Reveal Unseen Iron Oxides on Steel Surfaces
Why Test for Iron Contamination on Stainless Steel?
Iron contamination occurs when there is a deposit on the iron surface of stainless steel. This is typically 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 is used for the other without adequately cleaning between batches.
In most cases, the free iron on the stainless steel surface readily oxidises in environments where the stainless steel alloy should be relatively immune to corrosion. This oxidation of iron will result in rust stains, which contain plain iron or steel particles that disrupt the protective oxide layer of a stainless steel workpiece, ruining its corrosion resistance and causing rust.
Due to the potential damage rust can cause, an iron contamination test for stainless steel pipes, coated or cladded surfaces and other components is particularly important in industries like oil & gas and marine, where piping systems must maintain long-term corrosion resistance in aggressive environments.
Common sources of iron contamination of stainless steel 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 can cause local damage to the passive layer.
The ferroxyl test for free iron can detect free iron on the stainless steel surface by converting it to a visible form with a ferroxyl indicator solution. 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 are absent.
Common Test Methods
– ASTM A380 / A380M (2017 Edition) Clause 7.3.4
– ASTM A967/A967M, Practice E
2) Copper Sulfate Test – Surface detection for passivation
The copper sulfate test for chrome plating and passivation verification detects the presence of free iron, which is typically introduced to 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 surfaces of free iron particles.
It is a visual method for detecting the presence of free iron, specifically in stainless steel. In many cases, the copper sulfate test is used as a go/no-go test for passivation of a part or stock material. Still, it can also be used to determine whether passivation is necessary.
Passivation is a treatment that protects metal from corrosion by making the material “passive” to its surrounding environment. A thin layer of a new non-reactive chemical will form on the surface, which is less likely to react with air and cause chemical corrosion.
Common Test Methods
– ASTM A380 / A380M
– ASTM A967/A967M, Practice D
Ferroxyl Test vs Copper Sulfate Test
Between a ferroxyl test vs copper sulfate sensitivity test, both methods detect iron contamination, but they work differently and suit different applications:
Factor | Ferroxyl Test | Copper Sulfate Test | Detects | Free iron and (limited) iron oxide | Free iron only |
|---|---|---|
Indication | Blue discolouration | Copper-coloured deposit |
Sensitivity | High — detects trace contamination | Moderate — better for gross contamination |
Primary use | Post-weld cleaning verification; passive layer integrity | Passivation verification; go/no-go checks |
Surface types | All stainless steel grades | Primarily austenitic stainless steels |
Standard | ASTM A380 Clause 8.3.4 | ASTM A380 Clause 8.2.5.3 |
In comparison, ferroxyl is recommended when high sensitivity is needed, particularly after welding, grinding, or mechanical processing, where trace contamination is likely. Copper sulfate is better suited for verifying passivation effectiveness or for routine QC checks on passivated parts.
In some cases, both tests may be specified: ferroxyl for initial detection, copper sulfate for post-passivation confirmation.
The ASTM A380 Standard
The ASTM A380 standard for iron contamination testing is the go-to reference for cleaning, descaling, and passivation of stainless steel.
Adherence to the standard is how we ensure having proper cleaning methods for removing contamination (chemical, mechanical, electrochemical), passivation procedures for various stainless steel grades, and test methods to verify cleanliness and passivation effectiveness.
Clause | What It Covers | 8.3.4 – Ferroxyl Test | A procedure for detecting free iron using potassium ferricyanide solution. A blue indication indicates that contamination is present. |
|---|---|
8.2.5.1 – Water Wetting Test | A quick visual check where water should sheet evenly on a clean, passive surface. Beading suggests contamination or incomplete cleaning. |
8.2.5.2 – High Humidity Test | Exposes the surface to high humidity. If rust forms within 24 hours, this indicates the presence of free iron. |
8.2.5.3 – Copper Sulfate Test | Immersion or swab application of copper sulfate solution. Copper plating indicates free iron. |
ASTM A380 or equivalent standards are often referenced by classification societies (e.g., ABS, DNV, Lloyd’s) for marine and oil & gas projects in Singapore. Here at PTS, our test reports are formatted to meet these requirements.
Industries & Applications
Iron contamination testing is critical wherever stainless steel must maintain corrosion resistance over its service life:
- Marine & Offshore
- Oil & Gas
- Pharmaceutical & Food Processing
- Construction
Talk to Us Today
At Professional Testing Services, we offer stainless steel iron contamination testing services in Singapore, Malaysia, and Indonesia, as part of our broader non-destructive testing for steel structures. Our laboratory performs both the ferroxyl test for free iron detection and the copper sulfate test for chrome plating and passivation verification, helping you catch contamination before it leads to premature corrosion or rejection.
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.
