Types of Corrosion in Metals

## General (Uniform) Corrosion General corrosion attacks the metal surface at a relatively uniform rate over the exposed area. It is the most predictable form of corrosion because it can be quantified by measuring mass loss or penetration rate (in mm/year or mils per year, mpy), and components can be designed with a known corrosion allowance. Rusting of carbon steel in a humid atmosphere is the archetypal example. The annual corrosion rate of unprotected mild steel in a rural atmosphere is typically 0.05–0.1 mm/year; in an industrial or marine atmosphere, rates of 0.1–0.5 mm/year are common. Pressure vessel codes (e.g., ASME Section VIII) allow for corrosion allowances of 1.5–3.0 mm added to calculated wall thickness for carbon steel in wet hydrocarbon service. ## Pitting Corrosion Pitting is localized attack that produces small holes (pits) penetrating deep into the metal while leaving most of the surface apparently intact. It is treacherous precisely because it causes far greater structural damage than the visible corroded area suggests. Pitting requires a breakdown of the passive film at a local point, followed by an autocatalytic process where the pit chemistry becomes increasingly aggressive. Inside an active pit, the metal dissolves rapidly, generating metal ions and electrons. The anodic reaction inside the pit (metal dissolution) is balanced by cathodic reactions (oxygen reduction) on the surrounding passive surface. Chloride ions migrate into the pit, displacing hydroxide and lowering local pH, which prevents repassivation. Pitting resistance in stainless steels is characterized by the Pitting Resistance Equivalent Number (PREN): **PREN = %Cr + 3.3 × %Mo + 16 × %N** PREN values above 25 are considered necessary for use in seawater; grades like 316 (PREN ~26) sit near the minimum, while 6Mo superaustenitic stainless (PREN ~42) and superduplex 2507 (PREN ~43) offer substantially better pitting resistance. ## Crevice Corrosion Crevice corrosion occurs in geometrically restricted spaces where electrolyte is trapped and becomes stagnant: under boltheads, in threaded joints, under gaskets, beneath marine growth, and between overlapping plates. The mechanism is similar to pitting: oxygen depletion inside the crevice creates an anodic area, while the surrounding open surface acts as the cathode. Materials susceptible to pitting are generally also susceptible to crevice corrosion, and crevice corrosion typically initiates at lower chloride concentrations and higher pH than pitting. Design measures include eliminating crevices through full-penetration welds (rather than fillet welds with gaps), using solid gasket materials rather than spiral-wound type, and specifying a higher PREN grade than pitting resistance alone would require. ## Galvanic Corrosion When two metals of different electrochemical potential are in electrical contact in an electrolyte, the less noble (more active) metal corrodes preferentially while the nobler metal is protected. The driving force is the difference in electrode potential between the two metals in that electrolyte. The galvanic series ranks metals and alloys by their practical corrosion potential in flowing seawater. Key pairs to avoid: - Carbon steel adjacent to stainless steel in seawater: the carbon steel corrodes rapidly (large cathodic area on stainless relative to anodic area on carbon steel) - Aluminum adjacent to copper or brass: aluminum corrodes aggressively - Zinc adjacent to copper: zinc corrodes (used deliberately in sacrificial anodes) The severity of galvanic attack depends on the potential difference, the electrolyte conductivity, the relative areas of the two metals, and the distance between them. In fresh water (low conductivity), galvanic effects are limited to the immediate contact zone. In seawater (high conductivity), galvanic attack can extend centimeters to meters from the contact point. ## Intergranular Corrosion Intergranular (or intercrystalline) corrosion preferentially attacks metal along grain boundaries. In austenitic stainless steels, this occurs when carbon precipitates as chromium carbide (Cr₂₃C₆) at grain boundaries during slow cooling through 425–870 °C (sensitization), depleting the adjacent metal of chromium below the 10.5% threshold for passivity. The sensitized zones corrode while the grain interiors remain passive. Solutions: use low-carbon grades (304L, 316L with max 0.03% C), stabilized grades (321 with Ti, 347 with Nb) where carbon is tied up in stable carbides, or solution anneal after welding (practical only for small components). ## Stress Corrosion Cracking (SCC) SCC requires the simultaneous presence of three conditions: a susceptible material, a specific corrosive environment, and tensile stress (applied or residual). Remove any one of the three and SCC does not occur. Classic examples: - Austenitic stainless steels crack in hot chloride solutions (even dilute concentrations at temperatures above ~60 °C) - High-strength carbon and alloy steels crack in hydrogen sulfide environments (sulfide stress cracking, SSC) - High-strength aluminum alloys (7xxx series, T6 temper) crack in marine environments; the T73 overaged temper eliminates susceptibility at the cost of about 10-15% strength - Brasses crack in ammonia-containing environments (season cracking) ## Erosion Corrosion Erosion corrosion is accelerated attack resulting from the combined action of mechanical erosion (from flowing fluid, suspended particles, or cavitation) and corrosion. The erosion continuously removes the passive film or corrosion products, exposing fresh metal surface. Elbows, pump impellers, valve seats, and turbine blades in particle-laden or high-velocity flows are typical locations.