# What is corrosion?

> Corrosion is the gradual deterioration of a metal by chemical or electrochemical reaction with its surroundings; ISO 8044:2024, the international corrosion vocabulary, frames it as an interaction that changes the metal's properties and can impair the equipment it belongs to. For oil and gas plants, API RP 571 describes the specific mechanisms behind it.

- Label: Fact
- Type: Explainer
- Published: 2026-10-25T11:49:46+09:00
- By: Jun Zhong
- Researched and drafted by: Beacon's AI newsroom
- Checked by: Jun Zhong on 2026-10-25
- Topics: Pipelines, Refining
- Canonical: https://beacon.winda.ai/library/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](/library/corrosion-under-insulation/), [microbiologically influenced corrosion](/library/microbiologically-influenced-corrosion/), [pitting corrosion](/library/pitting-corrosion/) and [galvanic corrosion](/library/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](/library/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].

## Sources

1. [ISO 8044:2024, Corrosion of metals and alloys: Vocabulary (catalogue entry, published 2024-12-16), Standards Council of Canada standards database](https://ccn-scc.ca/standardsdb/standards/8187264), 2024-12-16
2. [API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, Third Edition, March 2020 (edition and section numbers as listed in the API 570 Exam Publications Effectivity Sheet for February and June 2027), American Petroleum Institute](https://www.api.org/-/media/Files/Certification/ICP/ICP-Certification-Programs/570/2027/570_Publications%20EffectivitySheet_Final_Feb_June%202027.pdf)
3. [Pipeline Corrosion: Final Report, Integrity Management Program, Delivery Order DTRS56-02-D-70036, November 2008, US Pipeline and Hazardous Materials Safety Administration](https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/docs/technical-resources/pipeline/gas-transmission-integrity-management/65341/finalreportpipelinecorrosion.pdf)
4. [International Measures of Prevention, Application and Economics of Corrosion Technology (IMPACT), 2016: Economic Impact, NACE International (now AMPP)](http://impact.nace.org/economic-impact.aspx)
5. [Proceedings of the Materials Forum 2007: Corrosion Education for the 21st Century, Session I: Motivation, National Research Council (National Academies Press)](https://www.nationalacademies.org/read/11948/chapter/2)
6. [Fontana's Eight Forms of Corrosion, AMPP](https://ampp.org/technical-research/impact/corrosion-basics/group-1/eight-forms-of-corrosion)
7. [Forms of Corrosion, NASA Kennedy Space Center Corrosion Technology Laboratory](https://public.ksc.nasa.gov/corrosion/forms-of-corrosion/)
8. [What is Corrosion? Definition and Prevention, TWI](https://www.twi-global.com/technical-knowledge/faqs/what-is-corrosion)

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