Stainless steel pervades many aspects of our lives. The transportation industry uses it for ship containers, car exhaust systems, road tankers and countless other applications. The oil and gas sector utilizes stainless steel for cable trays, subsea pipelines and platform components. Chemical and pharma players rely on it for pressure vessels and process piping. For the medical sector, stainless steel has been a mainstay for MRI scanners, surgical implants and surgical instruments.
In architectural and civil engineering spheres, stainless steel turns up in cladding, structural sections, lintels and lighting columns. And where would the water industry be without stainless steel for sewage treatment components, water tubing and hot water tanks? And that’s just the tip of the iceberg. Think of stainless steel applications in the food and drink sector and in the domestic products used daily. The applications are countless.
What is stainless steel?
Stainless steel is an alloy of iron with a minimum of 10.5 percent chromium. Chromium produces a thin layer of oxide on the surface of the steel – the “passive” layer – that prevents surface corrosion. Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. The iron oxide film (rust) is “active” and accelerates corrosion by making it easier for more iron oxide to form.
By comparison, stainless steel contains sufficient chromium to undergo passivation – the forming of an inert film of chromium oxide – on its surface. Passivation occurs only if oxygen is present and the proportion of chromium is high enough.
Increasing the amount of chromium increases resistance to corrosion. In addition to chromium, stainless steel also contains varying amounts of carbon, silicon and manganese. Other elements such as nickel and molybdenum may be added to impart other useful properties, such as enhanced formability and increased corrosion resistance.
Depending on its composition, stainless steel divides into five main types: austenitic (the most common), ferritic, martensitic, duplex and precipitation hardening or PH. Each have varying properties suited to varying applications. In all, there are more than 150 grades of stainless steel, of which 15 are used regularly.
What is passivation?
Passivation is an essential process in the manufacture and quality assurance of varying grades of stainless steel. In physical chemistry and engineering, passivation refers to a material becoming passive or less affected by the environment of future use. Put another way, passivation means to make something chemically passive or inactive. “Active” surfaces react readily whereas passive surfaces are resistant to reactions, including chemical reactions.
Stainless steel is corrosion-resistant by nature, which might suggest that passivating it would be unnecessary, yet it is not completely impervious to rusting. Some metals, such as gold and titanium, are self-passivating where exposed surface atoms readily react with oxygen in ambient air to form a stable layer of passive metal oxide. However, if steel tools are used on such metals, trace amounts of free iron (ferric material) can be left on the surface and the iron will corrode just as any iron would.
To passivate stainless steel, the surface of the material must be completely free of contaminants, such as free iron. If so, chromium content in stainless steel then reacts with oxygen in the ambient air to form an inert – or passive – layer of chromium oxide on the steel’s surface. This chromium oxide micro-coating acts as a barrier between the iron-dense alloy and the ambient air.
Passivation begins immediately after surface contaminants are completely removed. It typically takes 24 to 48 hours to achieve a uniform and stable passive layer of stainless steel. Because it’s uncommon for the passive oxide layer of stainless steel to sustain damage through any number of mechanical, industrial and environmental processes, passivation is the final step when manufacturing stainless steel components.
Testing, validating
The importance of validating the passivation of stainless steel surfaces cannot be overstated. Impossible to detect with the naked eye, passivation indicates that a protective layer of chromium oxide exists on a stainless steel surface – the essential ingredient that guarantees stainless steel will resist corrosion.
ASTM International, the American Society for Testing and Materials, sets out five best practices for chemical passivation of treatments of stainless steel components, which are listed below. It’s important to note that not all of these tests are suitable for all grades of stainless steel.
1. Water immersion test
The water immersion test is used to detect anodic surface contamination, including free iron on stainless steel. The test exposes passivated components to distilled water for intervals of one hour submerged in water and one hour without being submerged for at least 24 hours to determine the effectiveness of the passivation process. Although water is easily accessible, access to specialized immersion chambers can require a significant capital investment. The test’s drawbacks or challenges include:
- Water must be clean, distilled and free of chemicals
- Costly plumbing may be required
- Inadequate plumbing can falsely indicate trace iron on tested surfaces
- A minimum 24-hour testing cycle is required to comply with the ASTM standard
- Failed components may require reworking and further decontamination
2. High humidity test
The high humidity test is used to detect free iron or any other anodic surface contaminants on stainless steel. The test is performed in a humidity cabinet capable of maintaining 97 ±3 percent humidity at 100 ±5 degrees F (38 ±3 degrees C) for a minimum of 24 hours. The test sample is acceptable if there is no evidence of rust stains or other corrosion products after completion of the test. To comply with ASTM standards, components also require immersion in acetone or methyl alcohol and dried in an inert atmosphere or desiccated container. The test’s drawbacks or challenges include:
- Costly investment in humidity chamber and specialized lab equipment
- Chamber may be unsuitable for large stainless steel components
- Test cannot be tailored to different stainless steel grades
- A minimum 24-hour testing cycle is required to comply with ASTM standard
• Failed components may require reworking and further decontamination
3. Salt spray test
The salt spray test is an accelerated laboratory test that provides a controlled corrosive environment to determine the corrosion resistance of stainless steel. The test exposes components to a salt spray (fog) solution of 5 percent sodium chloride in a test chamber heated to 35 degrees C. Although the test duration is short compared with the natural environment, it is generally not possible to assess the behavior of a material, especially stainless steel, in a natural environment from the results of a salt spray test.
