Special Processes Institute

Corrosion Science & Engineering Knowledge Guide

Corrosion Science & Engineering Knowledge App

Learn why metals corrode, how electrochemical cells form, how environments and design features accelerate attack, how to select materials and protective systems, and how to investigate, control and prevent corrosion throughout the product lifecycle.

Mobile-first learning Interactive calculations Design and failure insight

Elements needed for an electrochemical corrosion cell

20+

Corrosion mechanisms and damage forms explained

Interactive engineering calculators

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The Electrochemical Corrosion Cell

All four elements are required

What corrosion means

Corrosion is the deterioration of a material through chemical or electrochemical interaction with its environment. For metals, it usually represents a return from a high-energy refined state toward more stable compounds such as oxides, hydroxides, sulphides or salts.

1. Anode

The location where metal oxidation occurs and metal atoms lose electrons. This is where metal loss takes place.

2. Cathode

The location where a reduction reaction consumes electrons. The cathode itself may remain relatively protected while driving attack elsewhere.

3. Metallic path

A conductive path allows electrons to move from anodic to cathodic sites.

4. Electrolyte

A conductive liquid film, bulk solution, damp deposit or soil allows ionic current to complete the circuit.

Prevention principle: corrosion can be reduced by removing or weakening any part of the cell, reducing reaction kinetics, stabilising a protective film, or controlling the environment.

Thermodynamics

Indicates whether a reaction is energetically possible and which states are stable.

Kinetics

Determines how fast corrosion proceeds. A possible reaction may still be extremely slow.

Transport

Movement of oxygen, ions, water and species can limit or accelerate the rate.

Surface films

Oxides and corrosion products may protect, remain porous, crack, detach or become locally unstable.

Corrosion terminology

Term	Meaning	Engineering significance
Corrosion potential, E_corr	Mixed potential where total anodic and cathodic currents balance.	Indicates operating potential, not corrosion rate by itself.
Corrosion current density, i_corr	Equivalent electrochemical current per area associated with corrosion.	Can be converted to penetration rate using Faraday’s law.
Passivity	Low corrosion rate caused by a thin protective surface film.	Central to stainless steels, aluminium, titanium and many alloys.
Polarisation	Shift of electrode potential away from its equilibrium value due to current.	Controls reaction kinetics and mixed-potential behaviour.
Overpotential	Additional potential required to drive a reaction at a given rate.	Explains why actual reactions differ from equilibrium predictions.
Depolariser	Species consumed by the cathodic reaction, commonly oxygen or hydrogen ions.	Availability often controls corrosion rate.
Critical relative humidity	Humidity above which a sufficiently conductive surface film forms.	Important for atmospheric storage and packaging.

Anodic and Cathodic Reactions

Anode: M → Mⁿ⁺ + ne⁻

Neutral aerated water: O₂ + 2H₂O + 4e⁻ → 4OH⁻

Acid solution: 2H⁺ + 2e⁻ → H₂

The overall rate is controlled by the slowest combination of charge transfer, mass transport, film growth, reaction-product removal and ohmic resistance.

Standard potentials: useful but limited

Standard electrode potentials rank equilibrium tendencies under defined reference conditions. Real engineering systems differ in concentration, pH, temperature, aeration, films, stress, geometry and flow.

Do not use a galvanic series or standard-potential table as a standalone material-selection rule. Actual potential, area ratio, environment, polarisation and film stability determine severity.

Mixed Potential and Polarisation

The corrosion potential is where anodic and cathodic partial currents are equal. Changing either reaction—through oxygen concentration, inhibitors, coatings, alloy composition or cathodic protection—moves the intersection and changes the rate.

Pourbaix diagrams

Potential–pH diagrams identify regions where a metal is thermodynamically immune, actively corroding or capable of forming stable oxide/hydroxide products.

Immunity: metal is the stable state.
Corrosion: dissolved ionic species are stable.
Passivation: a solid oxide/hydroxide is stable.

