Revealing Iron oxides on the steel surfaces
Why is there a need for Iron Contamination on stainless steel?
Iron contamination occurs when there is a deposit on the iron surface of stainless steel. This typically is caused by abrasion or scratching the stainless steel by iron or carbon steel during fabrication or handling. Iron residue might get transplanted onto stainless steel when the equipment used to process one type of material may be used for the other without adequately cleaning between batches.
In most cases, the free iron on the stainless steel surface will be readily oxidised in environments where the stainless steel alloy should be relatively immune to any corrosive attack. The oxidation of iron on the stainless steel surface will result in the accumulation of rust stains that will indicate that the stainless steel is being attacked.
These plain iron or steel particles can disrupt the protective oxide layer of a stainless steel workpiece by ruining its corrosion resistance so that it starts to rust.
Familiar sources of iron contamination include:
- Stainless steel coming into contact with any other steel or iron materials
- Tools and parts previously used on steel or iron
- Environment and atmosphere – the area the stainless steel is kept in, including the air, water or other chemicals used to clean the surface
- Finished products that have been coated but in an environment where iron is readily available
Type of Iron contamination tests
1) Ferroxyl Test – Detecting Free Iron
Free iron cannot be seen on the surfaces of contaminated parts. However, free iron particles left behind after welding can lead to the formation of iron ions in an aqueous solution. These particles are then able to cause local damage to the passive layer.
The ferroxyl test can show free iron on the stainless steel surface by converting it into a visible form. The ferroxyl test can show free iron on the stainless steel surface.
The test aims to check the integrity of the passive layer on the stainless steel surface and guarantee that free iron and (to a limited extent) Iron oxide is absent.
Common Test Methods
– ASTM A380 / A380M (2017 Edition) Clause 7.3.4
2) Copper Sulfate Test – Surface detection for passivation
The copper sulfate test aims to determine the presence of free iron, which is usually induced onto the surface of a part during fabrication with steel components. The test principle is based on an oxidation-reduction reaction that causes the dissolved copper ions to deposit or plate out onto the locations of free iron particles. It is a visual method of determining the existence of free iron, specifically stainless steel. In many cases, the copper sulfate test is used as a go/no-go examination of a passivation process on a part or stock material. Still, it can also be used as a test to determine if passivation is necessary in the first place.
Passivation is a treatment method to protect the metal from corroding by making the material “passive” to the surrounding environment. However, it encourages corrosion to occur. A thin layer of a new non-reactive chemical will appear on the surface, which is less likely to react with air and cause corrosion chemically.
Common Test Methods
– ASTM A380 / A380M
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Our expert testing team can confirm passivation using the Copper sulphate test and detect free iron with the Ferroxyl test. For more information on our iron contamination test, call us or send us an enquiry today.