Types of industrial corrosion: how to recognise and treat them

Talking about "corrosion" in the singular is misleading. Behind that one word sit around a dozen distinct mechanisms, which do not show up in the same place, do not progress at the same speed and do not call for the same treatment. Confusing two of them means applying the wrong defence and choosing the wrong inspection method. Identifying the mechanism governs everything that follows.

The fundamental distinction: general or localised

General corrosion removes metal everywhere at roughly the same rate. It leaves room to manoeuvre: you measure it, you track its progression, you replace before the end. It is rarely the cause of sudden failures.

Localised corrosion does the opposite. The quantity of metal lost is negligible, but it is removed in the same place, which produces a through-wall pit or a crack where the rest of the wall is intact. A thickness campaign that does not sample exactly the right point will conclude that all is well. It is this contrast that makes identifying the mechanism the priority.

The main mechanisms, one by one

For each one, four markers matter: what it looks like (morphology), what causes it, where it appears, and how you protect against it.

Uniform corrosion (general)

Morphology: even loss of thickness across the whole exposed surface, matt appearance, homogeneous oxide layer. Progression is slow and predictable.

Typical cause: distributed electrochemical attack by a moderately aggressive but permanent environment (atmosphere, water, dilute acid).

Where: tanks, structural steelwork, carbon-steel piping in ordinary service.

Prevention and treatment: this is the easiest form to manage. Corrosion allowance, coatings, cathodic protection, regular thickness monitoring. Because the degradation is close to linear, extrapolating a deadline is reliable: see calculate the corrosion rate.

Pitting corrosion

Morphology: narrow, deep cavities, often hidden by a corrosion product or a deposit that plugs the opening. Low mass loss, rapid perforation.

Typical cause: local breakdown of the passive film by chloride ions, on passivating alloys such as stainless steels.

Where: austenitic stainless steels in contact with chlorides (seawater, brines, condensates), stagnation zones, heat exchangers.

Prevention and treatment: choose an alloy with a suitable pitting resistance number (PREN), limit chlorides and stagnation, avoid deposits. Detection is the real challenge: an average thickness never reveals a pit.

Galvanic corrosion

Morphology: accentuated attack of the less noble metal in the immediate vicinity of its junction with a more noble metal, which is itself protected.

Typical cause: coupling of two metals of different potentials in the presence of an electrolyte. An unfavourable surface-area ratio (small anode, large cathode) sharply worsens the attack.

Where: mixed steel/stainless or aluminium/copper assemblies, bolting, flanges, exchanger bundles.

Prevention and treatment: electrical isolation of the couples, choice of metals close together in the galvanic series, management of the surface-area ratio, coating applied preferably to the cathode.

Crevice corrosion

Morphology: attack confined to zones where the environment stagnates: under joints, under deposits, in threads and overlaps.

Typical cause: oxygen depletion and acidification inside the gap, with chloride concentration. The mechanism is close to pitting, but initiated by geometry rather than by a passivation defect.

Where: flange faces, under washers, overlap zones, under degraded insulation.

Prevention and treatment: design without gaps, drain, choose suitable joints and alloys, avoid retention zones.

Stress corrosion cracking (SCC)

Morphology: fine, branched cracking, often undetectable to the eye, with very little loss of material. It can lead to sudden fracture with no prior sign of thinning.

Typical cause: the simultaneous occurrence of three conditions, a susceptible material, a specific environment and a tensile stress (residual or in service). Classic examples: chlorides on austenitic stainless steel, caustic soda on carbon steel, ammonia on brass.

Where: welds and their residual stresses, restrained zones, stressed bends.

Prevention and treatment: stress relief after welding, choice of a non-susceptible material, control of the environment, reduction of stresses. Detection relies on surface methods (dye penetrant) or volumetric methods (ultrasonics).

Intergranular corrosion

Morphology: attack along the grain boundaries, which loosens the grains and makes the metal lose its cohesion with no visible surface thinning.

