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Sulfuric Acid Corrosion

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Sulphuric Acid Corrosion is a well-known damage mechanism that manifests as general or localized corrosion in carbon steel and other alloys in acidic environments. Sulfuric acid corrosion is most commonly seen in sulfuric acid alkylation units and wastewater treatment plants where acid concentration, fluid velocity, operating temperatures, and acid carryover are not carefully controlled.  

Susceptible Areas

Acid carryover into equipment and piping systems that were not constructed to resist sulfuric acid at certain concentrations and temperatures can lead to higher corrosion rates. Reactor effluent lines, reboilers, deisobutanizer overheads, and caustic treating systems are particularly vulnerable. Vessels handling sulfuric acid can also become severely corroded when cleaned for internal inspection.

While sulfuric acid corrosion most often manifests as general thinning, it can also be highly localized to weldments (especially in heat affected zones), crevices, and liquid vapor interfaces. The latter is most often the case in flare lines because of the ingress of oxygen. It is not uncommon to see a knife-like line of corrosion at the tops of lines carrying concentrated sulfuric acid and at elbows, especially in areas where maximum allowable velocities are exceeded.

Prevention/Mitigation

Sulfuric acid corrosion can be mitigated through proper material selection. While most process equipment can be constructed cost effectively out of carbon steel, areas where sulfuric acid is anticipated may require upgrades to more resistant alloys like Alloy 20, Alloy 904L, and Alloy C276. Moreover, the risk of sulfuric acid corrosion can be minimized if acid concentration levels, operating temperatures, flow velocities, and oxidizers/contaminants are properly managed. 

Inspection Techniques

Ultrasonic Testing (UT) and Radiographic Testing (RT) are two nondestructive testing (NDT) techniques that have proven to be effective for detecting and measuring localized corrosion due to sulfuric acid. Permanently mounted thickness monitoring sensors can also be utilized to monitor corrosion rates. Vessels containing sulfuric acid should be considered for non-intrusive inspection to avoid the heightened risk for severe corrosion when introducing cleaning water. When clean-out is unavoidable, a post-cleaning inspection should be conducted to ensure no significant corrosion has developed prior to returning to service.

References

  1. API RP 571, “Damage Mechanisms Affecting Fixed Equipment in the Refining Industry”, Third Edition, American Petroleum Institute, 2020.
  2. Reynolds, J., 2004, “99 Diseases of Pressure Equipment: Sulfuric Acid,” Inspectioneering Journal, The Woodlands, Texas.

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Articles about Sulfuric Acid Corrosion
September/October 2004 Inspectioneering Journal

Unlike NAC where we’re still on a learning curve, the knowledge of corrosion by sulfuric acid has not changed much in the last quarter century, and there are many good references for it included in API RP 571.

Authors: John Reynolds
Partner Content

FFS assessment techniques are applicable to a wide range of damage types: LTA's, cracks, creep damage, dents, and more. These are very powerful analytical tools that often allow operators to not only keep the plant running, but to keep it running...

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