What causes pitting corrosion?
Pitting corrosion is an electrochemical process initiated when the passive oxide film on a metal surface is locally disrupted — typically by aggressive anions, most commonly chloride ions. The mechanism in carbon steel is:
- Chloride ions concentrate at a surface defect, inclusion, or grain boundary, disrupting the iron oxide surface film
- An anodic pit forms: iron dissolves rapidly within the pit cavity
- The pit geometry creates a differential aeration cell — the pit interior becomes oxygen-depleted, accelerating anodic dissolution; the surrounding surface acts as the cathode
- The pit chemistry becomes progressively more acidic and chloride-rich as the reaction products accumulate, further accelerating corrosion within the pit
- A dark corrosion product crust forms over the pit mouth, which can slow the visible progression while corrosion continues to advance beneath
In practical terms: pitting is self-reinforcing. Once initiated, a pit creates the chemical conditions that accelerate its own growth. This is why pitting progresses rapidly in aggressive environments — offshore, marine, chemical processing, and high-chloride atmospheric conditions — and why early detection and treatment is critical.
Where pitting corrosion occurs in industrial assets
- Offshore structures — splash zone and tidal zone on jacket legs; topsides at areas of coating breakdown where chloride deposition is high
- Marine vessels — ballast tanks (particularly at low points where saline water accumulates), cargo hold structures, hull plating at coating defects
- Pipelines — external surface at disbonded coating locations; internal surface in sweet and sour service where CO₂ and H₂S promote pitting
- Storage tanks — internal floor plates and lower shell courses in contact with process fluids or tank bottoms accumulating water
- Heat exchangers and pressure vessels — shell-side surfaces in contact with chloride-containing process fluids or cooling water
- Mining structures — process vessels, structural steel, and piping in contact with acidic process fluids and chloride-rich process water
Why pitting creates surface preparation challenges
Standard abrasive blast cleaning — even at SSPC-SP10 / Sa 2½ (near-white metal) — addresses surface contamination and anchor profile on the general steel surface, but does not fully resolve the challenges that deep pitting creates:
1. Pits retain corrosion products after blasting
Blast media impacts the surface at high velocity, but the geometry of a deep, narrow pit limits the media’s ability to clean its interior surfaces. Corrosion products — iron oxides, iron chlorides, and iron sulfates — can remain in the lower portion of pits after blast cleaning that produces Sa 2½ on the surrounding steel surface. These residual products continue to corrode the steel beneath a subsequently applied coating and contribute to osmotic blistering through their soluble salt content.
2. Pits retain soluble salt contamination
Chloride ions are hygroscopic — they attract moisture. Once chloride-rich corrosion products accumulate in pits, they draw moisture through any overlying coating by osmosis. This is the mechanism of osmotic blistering: the chloride concentration in the pit creates a local driving force that pulls water through the coating film, building pressure until the coating delaminates. Standard dry blasting does not remove soluble salts from pit interiors. UHP water jetting (SSPC-WJ grades) is significantly more effective at removing salt-laden corrosion products from pitting.
3. Deep pits create coating thickness deficiencies
An applied coating film follows the contour of the steel surface. A pit 2–5 mm deep covered by a coating applied to the surrounding flat surface will have significantly thinner coverage at the base and walls of the pit than over the surrounding steel. In severe pitting, the coating may not even bridge the pit mouth — creating a void beneath an apparently intact coating surface. These thin-film areas are the first to fail in service.
4. Pitting reduces effective section thickness
While not strictly a surface preparation issue, pitting that has reduced steel to below minimum acceptable wall thickness must be identified before coating and may require weld repair before coating application. Ultrasonic thickness measurement (UTM) across pitted areas is standard practice for fitness-for-service assessment in the energy and marine sectors.
How to address pitting corrosion in surface preparation
Step 1: Assess pit depth and extent
Before selecting a preparation method, characterise the pitting: depth, diameter, density (pits per unit area), and the presence of corrosion product crusts covering pit mouths. This determines whether the preparation requirement is within the scope of blast cleaning alone, or requires a preliminary mechanical removal step.
Step 2: Remove bulk corrosion from deep pits — Tercoo®
For steel with deep laminar corrosion, thick corrosion product crusts, or heavy pitting with intact corrosion product over the pit mouths, mechanical removal of the bulk corrosion volume before blast or mechanical preparation is the technically correct approach. The Tercoo® is designed for this application: it removes heavy rust, laminar scale, and existing coating efficiently, exposing the steel surface beneath the corrosion crust and opening pit mouths for subsequent cleaning.
Step 3: Achieve target cleanliness and profile — Bristle Blaster® or abrasive blasting
After bulk corrosion removal, the surface is brought to the target cleanliness grade and anchor profile:
- Abrasive blasting achieves the highest production rate and the most consistent result across large areas
- The Bristle Blaster® achieves cleanliness comparable to Sa 2½ (ISO 8501-1) / SSPC-SP 10 and 65–85 µm Rz anchor profile on maintenance surfaces without grit — applicable in ATEX zones, confined spaces, and remote locations where blasting is not viable
Step 4: Control soluble salt contamination
On pitted steel with a history of chloride exposure, soluble salt testing is mandatory before coating application. If chloride levels in the prepared surface exceed specification limits (typically 20–30 µg/cm² for offshore and immersion service), a water wash or UHP jetting treatment is required to reduce salt levels before coating. Verify with a post-wash Bresle patch test that levels are within specification before priming.
Step 5: Consider pit filling for deep pitting
Very deep pits — greater than approximately 1–2 mm — may require filling with a compatible surface filler or stripe coat of the primer before the full coating system is applied, to ensure adequate film thickness at the pit base and walls. This must be compatible with the full coating system and specified in the project’s coating specification. Consult the coating manufacturer’s TDS for guidance.
Key takeaways
- Pitting corrosion concentrates attack in localised deep cavities driven by chloride-induced breakdown of the passive film. It is self-reinforcing: the pit chemistry accelerates its own growth.
- Standard abrasive blast cleaning to Sa 2½ / SSPC-SP10 does not fully resolve the surface preparation challenge of deep pitting: corrosion products and soluble salt contamination can remain in pit interiors after blasting.
- On heavily pitted steel, the most effective approach is a two-step preparation: Tercoo® for bulk corrosion and crust removal, followed by Bristle Blaster® or abrasive blasting to achieve target cleanliness and anchor profile.
- Soluble salt control is critical for pitted steel — chloride-rich corrosion products in pits drive osmotic blistering. Test and treat before coating application.
- Pitting that has reduced steel below minimum wall thickness requires structural assessment and weld repair before coating — coating cannot restore lost structural section.