A Pourbaix diagram does not predict corrosion rate, film protectiveness, chloride breakdown, local chemistry, stress effects or non-equilibrium behaviour.

Concentration cells

Potential differences can develop on the same alloy when local chemistry differs. Oxygen concentration cells are especially common: poorly aerated crevices or deposits become anodic relative to better-aerated exposed surfaces.

Differential aeration
Metal-ion concentration differences
Temperature gradients
pH gradients
Salt concentration gradients

Factors affecting electrochemical rate

Factor	Typical effect	Important qualification
Temperature	Usually accelerates reaction and transport.	May reduce dissolved oxygen, change films, dry the surface or alter chemistry.
Oxygen	Often accelerates cathodic reduction.	Can also support protective passive films.
Chloride	Raises conductivity and destabilises passive films.	Threshold depends on alloy, pH, temperature, potential and surface condition.
pH	Changes reaction equilibria, film stability and hydrogen evolution.	Local pH may differ greatly from bulk pH.
Velocity	Improves oxygen transport and removes products.	May reduce deposits at moderate flow but cause erosion-corrosion at high turbulence.
Conductivity	Reduces ohmic resistance and can extend galvanic interaction.	Very pure water can still be aggressive after contamination or oxygen concentration develops.

Corrosion Mechanisms and Damage Forms

Morphology is evidence

Mechanism comparison

Mechanism	Typical appearance	Primary drivers	Detection challenge	Common controls
Uniform corrosion	Broad, relatively even thinning	General environment/material incompatibility	Often predictable but may accelerate under deposits	Allowance, coating, inhibitor, material change
Pitting	Discrete deep cavities, often under deposits	Passive-film breakdown, chlorides, oxidising potential	Small mass loss can cause perforation	Alloy upgrade, crevice-free design, cleanliness, coatings, potential control
Crevice corrosion	Attack beneath gaskets, lap joints, deposits and washers	Restricted transport and local acidification/chloride concentration	Hidden until disassembly or leakage	Seal/weld crevices, drainage, alloy selection, maintenance
Galvanic corrosion	Accelerated attack near dissimilar-metal connection	Potential difference, electrical contact, electrolyte, area ratio	May be local and assembly-specific	Isolate, coat cathode, choose compatible couples, control area ratio
Intergranular corrosion	Attack along grain boundaries	Precipitation, segregation or depletion at boundaries	Surface may appear acceptable	Correct grade, heat treatment and welding procedure
SCC	Branched cracks with limited general attack	Susceptible alloy + tensile stress + specific environment	Can fail suddenly below yield stress	Material/environment/stress change, compressive stress, inspection
Corrosion fatigue	Fatigue cracks accelerated by environment	Cyclic stress plus corrosive environment	No true endurance limit in many environments	Reduce stress, improve finish, coatings, environment control
Hydrogen damage	Delayed cracking, blistering or loss of ductility	Hydrogen generation/entry, susceptible microstructure and stress	Cracks may occur after processing or loading	Process controls, baking where applicable, hardness limits, alternative coatings

Material Corrosion Behaviour

Search by alloy or environment

Material-selection principles

Match the actual environment
Composition, concentration, temperature, aeration, flow, deposits, cleaning chemicals, stress and shutdown conditions all matter.

Do not select by family name alone
“Stainless”, “aluminium” or “nickel alloy” covers many grades with very different behaviour.

Control condition and processing
Heat treatment, cold work, welding, sensitisation, inclusions, surface finish and contamination can dominate performance.

Typical alloy considerations

Material family	Protective behaviour	Common vulnerabilities	Important controls

Atmospheric

Controlled by time-of-wetness, salts, pollutants, temperature cycling, sheltering, deposits and condensation.

Marine

High chloride, wet–dry cycling, splash, immersion, biofouling and galvanic interaction create severe localised attack.

Soil

Moisture, resistivity, oxygen gradients, chlorides, sulphates, microbes and stray current influence buried assets.

Fresh water

Oxygen, hardness, alkalinity, chloride, flow, deposits and treatment chemistry determine scaling or corrosion tendency.

