Pitting Corrosion in Stainless and Carbon Steel: Causes, Risks and How to Detect It

Pitting Corrosion: Causes, Risks and Detection Methods

Pitting corrosion is one of the most difficult forms of corrosion to detect, and one of the most dangerous. Unlike general corrosion, which reduces wall thickness evenly across a surface, pitting corrosion creates small, localised cavities that can penetrate deep into a material while the surrounding surface looks largely intact. Corrosion in all its forms is estimated to cost the global economy $2.5 trillion annually, or 3.4% of global GDP, and pitting is among the hardest variants to catch before it becomes a structural problem.

Carbon steel and stainless steel are both widely used across aerospace, marine, energy and industrial applications, but each brings different pitting corrosion risks, and different inspection challenges.

What Causes Pitting Corrosion?

Pitting corrosion typically begins when a material’s protective surface layer is locally broken down, often by chloride exposure, stagnant moisture, mechanical damage, or weaknesses introduced during welding or manufacturing. Once that protective layer fails at a specific point, corrosion concentrates there rather than spreading evenly, gradually boring into the material while leaving most of the surrounding surface unaffected.

Carbon steel is one of the most widely used engineering materials, valued for its strength and low cost, but it’s also naturally more prone to corrosion than corrosion resistant alternatives. Stainless steel is often chosen specifically to resist this, but it isn’t immune. Stainless steels can still pit in certain service environments, particularly at high temperatures or in the presence of chlorine, which is why they’re still closely monitored across power generation, construction and petrochemical applications.

Why Pitting Corrosion Is Difficult to Detect

Because pitting corrosion doesn’t produce a flat, even loss of material, it lacks the planar reflective surfaces that ultrasonic inspection normally relies on for accurate thickness measurement. This makes it harder to characterise than general corrosion, even though it can be just as structurally significant, if not more so, since deep localised pits can compromise a structure while the surrounding wall thickness still measures well within tolerance.

Stainless steel adds a further complication. Its larger grain structure causes more attenuation of the ultrasound beam than carbon steel, making high resolution inspection more demanding. On large structures like ship hulls, storage tanks, and bridges, the sheer scale of the surface to be inspected adds a further practical challenge, since manually locating and measuring every pit across a large area is slow and easy to miss.

How Ultrasonic NDT Detects Pitting Corrosion

Matrix array ultrasonic testing (MAUT), as used in the dolphicam2, addresses corrosion‑mapping challenges directly. Rather than relying on a single reflected signal, the dolphicam2 captures full waveform data across a two‑dimensional aperture, which can then be enhanced using ARCUS, the latest advanced post‑processing software designed to improve imaging clarity and reflector definition. This enables reliable characterisation of pitting depth and remaining wall thickness, even where no clean, flat reflective surface exists. Different transducer frequencies support a wide range of inspection scenarios: higher frequencies deliver fine detail for thin components, while mid‑range frequencies provide the ideal balance of penetration and resolution for general wall‑thickness surveys on thicker carbon steels. For large structures, the dolphicam2 can be paired with scanning tools to map corrosion across wide areas efficiently, producing full digital records that can be reviewed and compared over time.

Conventional linear arrays remain highly capable for corrosion pit sizing, but they depend on encoded scanning, controlled probe movement, and post‑processing to generate corrosion maps. This workflow is effective but inherently slower, more complex, and more sensitive to operator technique. Any deviation in scanning paths can lead to inconsistent coverage or missed areas, and building a complete corrosion map requires careful planning and mechanical encoding.

MAUT takes a fundamentally different approach. By capturing data over a matrix aperture, the dolphicam2 produces a live, real‑time C‑scan without the need for encoding or mapping, allowing inspectors to see corrosion features appear instantly as they move across the surface. This dramatically simplifies corrosion surveys, reduces setup time, and ensures full coverage even in awkward or constrained access areas. Combined with ARCUS post‑processing, the system delivers enhanced imaging performance and a complete digital record that can be reviewed, shared, and compared over time.

For operators who need fast, intuitive corrosion mapping across broad areas—and a repeatable digital record that supports long‑term asset monitoring—MAUT offers a more efficient and user‑friendly solution than traditional linear arrays, while still providing the volumetric detail needed to characterise corrosion confidently.

Real World Examples

We’ve used the dolphicam2 to detect and characterise pitting corrosion across a range of materials and structure sizes:

Pitting Corrosion in Carbon Steel: using TFM to resolve pitting features on a calibration block with a 2 to 15mm thickness range, enabling rapid accept/reject decisions and remaining life estimation.

Pitting Corrosion in Stainless Steel: successfully resolving pitting corrosion and distinguishing it from general surface corrosion, despite the additional signal attenuation stainless steel presents.

Corrosion in Large Structures: mapping both general and pitting corrosion across a large carbon steel plate using an X/Y scanner, demonstrating how large area inspection can be carried out efficiently with full digital records.