Glossary term
Corrosion Rate
The speed at which a material deteriorates by chemical or electrochemical attack under specified environmental and service conditions.
Definition
metricThe speed at which a material deteriorates by chemical or electrochemical attack under specified environmental and service conditions.
Corrosion rate converts material loss into a design and maintenance metric. It is used for wall-thickness allowance, inspection interval, coating selection, material selection, asset integrity, and remaining-life assessment.
Corrosion rate measures how quickly a material is consumed or degraded by its environment. It is commonly expressed as thickness loss per time, such as millimetres per year, or inferred from electrochemical current density. The value is meaningful only when tied to a material, environment, temperature, flow condition, chemistry, stress state, and exposure time.
Engineering role
Corrosion rate is used to select materials, set corrosion allowance, determine inspection intervals, evaluate coatings, compare inhibitors, estimate remaining wall thickness, and assess risk in pipelines, tanks, pressure vessels, marine structures, heat exchangers, reinforced concrete, and chemical plants.
Uniform and localized corrosion
Average corrosion rate is useful for uniform wall loss, but it can be misleading when localized corrosion dominates. Pitting, crevice corrosion, galvanic corrosion, erosion-corrosion, stress-corrosion cracking, and microbiologically influenced corrosion may produce severe local damage while the average metal loss appears modest. In those cases, maximum pit depth or crack growth may control integrity more than average rate.
Measurement
Corrosion rate can be estimated from weight-loss coupons, electrical resistance probes, linear polarization resistance, ultrasonic thickness readings, inspection data, or laboratory exposure tests. Each method has limitations. Coupons may not represent the most severe location. Electrochemical measurements may require conductive electrolytes and correct interpretation. Ultrasonic inspection depends on coverage, repeatability, and surface condition.
Design considerations
Corrosion rate changes with pH, oxygen, chlorides, temperature, velocity, deposits, wet-dry cycling, microbial activity, galvanic coupling, and protective films. A rate measured under one operating condition should not be extrapolated blindly to startup, shutdown, upset, stagnant, or high-temperature conditions. Corrosion allowance should be paired with inspection and mitigation, not treated as a complete solution.
Common mistakes
Common mistakes include using a single average rate for a system with localized attack, ignoring galvanic couples, assuming stainless steel is universally corrosion-proof, and extrapolating short laboratory tests to long service lives without considering passivation or breakdown. Engineers should also check whether corrosion products protect the surface or accelerate under-deposit attack.