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Free sample · API 571

Try a full lesson: Metal loss in the atmosphere, soil and water

This is a complete lesson from the course (module 2 of 6, Outline topic C.2.2, C.3.2, C.4.2, C.5.2, C.6.2), followed by 10 practice questions with full explanations. Nothing to sign up for.

What this module covers

This module covers metal loss from water in its everyday forms: rain and humidity, water under insulation, wet soil, process cooling water, boiler water and condensate, and moving fluids. These mechanisms affect almost every unit, so expect many questions. For each, learn the eight attributes from module c-m1, with the most weight on critical factors and appearance.

Atmospheric corrosion and corrosion under insulation

Atmospheric corrosion (RP 571 3.8) is general or localized loss on exposed steel from moisture in the air. Carbon steel, low alloy steel and copper alloys suffer it.

  • Critical factors: how long the surface stays wet, chlorides from sea air or cooling tower drift, sulfur compounds in industrial air, and wet and dry cycles. Rates are highest in coastal, humid sites and lowest in dry inland sites.
  • Where: piping and equipment in the open, especially where water collects: supports, crevices, under clamps and at poorly drained surfaces.
  • Prevention: coatings kept in good repair and design that sheds water. Inspection: visual inspection and thickness readings where damage shows.

Corrosion under insulation (CUI, RP 571 3.22) happens when water gets into insulation and stays against the metal.

  • Materials: carbon and low alloy steel suffer wall loss; 300 series stainless can suffer chloride stress corrosion cracking under wet insulation.
  • Critical factors: a metal temperature range where water stays liquid, cycling in and out of that range, damaged jacketing, absorbent insulation and chlorides leached from the insulation or the air.
  • Where: jacket penetrations, terminations, low points, supports, and lines that run intermittently.
  • Appearance: loose flaky scale over pitted or thinned metal, worst in wet spots.
  • Prevention: a good coating under the insulation, sealed jacketing, insulation that does not hold water and removing insulation that is not needed.
  • Inspection: stripping insulation at likely spots, profile radiography, guided wave screening, and thermography or neutron backscatter to find wet insulation.

Soil, galvanic and concentration cell corrosion

Soil corrosion (RP 571 3.57) attacks buried steel and steel resting on soil, such as tank bottoms. Factors: moisture, oxygen, soil resistivity, dissolved salts, pH, temperature and stray currents. Damage is worst where the coating fails, at the soil-to-air interface and where soils change. Prevention is coating plus cathodic protection; inspection uses CP surveys, excavation, guided wave and bottom scanning.

Galvanic corrosion (RP 571 3.31) needs two different metals joined in a conductive liquid. The less noble metal (the anode) corrodes faster, and the damage is worst near the joint. A large cathode with a small anode is the worst case. Prevention: insulating the joint, choosing close metals, and coating the cathode, never only the anode.

Concentration cell corrosion (RP 571 3.19) forms on a single metal where two areas see different oxygen or ion levels. The low-oxygen area, such as inside a crevice or under a deposit, corrodes. It shows as pitting in crevices, under gaskets and deposits, and under tank bottom edges. Prevention: seal welds instead of crevices, keep surfaces clean, and keep water from standing.

Microbiologically influenced corrosion

MIC (RP 571 3.45) is corrosion caused by living organisms: bacteria, algae or fungi.

  • Materials: most common alloys, including carbon steel, stainless steel and copper alloys.
  • Critical factors: water that stands or flows slowly, nutrients, and conditions organisms can tolerate. It is common in hydrotest water left in equipment.
  • Where: cooling water systems, tank bottoms and water draws, fire water, dead legs and lines left wet after a hydrotest.
  • Appearance: cup-shaped pits under nodules or tubercles, sometimes with slime and odor.
  • Prevention: biocides, keeping flow up, draining and drying after hydrotests, cleaning and coatings. Inspection: monitoring bacteria counts and deposits, and UT where pits are suspected.

Cooling water, boiler water and condensate, and CO2 corrosion

Cooling water corrosion is general or pitting loss in water-cooled heat exchangers and piping. Factors: water temperature, dissolved oxygen, chlorides, velocity (too slow lets deposits form, too fast erodes), and water treatment. Prevention is water treatment and design for proper velocity; inspection uses UT and tube methods such as eddy current or IRIS.

Boiler water and steam condensate corrosion (RP 571 3.9) has two main forms: oxygen pitting where dissolved oxygen is not removed, and carbonic acid attack in condensate return lines, which often shows as grooving along the bottom of the pipe. Prevention: deaeration and an oxygen scavenger, plus amine treatment to raise condensate pH. Monitoring: water chemistry, including pH, oxygen and dissolved iron.

