Why does it matter whether corrosion is uniform or localized?
Whether corrosion is uniform or localized matters because the word corrosion covers two different engineering problems, and they lead to different documents and different responsible parties. Metal that thins evenly behaves like a consumable: it has a rate, the rate can be measured, and a designer can add thickness to absorb it. Metal attacked at a single point does not. Localized corrosion can perforate a wall that is otherwise nearly intact, on a timescale nobody budgeted for. Separating uniform corrosion from localized attack is usually the first substantive question in a corrosion claim.
How do design codes deal with uniform corrosion?
Design codes deal with uniform corrosion, where an environment removes metal broadly at a roughly steady rate, through a corrosion allowance — wall thickness beyond what pressure and load require — paired with an inspection interval short enough to confirm that the assumed steady corrosion rate holds. General corrosion is the case the design codes were built around.
That makes uniform metal loss a documentary problem as much as a metallurgical one. The corrosion allowance, the assumed corrosion rate and the inspection frequency were written down somewhere. Where a wall thinned predictably by uniform corrosion and still failed, the questions are whether those assumptions were reasonable, whether the service stayed inside them, and whether anyone measured.
Why doesn’t an average corrosion rate work for pitting?
An average corrosion rate in millimeters per year says nothing useful about pitting, because pitting differs in kind from uniform corrosion: its character is that penetration concentrates where the average never looks. In pitting, a passive film breaks down at a discrete site, commonly under chloride, and the pit becomes its own micro-environment: oxygen-starved, acidified by hydrolysis of dissolved metal ions, and more aggressive as it deepens. The chemistry driving pitting is generated by the attack itself, so a pit accelerates rather than tapering off.
Pit initiation is also stochastic. Identical components in the same service pit at very different times.
Why is crevice corrosion considered a geometry problem?
Crevice corrosion is a geometry problem because it develops where a gasket face, a lap joint, a thread, a tube-to-tubesheet gap or a deposit on a tank floor creates a gap tight enough to restrict oxygen replenishment while still holding electrolyte. Crevice corrosion is governed by that geometry rather than by surface condition, and crevice attack in such a gap produces occluded-cell chemistry similar to that of pitting.
So a material can perform for years on open surfaces and fail by crevice corrosion where it is bolted, sealed or fouled. Attention then shifts to joint design, gasket selection and deposit control — decisions usually made by parties other than whoever specified the alloy.
How is general corrosion wall loss measured?
General corrosion wall loss is quantified by thickness. Ultrasonic surveys on a defined grid, compared against nominal thickness and against prior surveys, give the remaining wall and an implied corrosion rate, and it is repeat measurement at the same locations that makes that rate meaningful.
Where a corrosion coupon or a recovered component exists, mass loss over a known exposure gives an independent figure for general corrosion. A mass-loss figure depends on removing corrosion product without removing sound metal, and ASTM G1 sets out the cleaning procedures for it, including the repeated-cleaning approach that separates product removal from attack on the base metal.
How is pitting corrosion measured and reported?
Pitting corrosion is described by pit density, size and depth, and ASTM G46 provides the standard guide and rating charts for doing that consistently. ASTM G46 also provides the pitting factor — deepest penetration divided by average penetration — which is the number expressing how badly an average understates the worst case.
Pit depth is measured by microscope focus, by depth gauge, or by sectioning through the pit and examining it metallographically. Metallographic sectioning of a pit also shows whether the attack followed grain boundaries or a weld-affected region. Undercut and subsurface pits present far less at the surface than they have taken from the wall.
How do you test whether an alloy was suited to its corrosion service?
Whether an alloy suited its service is tested with standardized tests that characterize the alloy rather than the part. ASTM G48 uses ferric chloride solutions to rank the pitting and crevice resistance of stainless steels and related alloys, with distinct methods for pitting and for crevice resistance.
For austenitic stainless steels, the adjacent question is sensitization — chromium carbide precipitation at grain boundaries after welding or slow cooling, leaving neighboring metal depleted and open to intergranular attack. ASTM A262 sets out the practices for detecting that susceptibility to intergranular attack in austenitic stainless steels, from oxalic acid etch screening through the acid immersion tests.
Does ASTM B117 salt spray testing predict how long a coating will last?
No. The ASTM B117 standard is explicit that salt spray results are not a prediction of service life. Salt spray testing to ASTM B117 appears in specifications constantly and is widely over-read: it is a comparative, quality-control test run in a continuously wet, constant-condition chamber, which is why an accelerated exposure number rarely settles anything.
Real exposure is cyclic — wetting and drying, temperature swings, varying chloride loading — and that cycling governs both the rate of attack and how protective the corrosion products formed turn out to be. ASTM B117 cabinet hours can show that one coating outperformed another under one artificial condition, not how long either coating lasts on a structure.
Why can average remaining wall thickness overstate the safety margin?
Average remaining wall thickness can overstate the margin because a component with generous average remaining wall can still be one pit from a through-wall leak. Fitness-for-service assessment exists because of the distinction between general and local metal loss: inspection codes such as API 570 for piping, and the assessment procedures in API 579-1/ASME FFS-1, treat general and local metal loss under separate rules.
A remaining-life calculation built on averaged wall-thickness readings therefore answers a question nobody asked. What matters is the deepest verified penetration and whether the inspection survey was dense enough to have found it.
What should a corrosion failure report separate?
A useful corrosion report separates measurement from inference: which readings were taken where, how the specimen was cleaned, how pits were sized, and what any implied corrosion rate rests on. A corrosion report should also name the mechanisms it excluded and why.
Naming excluded mechanisms matters most at the boundaries. Stress-corrosion cracking can occur at or beneath a corroded surface; it is a separate mechanism with separate evidence, handled on its own terms rather than folded into a wall-loss discussion.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.