What is corrosion?

Why does corrosion happen?

Refining ore into metal stores energy in it, and corrosion is the slow return of that metal toward a more stable compound such as an oxide, hydroxide or sulfide ⁸. A metal can only oxidize if something else is reduced at the same time, most often dissolved oxygen or the hydrogen ions of an acid ⁷. The terms engineers use for all of this come from the international corrosion vocabulary, ISO 8044:2024, in which "metal" also covers alloys ¹.

In steel this works like a small battery, and four parts have to be present together ³:

• An anode, where iron leaves the metal as positively charged ions and gives up electrons ³.
• A cathode, where the electrons are consumed, typically by oxygen and water or by water releasing hydrogen ³.
• A metallic path, the pipe or vessel wall itself, carrying electrons from the anode to the cathode ³.
• An electrolyte, such as wet soil, seawater or produced water, carrying ions and closing the circuit ³.

The anode and cathode may be right next to each other or several feet apart. What forms depends on the water: with oxygen, iron ends up as rust, while fluids carrying carbon dioxide or hydrogen sulfide can leave iron carbonate or iron sulfide instead ³.

Corrosion in pipelines usually shows up as pits rather than even thinning. The liquid over an anodic spot turns more acidic, so once a pit starts, the attack keeps concentrating there ³. Across US pipelines, corrosion was the cause of 18 percent of significant incidents between 1988 and 2008, onshore and offshore together ³.

What are the main forms of corrosion?

Engineers have long sorted corrosion by what it looks like on the metal; the scheme published by Fontana and Greene in 1967 has eight forms ⁶. API Recommended Practice (RP) 571, third edition (March 2020), sorts damage by mechanism instead, one numbered section each ². The table below lines the two up.

Form	What it looks like	Typical example	API RP 571 (2020) section
Uniform attack	Even thinning over the exposed surface	Steel exposed to an acid	-
Galvanic	The more active of two joined metals corrodes faster	Aluminium or magnesium against steel in an electrolyte	3.31 Galvanic Corrosion
Crevice	Intense local attack in a shielded gap with stagnant liquid	Under gaskets, lap joints, deposits, bolt heads	-
Pitting	Small, deep holes; perforation with little weight loss	Pits under biofilms in stagnant water	3.45 Microbiologically Influenced Corrosion
Intergranular	Attack along grain boundaries	Stainless steel next to welds	-
Selective leaching	One element removed from an alloy	Dezincified brass, graphitized cast iron	-
Erosion-corrosion	Grooves, waves or rounded holes that follow the flow	Sand or multiphase flow, collapsing bubbles	3.27 Erosion/Erosion-Corrosion
Stress corrosion cracking	Branched cracks under tensile stress in a specific environment	Chloride from wet insulation on hot stainless lines	3.17 Chloride SCC; 3.15 Caustic SCC; 3.3 Amine SCC

Forms and examples come from AMPP's account of the Fontana scheme and NASA's corrosion laboratory; section numbers are the ones API lists on its 2027 API 570 exam sheet, so a dash means no section is listed there, not that the full RP is silent ^{6 7 2}.

Some damage mechanisms are not forms in Fontana's sense but still have their own sections, such as 3.22 Corrosion Under Insulation and 3.61 Sulfidation ². The Library explains several of them in depth: corrosion under insulation, microbiologically influenced corrosion, pitting corrosion and galvanic corrosion ².

How much does corrosion cost?

Two studies set the scale. They used different methods and base years, so their figures should not be read as a trend ⁴.

Study	Coverage	Estimate	Cost basis year
US Federal Highway Administration, FHWA-RD-01-156 (2002)	United States, direct costs	US$276 billion a year, 3.1% of GDP	1998
NACE IMPACT (2016)	World, built from five national studies	US$2.5 trillion a year, 3.4% of global GDP	2013
NACE IMPACT (2016)	Saving possible with existing control practice	15 to 35%, US$375 to 875 billion a year	2013

The US study was normalized to 1998 costs and reported a total of about US$276 billion, 3.1 percent of gross domestic product ⁵. IMPACT combined national studies from India (2011 to 2012), the United States (1998), Japan (1997), Kuwait (1987) and the United Kingdom (1970), and its totals leave out safety and environmental consequences ⁴.

For oil and gas, PHMSA's 2008 report put the share for onshore gas and liquid transmission pipelines at about US$7 billion of the US$276 billion total, with operations and maintenance making up roughly half of that pipeline figure ³.

How is corrosion found?

For pipelines, the US federal rules for gas (49 CFR 192) and liquids (49 CFR 195) accept three ways of assessing corrosion: in-line inspection (ILI), hydrostatic pressure testing and direct assessment ³. ILI tools, also called pigs, measure geometry, magnetic flux leakage or ultrasonic wall thickness from inside the line. They can find defects too small to fail a pressure test, though some defects may be missed or sized wrongly ³.

Between inspections, operators monitor. Removable coupons placed where water enters or collects show corrosion rates, but only if they sit in the water. Fixed ultrasonic probes track general thinning well and can miss pits ³. On cathodically protected gas lines, 49 CFR 192 calls for pipe-to-soil readings at every test station each year, with no more than 15 months between readings ³.

Pitting is the hardest form to find, measure or predict ⁶. A small opening at the surface can hide a much larger cavity below it ⁷. In process plants, insulation and fireproofing hide the metal altogether, which is why corrosion under insulation has inspection methods of its own ².

Is corrosion the same as erosion?

No, although the two often act together. Corrosion is chemical or electrochemical; erosion is mechanical removal by fast fluid, entrained sand or collapsing bubbles ⁷. When flow strips away the protective scale and the bare metal then corrodes, the result is erosion-corrosion: grooves and rounded holes that follow the flow direction, often on surfaces that look clean ^{6 7}. Stress corrosion cracking is a different case again, needing both tensile stress and a specific environment ⁶.