron “falls out” exposed onto the items of stainless steel products.
Copper Sulphate Test
This method is recommended for the detection of metallic iron or iron oxide on the surface of
- austenitic 200 and 300 Series, the precipitation hardening
- alloys, and the ferritic 400 Series stainless steels containing16% chromium or more.
It is not recommended for the:
- martensitic and lower chromium ferritic stainless steels of the 400 Series since the test will show a positive reaction on these materials.
- This test is hypersensitive and should be used and interpreted only by personnel familiar with its limitations.
Spray the surface with a solution of copper sulphate in water. If free iron is present, a copper film will form. In this test, the chemical reaction is: Fe + CuSO4 = FeSO4 + Cu. The copper film is immediately visible.
Ferricyanide Test (Ferroxyl Test)
Ferroxyl Test for Free Iron is a highly sensitive test and should be used only when even traces of free iron or iron oxide might be objectionable. It should be made only by personnel familiar with its limitations.
The test can be used on stainless steel to detect iron contamination, including iron-tool marks, residual-iron salts from pickling solutions, iron dust, iron deposits in welds, embedded iron or iron oxide, etc.
Spray the surface with a solution of potassium ferricyanide. If free iron is present, a blue colour will appear.
Due to the hazardous nature of potassium ferricyanide, this test shall only be conducted by well-trained personnel in strictly controlled conditions.
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| Sample condition before apply the test solution |
Sample after apply the test solution. Blue stain indicated of free iron presented on surface |