Types of industrial corrosion: how to recognize, diagnose and prevent each form

What industrial corrosion is

Industrial corrosion is the electrochemical degradation of a metal by its environment: at anodic sites the metal oxidizes and dissolves, at cathodic sites oxygen or hydrogen ions consume the released electrons. It takes about a dozen recognizable forms, and each form calls for a different prevention strategy.

Four conditions make a corrosion cell: an anode, a cathode, an electrolyte (a water film, condensate, process fluid or wet soil) and a metallic path between anode and cathode. On steel, the anodic reaction is Fe → Fe²⁺ + 2e⁻. In neutral, aerated water the main cathodic reaction is oxygen reduction, O₂ + 2H₂O + 4e⁻ → 4OH⁻; in acids it is hydrogen evolution, 2H⁺ + 2e⁻ → H₂. Rust is what forms when the dissolved iron meets hydroxide and oxygen. Remove any one of the four conditions and the cell stops — which is what every prevention method described below sets out to do.

The distinction that matters on site is where the anodes and cathodes sit. When they move constantly across the surface, the metal thins evenly: uniform corrosion, predictable and easy to measure. When they stay fixed — because of a dissimilar metal, a crevice, a break in a passive film, a stress concentration or a bacterial colony — the attack concentrates, and a small loss of metal can perforate a wall or crack a component. Most expensive plant failures are of this localized kind. Terms here follow ISO 8044, the corrosion vocabulary standard.

The types of corrosion: what they look like and where to find them

The forms rarely act alone. A deposit creates a crevice, and the crevice shelters bacteria. A galvanic couple accelerates pitting on the anode. Corrosion under insulation on austenitic stainless steel typically shows up as chloride stress corrosion cracking rather than wall loss. In heat exchangers, galvanic attack on the tube sheet and erosion at tube inlets frequently coexist, as described in galvanic corrosion in heat exchangers. Treat the dominant mechanism, or the damage returns.

Environmental factors that drive corrosion

Six variables explain why the same steel lasts in one location and fails in another.

• Humidity and wetness. Atmospheric corrosion only runs while an electrolyte film is present. What counts is how long the surface stays wet.
• Chlorides. Chloride ions break down passive films, which is why they drive pitting, crevice corrosion and stress corrosion cracking of stainless steels. In Quebec and Ontario, de-icing salt is a major source.
• Temperature. Higher temperature generally accelerates corrosion reactions. In open systems it also drives oxygen out of the water, and it governs the susceptibility band for corrosion under insulation and for chloride stress corrosion cracking.
• pH. Carbon steel corrodes rapidly in acids and is passive in strongly alkaline media such as sound concrete. Amphoteric metals like aluminum and zinc are attacked at both ends of the scale.
• Oxygen. Dissolved oxygen feeds the cathodic reaction, and differences in oxygen concentration create differential aeration cells: at waterlines, under deposits and inside crevices.
• Flow. Stagnant fluid lets deposits settle and bacteria colonize; excessive velocity strips protective films and causes erosion-corrosion. Both extremes are corrosion problems.

Corrosivity categories: ISO 9223 and ISO 12944 in brief

ISO 9223 classifies atmospheric corrosivity into six categories, from C1 (very low) to CX (extreme). The category is determined from the first-year corrosion loss of standard metal specimens, or estimated from environmental data: time of wetness, sulfur dioxide and chloride deposition. ISO 12944-2 uses the same atmospheric categories for paint systems on steel structures and adds Im1 to Im4 for immersed and buried service. The category, combined with the required durability, drives coating system selection; the details are in our article on ISO 12944 categories and coating systems.

These categories cover the atmospheric side only. Process-side corrosivity inside a vessel or pipe depends on fluid, temperature and flow, and is assessed from operating data and inspection history.

The cost of corrosion

The IMPACT study published by NACE International (now AMPP) in 2016 estimated the global cost of corrosion as a significant share of world economic output, and concluded that a substantial part of it could be avoided by applying corrosion control practices that already exist. It also found that savings come when corrosion management is built into the organization's management systems, not left to individual repairs.

In a plant, the metal itself is rarely the main cost: unplanned shutdowns, leaks, safety and environmental incidents, and repeat repairs are.

