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Phosphoric Acid Corrosion

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Phosphoric acid corrosion refers to the deterioration of metals and alloys due to exposure to phosphoric acid. Metals can experience corrosion, including localized pitting, general corrosion, and stress corrosion cracking, when exposed to phosphoric acid. The corrosion process can compromise the structural integrity of the metal, potentially leading to equipment failure if not addressed.

Acid concentration, water content, temperature, and contaminants are crucial factors of phosphoric acid corrosion. Solid phosphoric acid catalysts are non-corrosive to carbon steel without free water, but water presence can lead to severe corrosion. Corrosion rates escalate with rising temperatures or when contaminants like chlorides, fluorides, and other halide salts are introduced. Corrosion is often associated with water washing during shutdowns, with low points susceptible to localized corrosion.

Susceptible Areas

Phosphoric acid corrosion primarily affects polymerization units employing phosphoric acid as a catalyst. Equipment made of carbon steel, stainless steel, aluminum, and certain alloys may be susceptible to phosphoric acid corrosion, including heat exchangers, piping, and storage tanks. However, corrosion typically occurs in low-velocity areas and low points where water mixes with the catalyst. Examples include piping manifolds, bypass lines, dead-legs, partial penetration welds, kettle-type reboilers, and exchangers with sufficient residence time for acid droplets to settle.

Materials commonly used in polymerization units, ranked in increasing order of resistance to phosphoric acid corrosion, include carbon steel, 304L stainless steel (SS), 316L SS, Alloy 20, and Alloy 825. Low-carbon stainless steel grades are preferred to mitigate the risk of sensitization and intergranular corrosion. 

Prevention/Mitigation

Phosphoric acid corrosion can be prevented or mitigated by implementing some or all of the following measures depending on where you are at in the asset’s life cycle.

  • Ensure Proper Design: Ensure proper equipment design to minimize areas where phosphoric acid can accumulate and corrode. Consider factors such as fluid flow and the geometry of the equipment.
  • Perform Material Compatibility Tests: Before using a specific metal or alloy, conduct compatibility tests with the intended phosphoric acid concentration and temperature to assess corrosion resistance.
  • Proper Material Selection: Choose corrosion-resistant materials for equipment that may be exposed to phosphoric acid, such as alloys with high nickel or chromium content.
  • Apply Protective Coatings: Properly apply corrosion-resistant coatings, such as epoxy or polymer coatings, to create a barrier between the metal surface and phosphoric acid.
  • Use Corrosion Inhibitors: Utilize corrosion inhibitors that form a protective film on the metal surface to help prevent direct contact with phosphoric acid.
  • Control Temperature and Acid Concentration: Monitor and maintain control over temperature and acid concentration to minimize the corrosive effects. Lowering the acid concentration may help mitigate corrosion.
  • Conduct Regular Maintenance and Inspection: Implement a regular maintenance schedule, including visual inspections and NDT, to promptly identify and address corrosion issues as they arise.

Inspection and Monitoring

Inspection for phosphoric acid corrosion involves visual examination, non-destructive testing (NDT) techniques, and monitoring of key parameters such as acid concentration and temperature. Visual inspection may reveal signs of corrosion, such as discoloration, pitting, or surface roughness. NDT methods like ultrasonic testing or radiography can be effective tools for assessing internal corrosion or degradation. Permanently mounted sensors are also effective for detecting and measuring thickness loss.

References

  1. American Petroleum Institute, 2020, “API Recommended Practice 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry,” 3rd Edition, Washington DC.

  2. Reynolds, J., 2004, “Ninety-Nine Diseases of Pressure Equipment - Part 7: Phosphoric Acid,” Inspectioneering Journal, 10(5), pp. 1-6.

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Articles about Phosphoric Acid Corrosion
September/October 2004 Inspectioneering Journal

Corrosion from phosphoric acid is another “old knowledge” corrosion issue that effects only a few processes in the chemical and hydrocarbon process industry.

Authors: John Reynolds
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