Stainless Steel Corrosion: Types, Detection, Prevention

Stainless steel is synonymous with durability. Resistant to temperature, corrosion, and high loads, stainless steel is the go-to material for everything from kitchenware to large industrial heat exchangers and even the newest starships.

Yet, stainless steel isn’t fully immune to environmental damage or cracking under excessive stress. Learn why stainless steel corrosion emerges and how to best prevent it from this article.

Is Stainless Steel Corrosion-Resistant?

Stainless steel has great corrosion resistance thanks to high chromium content. Oxygen in the air reacts with chrome, creating an invisible protective oxide film. Although transparent and only two nanometers thick, this layer shields stainless steel from external impacts, making stainless steel electrochemically passive in corrosive environments.

Stainless steel corrosion resistance levels will depend on three factors:

• Chromium concentration: The chromium content of stainless steel varies between 10.5% and 30%. More chromium equals better resistance.

• The presence of other alloying elements: Nickel, molybdenum, nitrogen, and copper prevents stress-corrosion cracking, crevice corrosion, and pitting in chloride-based or high-temperature conditions.

• Operational and environmental factors: Corrosion resistance diminishes under reduced oxygen content, heavier loads, and higher pH levels, temperatures, and chloride contents.

Based on these factors, stainless steels are classified into five corrosion resistance classes as per DIN EN 1993-1-4:2015-10/Eurocode 3:

• I: Low – ferritic stainless steels, chromium, and iron alloy
• II: Moderate – austenitic stainless steels, rich in chromium, nickel, and iron
• III: Medium – austenitic stainless steel with molybdenum
• IV: Strong – super austenitic stainless steel with molybdenum; duplex stainless steels with chromium, nickel, molybdenum, copper, and iron
• V: Very strong – super austenitic and duplex are the most corrosion-resistant stainless steels

In other words, stainless steel isn’t fully immune to corrosion, which can develop under exposure to chlorides (mostly salt) and sulfur dioxide. In some cases, stainless steel can also become corroded due to heavy stress and load. Thus, one of the first steps in selecting a stainless steel product is determining the proper alloy per your operating conditions.

Types of Corrosion in Stainless Steel

High chloride environments, combined with high temperatures and residual stress create a perfect storm for stainless steel corrosion development. Generally, the most common types of corrosion in stainless steel are pitting corrosion, stress corrosion cracking, and galvanic corrosion.

1. Pitting Corrosion

Pitting corrosion manifests as small, but deep, sub-surface holes (pits), which compromise its structural integrity. A high temperature and chloride-rich environment can cause pitting corrosion on stainless steel. The resistance of alloys to pitting corrosion depends on chromium, molybdenum, and nitrogen contents in the material. Thus, ferritic stainless steels are the most vulnerable.

Because corrosion products often cover the pits, detection is challenging. It can also progress relatively fast in stainless steel. Moreover, under high-stress loads, pitting corrosion makes stainless steel susceptible to cracks.

2. Stress Corrosion Cracking

Stress corrosion cracking (SCC) emerges as a surface fissure that happens without any previous material deformation. SCC results from combined exposure to chloride environments, extreme temperatures, applied loads, and residual stresses. The main danger is that it’s very abrupt and, therefore, difficult to detect at early stages.

SCC can lead to leaks, ruptures, and, in the worst-case scenarios – severe asset failures. Therefore, for high-stress and high-load assets, it’s best to select SCC-resistant steel grades like duplex and ferritic stainless steels or protect non-resistant ones (e.g., austenitic) with extra coating.

3. Galvanic Corrosion Between Aluminum and Stainless Steel

Galvanic corrosion occurs when two adjacent metals with different electrode potentials interact with an electrolyte solution such as accumulated moisture. Metals that are positively charged (anodes) transfer their electrons and experience oxidation leading to corrosion. Galvanic corrosion is accelerated in high-moisture environments. For example, on offshore oil rigs, exposed to salty waters and frequent rain.

When the stainless steel and aluminum are used together, the latter takes the whole corrosion hit. In this assembly, aluminum acts as an anode and quickly deteriorates, especially when moisture, rainwater, or salty fluids are present. So, the general rule is to use stainless steel fasteners for aluminum components (not vice versa).

How to Detect Corrosion in Stainless Steel

Stainless steel assets can suffer from external and internal corrosion. Mapping and sizing its patterns requires different non-destructive testing methods and equipment.

External corrosion ranging from mild to severe is easily visible to the naked eye. For more nuanced damage or internal corrosion, you will need penetrative NDT methods, like ultrasonic inspection and eddy current testing among others.

Visual Inspection

Visual inspection involves a surface-level examination to pinpoint easily apparent discolorations, deformations, and other changes, indicative of corrosion. It is an important part of regular asset inspections and can augment other NDT methods (for preliminary checks, site preparations, or result validations).

Visual inspection helps detect stainless steel corrosion that begins on joints, welds, and exposed edges. To assess hard-to-reach, elevated, or confined objects, you can use specialized inspection drones with HD cameras. This way, you can access restricted areas faster, obtain more detailed footage, and maintain visual references for ongoing condition monitoring.