High temperature

Oxidation, sulfidation, carburisation, nitridation, hot corrosion and molten-salt attack replace aqueous mechanisms.

Industrial chemicals

Acid, alkali, solvent, halide, oxidiser and contaminant concentration must be considered over the full temperature range.

Atmospheric corrosivity drivers

Driver	Why it matters	Examples of controls
Time of wetness	Controls how long an electrolyte film exists.	Ventilation, heating above dew point, drainage, packaging.
Chloride deposition	Raises conductivity and destabilises passive films.	Washing, marine-grade systems, sealed storage, material upgrade.
SO₂ and industrial pollutants	Acidify moisture films and alter corrosion products.	Site classification, coatings, washdown, enclosed environment.
Condensation	Creates clean but highly oxygenated moisture, often in hidden areas.	Insulation control, vapour barriers, ventilation, desiccants.
Deposits	Retain moisture and create differential aeration.	Cleanable design, housekeeping, filtration, covers.

Microbiologically influenced corrosion

MIC is corrosion influenced by microbial activity, biofilms or their metabolic products. Microorganisms do not create a new electrochemical law; they alter local chemistry, oxygen, pH, sulphide, deposits and cathodic/anodic kinetics.

Sulphate-reducing organisms
Iron-oxidising bacteria
Acid-producing organisms
Biofilm-driven differential aeration

Diagnosis requires combined microbiological, chemical, metallurgical and operational evidence. Finding organisms alone does not prove causation.

Stray-current corrosion

Direct current leaving a metallic structure and entering the electrolyte produces anodic dissolution. Sources include rail systems, welding returns, impressed-current CP systems and electrical faults.

Attack can be rapid and highly localised.
Potential surveys and current measurements are essential.
Mitigation may involve bonding, drainage, isolation or source correction.

Designing to Prevent Corrosion

Design feature	Corrosion risk	Preferred response
Lap joint or unsealed seam	Crevice, trapped chemicals and bleed-out	Continuous weld, seal, redesign as open joint or provide drainage.
Dissimilar-metal connection	Galvanic attack of the anodic member	Choose compatible materials, electrically isolate, seal electrolyte path and control area ratio.
Small anode / large cathode	High anodic current density and severe local loss	Avoid unfavourable area ratio; coat cathodic area where practical.
Horizontal ledge	Water and deposit retention	Slope, drain, shield or provide access for cleaning.
Sharp geometry	Coating thinning and stress concentration	Use radii, edge stripe-coating and adequate access.
Inaccessible cavity	Uninspectable hidden corrosion	Provide access, inspection ports, removable covers or sealed dry volume.
Insulation or lagging	Corrosion under insulation from trapped water	Weatherproofing, drainage, suitable coating, inspection strategy.
Threaded fastener	Crevice, galvanic couple, coating damage and hydrogen risk	Compatible fastener/coating, sealant, controlled torque and replacement plan.

Drain

Prevent standing liquid and orient assemblies to empty fully after cleaning, rain, rinse or process exposure.

Vent

Allow pressure equalisation, drying and access of protective treatments without creating moisture paths.

Inspect

Design inspection access around known high-risk locations and make deterioration detectable before loss of function.

Corrosion-Control Hierarchy

Eliminate or reduce the aggressive environment

Drying, dehumidification, oxygen removal, salt control, water treatment, cleaning and contaminant exclusion.

Select a compatible material and condition

Choose alloy, heat treatment, weld consumable, hardness, surface condition and product form for the complete environment.

Design out traps, crevices and galvanic couples

Control drainage, access, area ratio, geometry, electrical isolation and maintainability.

Apply protective surface systems

Metallic coatings, conversion coatings, anodising, paints, sealants, inhibitors or temporary protectives.

Use electrochemical protection where appropriate

Sacrificial-anode or impressed-current cathodic protection, or less commonly anodic protection.

Inspect, monitor and maintain

Defined acceptance criteria, condition monitoring, washdown, repair, recoating and life-management plans.

