1. Introduction

Corrosion is the progressive degradation of metals by chemical or electrochemical reaction with their environment, producing oxides, hydroxides or salts. It causes loss of material, mechanical strength and function.

A generic anodic reaction for metal M can be written as:

2. Electrochemical Mechanism of Corrosion

Most corrosion in aqueous environments proceeds by coupled anodic (oxidation) and cathodic (reduction) half-reactions. The anodic site is where metal dissolves; the cathodic site is where electron acceptors (O₂, H⁺) are reduced.

2.1 Anodic Reaction

2.2 Cathodic Reactions (common)

Ionic conduction through electrolyte and electronic conduction through the metal complete the corrosion cell. Local differences in oxygen, concentration, or potential produce anodic and cathodic zones on the same component.

3. Major Types of Corrosion

3.1 Uniform (General) Corrosion

Metal loss occurs approximately uniformly over the exposed surface. It is predictable and often mitigated by coatings. Example: mild steel rusting in moist air.

3.2 Galvanic (Bimetallic) Corrosion

Occurs when two dissimilar metals are electrically connected in an electrolyte. The more active (anodic) metal corrodes faster, the noble metal becomes cathodic. Mitigation: electrical isolation, sacrificial anode selection, or insulating coatings.

3.3 Crevice Corrosion

Localized attack inside shielded regions (crevices, gasket seams, under deposits) where stagnant solution leads to differential chemistry (e.g., lower pH, higher chloride) and accelerates corrosion.

3.4 Intergranular Corrosion

Preferential attack along grain boundaries due to segregation or precipitate formation (e.g., chromium carbide precipitation in stainless steels), which depletes protective elements near boundaries.

3.5 Pitting Corrosion

Highly localized breach of a passive film produces deep small pits. Pitting often initiates at chloride ion sites and is dangerous because it causes failure with little overall mass loss.

3.6 Stress Corrosion Cracking (SCC)

Cracking due to the combined action of tensile stress (static or residual) and a corrosive environment. SCC can cause sudden, brittle-like failure below yield stress. Common systems: austenitic stainless steels in chloride solutions, high-strength alloys in caustic or embrittling species.

4. Water-line Corrosion

Water-line (or tide-line) corrosion occurs at the interface between liquid water and air in tanks or pipelines. The oxygen concentration difference between the wetted and the air-exposed regions establishes differential-aeration cells.

Mechanism summary:

• Above water: oxygen-rich cathodic region (cathodic reaction proceeds rapidly).
• At/near waterline: oxygen-depleted/anodic zone where metal dissolution is concentrated.
• Result: localized corrosion and possible undercutting at the water line.

Prevention: tank design to minimize trapped water, coatings, sacrificial anodes, and periodic cleaning.

5. Stress Corrosion (SCC)

SCC requires (i) a susceptible material, (ii) a specific corrosive environment, and (iii) tensile stress. Crack initiation and propagation occur by anodic dissolution, hydrogen embrittlement, or film rupture mechanisms depending on the alloy and environment.

Typical controls: residual stress relief (annealing), reducing tensile stresses, material selection, use of corrosion inhibitors, and environment modification (e.g., remove chlorides).

6. Pitting Corrosion

Pits nucleate where the protective oxide or passive film breaks down locally. Once initiated, pits become anodic microcells while the surrounding metal remains cathodic. Local acidification and chloride accumulation sustain rapid pit growth.

Key features:

• Small initiation sites, often below visible detection.
• Deep, narrow morphology — high stress concentration at pit base.
• Chloride ions strongly promote initiation and propagation.
• Mitigation: alloys with higher pitting resistance, protective coatings, chloride control, cathodic protection.

7. Prevention and Monitoring

Prevention methods

• Material selection: corrosion-resistant alloys, stainless steels, non-metallics.
• Protective coatings: paints, linings, galvanizing, ceramics.
• Cathodic protection: sacrificial anodes or impressed current systems.
• Environmental control: inhibitors, deoxygenation, pH control, remove aggressive ions.
• Design: avoid crevices, allow drainage, eliminate galvanic couples.

Monitoring & inspection

• Visual inspection, thickness gauging (ultrasound), and coupon tests.
• Electrochemical techniques: linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS).
• Non-destructive testing: radiography, eddy currents, dye penetrant for surface cracks.