The test is also of limited use for comparing the corrosion resistance of different stainless steel grades and for establishing a ranking or quantifying the differences in corrosion resistance. This is because the corrosive conditions of the test are fixed and cannot be adjusted to the resistance of the steel grades to be tested. Additional drawbacks or challenges include:
- Costly investment in humidity chamber and specialized lab equipment
- Chamber may be unsuitable for large stainless steel components
- Test cannot be tailored to different stainless steel grades
- Test does not predict behavior of stainless steel in natural environment
- Failed components may require reworking and further decontamination
4. Copper sulfate test
The copper sulfate test is rarely accepted in the food industry because of its toxic nature. In fact, ASTM bans the use of this test on stainless steel components “to be used in food processing.”
The test, which requires aqueous copper sulfate solutions “no more than two weeks old,” is used for specific grades of austenitic, martensitic, ferritic and PH steel where a minimum 16 percent of chromium is present. The test’s drawbacks or challenges include:
- Costly investment in specialized lab equipment and chemicals
- Aesthetic damage may appear on tested components
- Impractical for food industry-related components
- Failed components may require reworking and further decontamination
5. Potassium ferricyanide-nitric acid test
The potassium ferricyanide-nitric acid test is recommended when detection of very small amounts of free iron is required on austenitic 200 and 300 series stainless steel. As with the copper sulfate test, ASTM forbids use of this test on stainless steel components used in the food processing sector. The test’s drawbacks or challenges include:
- Costly investment in specialized lab equipment and chemicals
- Not recommended for ferritic or martensitic steel due to false positives
- Testing solution must be prepared daily
Operational obstacles
Welding creates a heat-affected zone, known as heat tint, in which the alloy structure is altered. Heat tint is a thickening of the naturally occurring oxide layer on the surface of the metal.
As heat tint colors are formed on stainless steel, chromium is drawn from below the surface of the metal to form a chromium-rich oxide surface layer. This leaves the metal just below the surface with a lower chromium level, which can have a negative effect on the corrosion resistance of the steel. Heat tint is a serious contaminant that must be removed from the surface, not only for aesthetic reasons, but specifically to allow stainless steel to self-passivate.
Sandblasting removes heat tint but embeds other contaminants in stainless steel. Grinding, although an effective method to remove heat tint, leaves traces of free iron, which cause pitting and corrosion. To eliminate free iron, a post chemical treatment with harsh acids is necessary. Both techniques are labor intensive. Galvanic (or bimetallic) corrosion may occur when dissimilar metals are in contact in a common electrolyte (e.g., rain or condensation).
Electrochemical weld cleaning
Electrochemical cleaning and polishing is the safest and most efficient method of removing heat tint and other contaminants from stainless steel surfaces. This process removes more iron and nickel, leaving the surface rich in chrome. Most commonly, phosphoric and sulfuric acids are used in conjunction with a high current density to clean and smooth (by metal removal) the surface of the metal.
Electropolishing preferentially attacks peaks and valleys on the material surface and raises the proportion of chromium at the surface. The technique has a substantial effect on the appearance, increasing luster and brightness while only changing the measured roughness by about 30 percent.
Walter Surface Technologies’ Surfox weld cleaning and polishing system removes heat tint from surfaces at 3 to 5 ft./min. The system uses food grade acids and electricity to remove heat tint and is a recognized method for achieving chemical passivation on stainless steel parts, as defined by ASTM A967-05/A380 for passivation. Surfox is a safer, faster and more cost-effective alternative to hazardous pickling pastes and abrasive weld cleaning processes, such as wire brushing and grinding.
The ASTM A380 standard provides definitions and describes best practices for cleaning, descaling and passivation of stainless steel parts, equipment and systems. The ASTM A967 standard provides tests with acceptance criteria to demonstrate that the passivation procedures have been successful.
As per ASTM International standards, “passivation testing is the process by which stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen containing environments. Passivation testing involves the removal of exogenous iron or iron compounds from the surface of the stainless steel, by means of a chemical dissolution, most typically with an acid solution that will remove the surface contamination but will not significantly affect the stainless steel itself.”
The Surfox system can create cost savings of 26 percent for users in the first year of use compared with pickling paste (see Figures 1 and 2). In following years, after an initial low-cost capital investment is completed, cost savings can triple to 78 percent.
Passivation test
Walter Surface Technologies has long exploited the benefits of open circuit potential as a reliable technique to qualify the stability and thickness of the passive chromium oxide layer of stainless steel. The Surfox Smart Passivation Tester works on the principal of open circuit potential whereby a user can measure the interface potential of the chromium oxide layer relative to the underlying steel.
Bluetooth-enabled, compact and portable, the device displays a numeric value of the quality of the passive layer of chromium oxide. A positive value indicates a stainless steel part is passivated. A negative value indicates a stainless steel part is not passivated. The higher the value, the thicker and more resistant the passive layer.
The Smart Passivation Tester allows users to quickly ensure passivation of stainless steel anytime and anywhere – including job sites – thereby reducing downtime by guaranteeing users that their stainless steel components are corrosion-free. What better way for stainless steel fabricators to avoid costly rework of rejected components and ship products with confidence?
The Smart Passivation Tester adds certainty, cost savings and convenience when compared with outsourcing passivation testing. Indeed, shipping stainless steel samples for laboratory-based passivation testing can be costly. This inconvenience and expense is no longer necessary, however, with the Smart Passivation Tester.
It goes without saying that passivation of stainless steel is a key concern for fabricators, welders and manufacturers that buy, sell or work with the ubiquitous and essential material. With the Surfox system, metalworkers have an exceptional set of tools in their arsenal to accelerate, detect and measure passivation as well as reduce costs associated with rework of rejected stainless steel products.