Typical cause: sensitisation of stainless steels by precipitation of chromium carbides at the grain boundaries, hence a local chromium depletion, after poorly controlled welding or heat treatment.

Where: heat-affected zones of stainless steel welds.

Prevention and treatment: low-carbon grades (L grades), titanium- or niobium-stabilised steels, solution annealing, control of the thermal cycle.

Erosion-corrosion

Morphology: polished, undulating surface, marked with grooves or horseshoe-shaped pits oriented in the direction of flow. The protective film is removed mechanically on a continuous basis.

Typical cause: fast flow, turbulence, cavitation or abrasive particles that strip the passive film faster than it can re-form.

Where: bends, tees, reducers, downstream of valves, pump casings, exchanger tubes.

Prevention and treatment: reduce velocity and turbulence, redesign the hydraulic singularities, select harder materials or resistant coatings.

Corrosion under insulation (CUI)

Morphology: corrosion of the steel beneath the insulation, general or in patches, completely invisible while the insulation stays in place. It is often discovered late, sometimes at the moment of a leak.

Typical cause: water trapped under the insulation (rain, condensation, washing) within a favourable temperature range, of the order of -4 to 175 °C for carbon steel. On stainless steel, chlorides leached from the insulation can initiate stress corrosion cracking.

Where: insulated piping and equipment, low points, penetrations, supports, water-retention zones.

Prevention and treatment: sealing barrier and coating under the insulation, but above all a dedicated inspection strategy, because conventional thickness measurement is not enough. The question of where to place the measurement points is covered in placing measurement points.

Microbiologically influenced corrosion (MIC)

Morphology: deep pits, often clustered under tubercles or a biofilm, sometimes accompanied by a smell of hydrogen sulphide. Beneath the deposit, the cavities are sharp and localised.

Typical cause: the activity of micro-organisms, notably sulphate-reducing bacteria, which alter the environment in contact with the metal. Stagnant water and poorly drained hydrostatic tests are frequent triggers.

Where: tank bottoms, cooling-water and fire-water networks, low points, circuits left full of water after testing.

Prevention and treatment: biocide treatment, purging and draining, removal of stagnation zones, control of the test-water quality.

Recognition table

Table scrolls horizontally on small screens.

The mechanism dictates the treatment and the inspection

The whole value of this classification rests on one idea: diagnosis comes before the remedy. A corrosion allowance protects against uniform attack and does nothing against stress corrosion cracking. Regular thickness monitoring keeps general corrosion under control and misses pitting. Getting the mechanism wrong amounts to spending an inspection budget on the wrong defect.

The same logic governs the choice of inspection method. A surface method will never see an internal defect, and an average thickness measurement will never see a pit. Matching the suspected mechanism to the detection technique is the real work: that link is developed in non-destructive testing methods. Once the mechanism has been identified and tracked, estimating a deadline becomes possible; that is the subject of remaining useful life, and tracking it over time is the subject of tracking degradation.

One practical difficulty remains, often underestimated. A mechanism rarely shows up just once: the chloride pitting observed on one exchanger has a good chance of affecting the other exchangers on the same circuit. But you still need to be able to find, in the history, every place where that mechanism has already been described. That assumes structured, searchable inspection reports, where a term such as "pitting" or "stress corrosion cracking" brings back all the past cases rather than a stack of PDFs to reopen one by one. That is the aim of the documentary intelligence for inspection reports offered by Integrity Loop: to make the history searchable by mechanism, not only by equipment.

• Reducing corrosion to a loss of thicknessthe localised mechanisms (pitting, SCC, MIC) perforate or crack with a negligible mass loss.
• Applying an average rate to a localised attackaveraging a pit or an under-deposit attack produces a reassuring and false figure.
• Choosing the inspection method before identifying the mechanisma surface method stays blind to an internal defect, whatever its quality.
• Treating CUI as visible external corrosionit is hidden under the insulation and demands a dedicated strategy, not a simple thickness survey.
• Neglecting the surface-area ratio in galvanic corrosiona small anode coupled to a large cathode is attacked far faster than anticipated.