What is corrosion?
Corrosion is the chemical or electrochemical reaction between a metal and its environment that degrades the metal’s properties. The environment provides the oxidising agent — most commonly oxygen dissolved in water, or water itself in combination with atmospheric oxygen. The fundamental corrosion reaction involves the metal being oxidised (losing electrons) at an anodic site, while the oxidising agent is reduced at a cathodic site. In the presence of an electrolyte — essentially any moisture containing dissolved ions — this reaction proceeds continuously.
Corrosion affects virtually all structural metals used in industrial applications: carbon steel, stainless steel, aluminium, copper, titanium, and zinc. Each corrodes through mechanisms that are specific to its chemistry and the environment it is exposed to.
What is rust?
Rust is the common name for the reddish-brown iron oxide compounds that form when iron or steel reacts with oxygen in the presence of water. The primary rust compounds are iron(III) oxide (Fe₂O₃) and iron(III) oxide-hydroxide (FeOOH). Rust is chemically distinct from the parent metal — it is porous, brittle, and non-adherent. Unlike the dense, adherent passive oxide layers that protect stainless steel and aluminium from further oxidation, rust does not arrest corrosion. As rust forms and flakes, it exposes fresh steel surface to further attack.
Key characteristics of rust:
- Only iron and steel rust — no other common structural metal produces the same reddish-brown iron oxide product
- Rust is volumetrically larger than the steel it replaces — rust occupies approximately 2–6 times the volume of the original iron, depending on the specific iron oxide compounds formed, which is why rusting steel causes cracking and spalling of surrounding concrete in reinforced concrete structures
- Rust is hygroscopic — it absorbs moisture, which accelerates further corrosion
- Rust in surface pits and crevices retains chloride and sulfate contamination, a significant factor in coating failure when rusted steel is incompletely prepared before recoating
Types of corrosion that affect steel structures
While rust is the most visible form of corrosion on steel, multiple distinct corrosion mechanisms affect industrial steel structures. Understanding which types are present determines the protective strategy:
| Corrosion type | Mechanism | Typical locations | Surface preparation implication |
|---|---|---|---|
| Uniform (general) corrosion | Even electrochemical attack across the surface; classic rusting of unprotected steel | Uncoated or de-coated atmospheric steel | Standard blast cleaning to target grade removes surface rust; soluble salt testing required |
| Pitting corrosion | Localised attack initiated by chlorides; creates deep cavities while general surface is relatively intact | Marine and offshore structures; chloride-rich environments | Blasting alone may not clean pit interiors; two-step preparation (Tercoo® + Bristle Blaster®) recommended; strict salt control |
| Galvanic corrosion | Accelerated corrosion at the junction of two dissimilar metals in electrolyte contact | Steel-aluminium connections; steel-copper grounding | Isolate dissimilar metals; apply protective coating to anodic metal; surface prep per coating TDS |
| Crevice corrosion | Accelerated attack in occluded spaces (lap joints, under gaskets, at fasteners) | Structural connections, flanged joints, overlapping plates | Stripe coat all crevice-forming joints; consider design modification to eliminate crevices; thorough crevice cleaning before coating |
| Erosion-corrosion | Combined mechanical and electrochemical attack in flowing fluid; surface film continuously removed, accelerating corrosion | Pipe bends, pump impellers, heat exchanger tubes, offshore splash zone | Harder, more abrasion-resistant coating systems; higher anchor profile required for mechanical interlocking; SP-10 minimum |
| Stress corrosion cracking (SCC) | Cracking under combined tensile stress and corrosive environment (H₂S, chlorides) | High-strength steel in sour gas service; stainless steel in chloride environments | Coating applied to exclude corrosive environment; surface prep to SP-10; special primers for H₂S environments |
| Microbiologically influenced corrosion (MIC) | Bacteria (sulfate-reducing, acid-producing) create locally corrosive microenvironments | Pipeline internals, offshore submerged zones, water infrastructure | Biocide treatment; thorough removal of biofilm before preparation; strict salt and contamination control |
Rust grades and surface preparation standards
ISO 8501-1 defines four rust grades (A, B, C, and D) that describe the initial condition of uncoated steel before surface preparation:
- Grade A — Steel with intact millscale; no rust visible. The millscale is a relatively inert steel oxide layer formed during hot rolling — but it is cathodic to the underlying steel and, if not completely removed, promotes galvanic corrosion at any break.
- Grade B — Steel with beginning of rust and some pitting; millscale beginning to flake. Most carbon steel structural sections in service for more than a few months in atmospheric environments fall into this category.
- Grade C — Steel with widespread rust; millscale has rusted away or can be scraped off. Significant pitting typically present.
- Grade D — Steel with extensive pitting; millscale has rusted away completely. Advanced corrosion requiring the most thorough preparation.
Surface preparation standards (Sa grades for blast cleaning, St grades for power tool cleaning) are assessed against these initial conditions. The required preparation grade depends on the coating system being applied — most high-performance industrial coating systems require Sa 2½ (SSPC-SP10), regardless of the initial rust grade. Achieving Sa 2½ on Grade D steel with deep pitting requires significantly more preparation effort and time than achieving it on Grade B steel.
What rust grade tells you about surface preparation requirements
The rust grade of a steel surface provides important practical information before preparing it for recoating:
- Grade A steel — Millscale must be removed completely for most high-performance coating systems. Millscale is adherent but cathodic to steel; any break in the millscale creates a galvanic cell. Blast cleaning is required; hand and power tool methods cannot reliably remove intact millscale.
- Grade B–C steel — Rusted but without severe pitting; achievable to Sa 2½ by standard blast cleaning or the Bristle Blaster® mechanical preparation method in maintenance scenarios.
- Grade D steel — Deep pitting present; two-step preparation (Tercoo® for bulk corrosion removal, Bristle Blaster® for final preparation) or blast cleaning required; soluble salt control critical.
Key takeaways
- Rust is a specific form of corrosion — the iron oxide product of steel reacting with oxygen and water. Corrosion is the broader category of electrochemical metal degradation that affects all structural metals.
- Multiple types of corrosion affect steel structures: uniform rust, pitting, galvanic, crevice, erosion-corrosion, stress corrosion cracking, and microbiologically influenced corrosion. Each has different causes, locations, and surface preparation implications.
- ISO 8501-1 rust grades A–D describe the initial condition of steel before preparation, from intact millscale (A) to severely pitted (D). The rust grade determines the preparation effort required to reach a given cleanliness standard.
- For all high-performance industrial coating systems, Sa 2½ (SSPC-SP10) is the effective minimum surface preparation standard — regardless of the initial rust grade.
- Deep pitting (Grade D) retains corrosion products and soluble salts that blasting alone may not fully address. A two-step preparation approach and strict soluble salt control are required for pitted steel in aggressive environments.