Inhibitors

Inhibitors reduce corrosion by suppressing anodic, cathodic or both reactions, forming protective films, scavenging oxygen or controlling pH.

Anodic inhibitors: promote passivation but can be dangerous if under-dosed.
Cathodic inhibitors: slow oxygen reduction or hydrogen evolution.
Mixed inhibitors: affect both branches.
Volatile inhibitors: protect enclosed spaces through vapour transport.

Compatibility, concentration, replenishment, toxicity, downstream processing and local depletion must be validated.

Environmental control

Control relative humidity and condensation.
Reduce chloride and contaminant deposition.
Maintain pH, alkalinity, hardness and inhibitor concentration.
Prevent stagnant zones and deposits.
Control flow to avoid both stagnation and erosion.
Separate wet and dry systems during shutdown.
Use preservation and packaging for storage.

Sacrificial-anode cathodic protection

A more active metal is electrically connected to the structure. The sacrificial anode supplies protective current and corrodes preferentially.

Simple and self-regulating
No external power supply
Limited driving voltage and current capacity
Anode consumption must be monitored

Impressed-current cathodic protection

An external DC source drives current from durable anodes through the electrolyte to the protected structure.

High and adjustable current capacity
Suitable for large structures
Requires monitoring, power and careful interference control
Overprotection can damage coatings or promote hydrogen-related problems

Cathodic protection concept

Key CP engineering considerations

Consideration	Why it matters
Protection criterion	Potential criteria depend on material, electrolyte, reference electrode, standard and operating condition.
Current demand	Depends on exposed area, coating breakdown, environment and polarisation.
Reference electrode	Potential is meaningful only with a known, correctly located and maintained reference.
IR drop	Voltage loss through electrolyte can distort measured structure-to-electrolyte potential.
Shielding	Disbonded coatings, deposits or geometry can prevent protective current reaching the surface.
Interference	Stray current can accelerate corrosion on neighbouring structures.
Hydrogen risk	Excessively negative potentials may promote hydrogen entry in susceptible materials.

Protective Coating Systems

System	Protection mechanism	Typical uses	Critical controls
Barrier paint system	Separates substrate from water, oxygen and ions.	Steel structures, vehicles, equipment	Surface preparation, profile, cleanliness, film thickness, cure, edge coverage and holidays
Zinc-rich primer	Barrier plus sacrificial action when electrical continuity is sufficient.	Steel structures and severe atmospheres	Zinc loading, mixing, surface prep, conductivity, overcoating window
Hot-dip galvanising	Metallic zinc barrier and sacrificial protection.	Structural steel, fasteners, outdoor hardware	Steel chemistry, drainage/venting, coating mass, handling and repair
Electroplated zinc or zinc alloy	Sacrificial metallic coating, often with passivate and sealer.	Fasteners, automotive and engineering parts	Thickness distribution, hydrogen embrittlement controls, passivation and topcoat
Cadmium	Sacrificial protection with lubricity and aerospace heritage.	Restricted aerospace/defence uses	Toxicity restrictions, approved need, hydrogen relief, chromate/post-treatment
Anodising	Thickens the protective aluminium oxide film.	Aluminium structures and components	Alloy, pre-treatment, thickness, sealing, fatigue and dimensional effects
Conversion coating	Forms a reaction film improving corrosion resistance and paint adhesion.	Aluminium, zinc and magnesium systems	Cleaning, chemistry, film weight, rinsing, drying and handling
Thermal spray metal	Thick metallic barrier, often sacrificial aluminium or zinc.	Marine, offshore, high-temperature structures	Surface profile, porosity, sealers, thickness and bond quality

Surface preparation

Coating life is strongly dependent on removal of oil, salts, rust, scale and weakly adherent material, plus the correct anchor profile.

Edges and welds

Coatings thin over sharp edges and may bridge weld defects. Radiusing and stripe coats improve coverage.

Holiday control

Pores, pinholes and damage can become intense local corrosion cells. Detection and repair criteria should match service severity.