How to diagnose corrosion on site

Choosing a remedy before identifying the mechanism is the most common mistake. A structured diagnosis follows six steps.

1. Collect the history. Material grade (confirmed by positive material identification if there is any doubt), service fluid, temperature, flow, age, previous repairs and when the damage was first seen.
2. Inspect and map before cleaning. Photographs with a scale, location relative to welds, flow direction, waterline, supports and insulation penetrations. The morphology in the table above is the first diagnostic clue.
3. Sample deposits and fluids before cleaning. Scrape deposits into clean, sealed containers and analyze them for chlorides, sulfides and other species. Where MIC is suspected, samples must be fresh and sent quickly for microbiological testing; cleaning first destroys the evidence.
4. Measure. Ultrasonic thickness readings, pit depth gauges, and crack detection by penetrant, magnetic particle or ultrasonic methods where stress corrosion or hydrogen damage is possible.
5. Run a failure analysis when a part is removed. Metallographic sections distinguish intergranular from transgranular cracking and branched stress corrosion cracks from fatigue; fractography and deposit analysis confirm the mechanism.
6. Assess fitness for service. Remaining wall and flaw size are evaluated against API 579-1/ASME FFS-1 to decide whether to repair, monitor or replace.

Corrosion prevention strategies for industry

Prevention works by removing one of the four conditions of the corrosion cell. The levers are listed from cheapest to apply, early in the equipment's life, to latest.

• Material selection. Matching the alloy to the environment: PREN-ranked stainless steels for chloride pitting, L grades for welded stainless, HIC-resistant steels for sour service.
• Design. Drainage with no standing liquid, no crevices in aggressive service, no dissimilar metals in contact without isolation, flow velocities within limits, access for inspection, and a corrosion allowance where uniform loss is expected.
• Coatings and linings. The barrier that separates metal from electrolyte. Performance depends on surface preparation, film thickness and the absence of holidays as much as on the product.
• Cathodic protection. Sacrificial anodes or impressed current make the structure the cathode of the cell. It protects immersed and buried surfaces only, and works best combined with a coating.
• Inhibitors and water treatment. Chemicals that slow the anodic or cathodic reaction, plus oxygen, pH and biological control in closed and cooling systems.
• Inspection and maintenance. Thickness monitoring, coating condition surveys and timely touch-up keep small defects from becoming failures.

How these families compare, and which mechanism each one actually stops, is detailed in the five families of metal corrosion protection.

FAQ

What are the main types of corrosion? The forms met most often in plants are uniform, galvanic, pitting, crevice, intergranular, stress corrosion cracking, erosion-corrosion, cavitation, selective leaching, microbiologically influenced corrosion, corrosion under insulation and hydrogen damage.

Which type of corrosion is the most dangerous? The localized forms, because a small amount of metal loss can cause a leak or a fracture. Stress corrosion cracking and hydrogen damage are the most treacherous: they can produce cracks with little visible corrosion and fail with limited warning.

How do I identify the type of corrosion on my equipment? Start with the appearance and location of the damage, then confirm with deposit sampling, thickness and crack inspection, and, when a part can be removed, metallographic analysis. A mechanism should be confirmed, not assumed, before a costly repair is specified.

Can a coating stop every type of corrosion? No. A well-applied barrier coating controls uniform and atmospheric corrosion and protects against pitting at the coated surface. It does not correct a cavitation problem caused by insufficient NPSH, nor relieve the stress behind stress corrosion cracking. On a galvanic couple, coating only the anode concentrates attack at any defect.

Does stainless steel corrode? Yes. Stainless steel resists uniform corrosion, but it is vulnerable to pitting and crevice corrosion in chlorides, chloride stress corrosion cracking, intergranular attack after poor welding, and MIC.

In practice

Identifying the mechanism first is what makes a corrosion repair last. Induscoat assesses corroded equipment on site — damage mapping, deposit sampling, thickness readings — and applies the matching solution through its protective coatings service and its industrial corrosion protection work for plants in Quebec and Ontario: surface preparation, barrier and erosion-resistant coatings, and cold-bonded rebuilding of pitted or eroded parts. Photos of the damage and the service conditions are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.