Ultrasonic Testing

Ultrasonic testing (UT) leverages high-frequency sound waves that penetrate the test object to identify thickness loss and sub-surface defects. UT is a relatively simple procedure requiring only a suitable transducer and couplant. Inspection of high-temperature or elevated assets, however, will call for a drone.

At Voliro, we developed a specialized inspection drone with an ultrasonic transducer payload (among several others!). The drone has a 360-degree freedom of movement, allowing easy access from every angle for performing A-scans. It measures thickness up to 2-150 mm/0.08-5.9 in and has a resolution of 0.06 mm/0.002 in. Voliro’s high-temp UT payload can detect the thickness of operational assets heated up to 0-260 °C/32-500 °F. With our technology, you can take precise thickness measurements two to four times faster than using conventional methods to detect stainless steel corrosion at early stages.

Other UT tools also allow B- and C-scans. B-scans provide a 2D cross-sectional view of the asset, aiding in defining the precise material’s depth. C-scans help create a 3D map of internal structures, displaying color-coded images of corrosion positioning and intensity.

Eddy Current Testing

Eddy current testing leverages electromagnetic induction to detect surface and subsurface defects in conductive materials. Corrosion and cracks are detected by sending alternating current into the stainless steel and creating a magnetic field.

The most common ECT tools are surface probes. They come in different diameters and frequency ranges, but all require direct access to the asset for precise measurements. Voliro is changing that with a drone-mounted pulsed eddy current probe, designed to detect corrosion under insulation (CUI). The probe can penetrate coatings and insulation as much as 100 mm thick, revealing hidden CUI in insulated stainless steel pipes. The precision of the result increases thanks to embedded noise shielding and canceling mechanisms, plus specialized data processing algorithms.

Other Corrosion Detection Methods for Stainless Steel

Visual inspection, ultrasonic scans, and eddy current testing are the most widely used methods for locating stainless steel corrosion. There are, however, other options that may come in handy:

• Dye penetrant testing involves dispersing dyed liquid on a test area. As liquid penetrates cracks and holes, it visualizes surface defects and corrosion.

• Radiographic testing places a test object between a radiation emitter and film, sending X-rays or gamma rays to inspect internal asset structures for signs of corrosion.

• Magnetic particle testing relies on colored magnetic particles spread on a magnetized test object. As magnetic particles attract to defected areas, they visualize the location of even minute cracks.

• Chloride testing tests chloride resistance of protective coating by applying a salt spray in a controlled corrosive environment.

Prevention Techniques for Stainless Steel Corrosion

With heavy use and exposure to aggressive agents, some stainless steel assets may develop corrosion abruptly. Optimal operating conditions, protective coatings, and surface treatments can help avoid or postpone corrosion onset.

Select the Right Stainless Steel Grade for the Environment

Choosing an optimal stainless steel grade is the first step toward corrosion prevention. The most common types of stainless steel found in industrial assets are austenitic grade 316 and 316L. They are used for storage tanks, flare stack chimneys, and all sorts of pressure vessels. The oil and gas industry, in turn, relies on duplex stainless steels like 2205, 2507, and 2507 for offshore structures and pipelines.

Here’s a quick refresher on your options.

Regularly Apply Protective Coatings and Surface Treatments

Epoxy, polyurethane, resin, and powder coatings protect stainless steel from moisture, chemicals, and mechanical wear. They are available as sprays and take 1-2 hours to take effect. Stainless steel, however, requires preparation and cleansing before the coating is applied. Poorly sealed coating leads to localized breakdown, increasing corrosion risk. So it’s important to take dry film thickness measurements right after application (something you can do with Voliro’s DFT probe).

If paint and coating don’t suffice, you can opt for stainless steel treatment, such as passivation and electropolishing. Passivation uses nitric acid to wash away free iron from the surface, renewing its protective oxide film. This chemical process typically takes about an hour. Also, NASA recently found that citric acid (found in oranges and lemons) can replace nitric acid for passivation. It does not harm beneficial heavy metals, is safer for the inspectors, and costs less.

Electropolishing uses electrical current to dissolve a thin layer of stainless steel into an electrolyte substance, thus eliminating microcracks, iron residues, and other minute imperfections. It creates a smooth, bright finish and keeps the surface corrosion-proof. The process takes only 20 minutes.

Minimize Contact with Desperate Metals

Two dissimilar metals combined in a humid environment trigger galvanic corrosion. One way to prevent galvanic corrosion between aluminum and stainless steel is by minimizing contact between the metals and keeping the area dry. To reduce their contact, you can use rubber gaskets, insulation rings, or anodic coating.

Additionally, you can select similar metals for an assembly using stainless steel bolts and joints. But be vary: different stainless steel grades have varying electrode potentials. So be sure to always select the right adjacent materials.

Conclusion

The softest water eventually wears away the hardest stone. Salt it up – and it ruins even stainless steel. To minimize the odds of stainless steel corrosion proliferation, select the right grades and perform frequent NDT inspections to detect early signs of asset failure.

With Voliro’s inspection technology, you can perform NDT tests on the fly (literally) in less time, with less equipment, and with smaller crews. Suitable for storage tanks, flare stacks, transmission lines, and pipelines, one Voliro drone can reduce inspection times from 3 days to two hours and cut inspection costs by 30%.