Why coatings fail

Osmotic blistering

Water is drawn through the coating toward soluble contamination or retained salts at the interface.

Undercutting and filiform corrosion

Corrosion propagates beneath a coating from defects or cut edges, often aided by humidity and surface contamination.

Poor adhesion

Can result from contamination, inadequate profile, condensation, incorrect cure, incompatible layers or exceeded overcoat window.

Cathodic disbondment

High local alkalinity and electrochemical reactions under CP can weaken coating adhesion around defects.

Mechanical damage

Impact, abrasion, flexing, assembly and fastener installation expose substrate or crack the protective system.

Corrosion Inspection and Characterisation

Method	Best suited to	Strengths	Limitations
Visual inspection	Rust, staining, blistering, deposits, cracking and coating damage	Fast, low cost, excellent for mapping morphology	Limited below coatings, insulation and inaccessible areas
Ultrasonic thickness	Wall loss and remaining thickness	Quantitative and one-sided access	Local pits may be missed without dense scanning; surface condition matters
Radiography	Wall loss, deposits and internal condition	Can inspect through insulation in some cases	Safety, geometry, cost and interpretation
Eddy current	Surface/near-surface defects and tube inspection	Sensitive, fast and no couplant	Conductive materials only; calibration and geometry sensitive
Potential survey	CP systems, galvanic activity and stray-current effects	Direct electrochemical information	Reference placement, IR drop and transient effects
Electrochemical methods	Corrosion rate, polarisation and coating performance	Mechanistic and often rapid	Requires representative cell, area and interpretation
Metallography	Attack path, dealloying, intergranular attack and crack morphology	Direct microstructural evidence	Destructive and location-sensitive
SEM/EDS	Fracture surfaces, corrosion products and local chemistry	High-resolution morphology plus elemental data	EDS does not directly identify compounds or prove causation

Failure-investigation workflow

Preserve evidence

Do not clean, scrape, wire-brush or chemically treat the key surfaces before documentation and sampling.

Collect history

Material, heat treatment, coating, environment, cleaning, stress, temperature, maintenance and timeline.

Map morphology and location

Relate attack to geometry, liquid level, joints, flow direction, deposits, dissimilar metals and heat-affected zones.

Verify material and condition

Composition, hardness, microstructure, coating thickness, surface treatment and traceability.

Characterise products and environment

Use chemistry, deposits, ions, pH, conductivity, microbial evidence and service samples.

Separate mechanism from root cause

Pitting may be the mechanism; chloride contamination, design trap or coating damage may be the root cause.

Mass-Loss Corrosion Rate

CR (mm/y) = 87.6 × W / (ρ × A × t)

Mass loss W (mg)

Density ρ (g/cm³)

Area A (cm²)

Exposure time t (hours)

Electrochemical Penetration Rate

CR (mm/y) = 0.00327 × i_corr × EW / ρ

i_corr (µA/cm²)

Equivalent weight EW

Density (g/cm³)

Remaining-Life Estimate

Current minimum thickness (mm)

Minimum allowable thickness (mm)

Corrosion rate (mm/year)

Design factor on rate

Galvanic Area Ratio

Anode area (cm²)

Cathode area (cm²)

Total galvanic current (mA)

Coating Material Quantity

Surface area (m²)

Target dry film thickness (µm)

Volume solids (%)

Application loss (%)

Calculator warning: outputs are educational estimates. Use controlled methods, validated constants, inspection data, applicable design codes and competent engineering judgement for real assets.

Corrosion Failure Explorer

Open a symptom to review likely mechanisms

Structured diagnosis questions

Where?

Crevice, liquid line, weld, heat-affected zone, fastener, deposit, edge, drain, flow disturbance or contact point?

When?

During operation, shutdown, storage, cleaning, transport, pressure test, coating cure or after maintenance?

What changed?

Material batch, supplier, chemical, temperature, flow, coating, water source, maintenance interval or cleaning method?