CO2 corrosion occurs when carbon dioxide dissolves in water to form carbonic acid, which attacks carbon and low alloy steel. Factors: CO2 partial pressure, temperature, pH and flow. Damage is localized thinning or pitting, often where water condenses. It is found in condensate systems and where wet CO2 streams cool. Prevention: inhibitors, raising pH or upgrading to stainless steel.

Erosion, erosion-corrosion and cavitation

Erosion and erosion-corrosion (RP 571 3.27). Erosion is mechanical removal of metal by solids, droplets or high-velocity flow. Erosion-corrosion is the combined case, where flow strips the protective scale and corrosion attacks the fresh metal.

  • Critical factors: velocity, solids content and hardness, droplet impact, and the fluid's corrosivity.
  • Where: elbows, tees, reducers, downstream of control valves and orifices, pump casings and impellers.
  • Appearance: smooth grooves, gullies, waves or horseshoe-shaped pits that line up with the flow.
  • Prevention: lower velocity, larger-radius bends, harder or more resistant alloys, impingement plates and removing solids. Inspection: UT and RT at known high-wear spots.

Cavitation happens when local pressure falls below the liquid's vapor pressure, bubbles form, and they collapse violently on the metal. It leaves sharp-edged, rough pits, typically on pump impellers and downstream of throttling valves. Prevention: enough suction head and better design. Noise and vibration are early signs.

Worked example: matching a finding to a mechanism (invented unit). A cooling water exchanger on a coastal site shows (a) pitted shell exterior under torn insulation, (b) cup-shaped pits under nodules in a cooling water line left full after a hydrotest, and (c) a horseshoe pattern on an elbow just after a control valve. Match: (a) CUI, (b) MIC, (c) erosion-corrosion. Each match rests on one clue: insulation, stagnant water with tubercles, and flow-aligned pits after a velocity change.

Common traps

  • Calling damage under wet insulation atmospheric corrosion.
  • Coating only the anode in a galvanic couple, which focuses attack at any coating flaw.
  • Treating MIC as a high-temperature mechanism.
  • Confusing cavitation (vapor bubbles) with erosion (particles or droplets).

How to study this module

Fill your eight-attribute table for each mechanism here. Then make a list of "one-clue" identifiers: insulation, two metals, crevice, tubercles, flow pattern, bubbles.

Check yourself

The three questions below cover what raises CUI risk, the coating mistake in a galvanic couple and the appearance of cavitation.

Where to look in your own books

  • API RP 571 3.8, 3.9, 3.19, 3.22, 3.27, 3.31, 3.45 and 3.57
  • API RP 571: cooling water corrosion, CO2 corrosion and cavitation, by mechanism title

Practice set

Print these questions for a class (PDF) Answer key on page 2. Free, no signup.

Pick an answer, then check it. Every wrong option is explained too.

1. Terms, definitions and acronyms

An inspection report says an austenitic stainless steel weld area is sensitized. What does that describe?

2. Damage mechanisms: description, affected materials and critical factors

Carbon dioxide dissolves in water that condenses inside a carbon steel line. What forms, and what does it do?

3. Damage mechanisms: description, affected materials and critical factors

Which group of materials generally lacks a ductile-to-brittle transition, so brittle fracture is not a usual concern at low temperature?

4. Damage mechanisms: description, affected materials and critical factors

Which combination most promotes ethanol stress corrosion cracking of carbon steel in fuel ethanol service?

5. Damage mechanisms: affected units and equipment

Where is ammonium bisulfide corrosion most often found in a hydroprocessing unit?

6. Damage mechanisms: appearance or morphology of damage

What does naphthenic acid corrosion typically look like in a high-velocity crude transfer line?

7. Damage mechanisms: appearance or morphology of damage

How do caustic stress corrosion cracks in carbon steel usually appear?

8. Damage mechanisms: prevention and mitigation

Which practice reduces the risk of brittle fracture during a cold startup of a thick carbon steel vessel?

9. Damage mechanisms: inspection and monitoring

Which method is best suited to find amine stress corrosion cracking at carbon steel welds?

10. Damage mechanisms: inspection and monitoring

Which technique helps find wet insulation along an insulated line without removing the insulation?

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See all 6 modules
  1. How RP 571 describes damage: terms, acronyms and the eight attributes
  2. Metal loss in the atmosphere, soil and water
  3. Mechanical and metallurgical damage
  4. High-temperature corrosion and hydrogen attack
  5. Environment-assisted cracking
  6. Process-unit corrosion in refining
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    Questions are original and written to API's published content outline. Code is cited by section number so you can look it up in your own copy. Think something is wrong? Report it.