Case: stainless fastener on aluminium

Observation: severe aluminium attack around a stainless fastener in a marine environment.

Mechanism: galvanic corrosion intensified by a large cathodic stainless area, small exposed aluminium area, chloride electrolyte and coating damage around the hole.

Prevention: compatible fastener system, electrical isolation, sealed joint, sacrificial finish, controlled touch-up and drainage.

Case: pitting beneath deposits

Observation: stainless pipe perforated beneath sludge despite low overall wall loss.

Mechanism: differential aeration and local chloride concentration destabilised the passive film beneath the deposit.

Prevention: flow/filtration improvement, deposit removal, water chemistry control, alloy review and targeted inspection.

Case: coating blisters after service

Observation: widespread blisters with rusting beneath an apparently intact paint film.

Mechanism: residual soluble salts and moisture drove osmotic transport and underfilm corrosion.

Prevention: verify salt removal, dew-point control, surface profile, cleanliness and coating cure.

Case: delayed fastener cracking

Observation: high-strength plated steel fasteners cracked hours or days after installation.

Mechanism: hydrogen introduced during cleaning/plating combined with high hardness and tensile stress.

Prevention: approved process route, hardness controls, prompt embrittlement-relief treatment where specified, coating alternative and verification testing.

Case: corrosion under insulation

Observation: external pipe wall loss hidden beneath insulation and weather cladding.

Mechanism: water ingress, wet–dry cycling and concentration of contaminants at elevated temperatures.

Prevention: weatherproofing, drainage, suitable coating, inspection windows and risk-based inspection.

Case: erosion-corrosion at elbow

Observation: local thinning downstream of an elbow.

Mechanism: turbulence, impingement and removal of protective films accelerated electrochemical dissolution.

Prevention: reduce velocity/turbulence, alter geometry, upgrade material, improve water chemistry and monitor thickness.

Corrosion Knowledge Test

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Standards, Methods and Verification

Always verify the current revision, contractual applicability, test exposure, specimen preparation and acceptance criteria before use. Accelerated tests rank systems under specified conditions; they do not directly equal service life.

Topic	Common standards families / methods	Purpose
Atmospheric corrosivity	ISO 9223 series	Classification of atmospheric corrosivity and related guiding data.
Salt spray	ISO 9227; ASTM B117	Controlled neutral/acetic/copper-accelerated salt-fog exposure as applicable.
Coating corrosion tests	ISO 12944 series; cyclic corrosion methods; customer specifications	Selection, qualification and testing of protective paint systems.
Corrosion coupons	ASTM G1, G31 and related practice	Cleaning, immersion testing and mass-loss evaluation.
Electrochemical testing	ASTM G5, G59, G61, G102 and related methods	Polarisation, pitting and conversion of electrochemical data to rate.
Galvanic corrosion	ASTM G71 and galvanic-series data for the actual environment	Evaluation of couples and current behaviour.
Stress-corrosion cracking	ASTM G30, G36, G39, G44 and material-specific methods	Controlled stress/environment susceptibility testing.
Hydrogen embrittlement	ASTM F519, F1624 and process/customer requirements	Assessment of coating-process or material susceptibility.
Cathodic protection	ISO 15589 series, AMPP/NACE practices and sector standards	Design, operation and monitoring of CP systems.
Coating inspection	ISO 8501/8502/8503, ISO 19840, ASTM D7091 and related methods	Surface preparation, cleanliness, profile and dry-film thickness.

Accelerated testing cautions

Salt spray may not reproduce UV, wet–dry cycling, pollutants, temperature or real galvanic geometry.
Ranking can change between test methods and service environments.
Scribes, edges, fixtures, specimen orientation and condensation strongly affect results.
Acceptance must be defined before testing.
Use field history and representative cyclic tests where possible.

Quality and process controls

Material and coating traceability
Surface-preparation verification
Bath and process control
Coating thickness and adhesion
Environmental monitoring
Inspection calibration and competency
Repair and maintenance criteria
Failure feedback into design and risk management