Sulfuric Acid Corrosion

Sulfuric acid is used as a supportive chemical in a variety of processes, mostly acting as a neutralizing agent, for example, in Condensate Polishing units, as a catalyst in alkylation units, and as a catalyst in esterification reactions. Despite relatively well-understood knowledge about the behavior of metallic materials in the presence of sulfuric acid (H2SO4), some unexpected failures still occur. This chapter provides general information on H2SO4 corrosion, the correlation between process parameters and corrosion, and some guidelines to mitigate the risk of failures in in H2SO4 systems.

General Information

Sulfuric acid has been well-known for centuries, with the earliest documented information dating back to the Sumerian period (2000-3000 years BC). Its significance in modern times, particularly during the 19th and 20th centuries, led to numerous extensive studies on its chemical properties, such as reactivity in processes like electrophilic aromatic substitution, as well as its corrosion reactions with common construction materials such as carbon steel, stainless steels, and high chromium and molybdenum alloys.

Corrosion of carbon steel and major austenitic stainless steels like 304L and 316L has been extensively studied over the last 60-70 years. These studies have provided a relatively good understanding of the correlation between temperature, velocity, acid concentration, and corrosion rate for the respective alloys.1 ,2 ,3 ,4 The development of new Corrosion Resistant Alloys (CRAs) such as Alloy B-2, B-3, Alloy-20, and Alloy 31 has virtually eliminated or at least minimized problems associated with sulfuric acid corrosion in refining and other industries.

The main challenge in operating with sulfuric acid in many applications arises during dilution, injection, or mixing, where the concentration of the acid may fluctuate from nearly 98% to nil. Only a few expensive alloys can handle situations where sulfuric acid changes its properties from a reducing to an oxidizing system and vice versa. The presence of contaminants like ferrous ions and halides may exacerbate this effect and accelerate the degradation of steels and alloys that are virtually immune to corrosion.

In the last decade, attention has been focused on the use of polymeric materials such as cross-linked low-density polyethylene (X-LDPE) and high-density polyethylene (HDPE) for sulfuric acid storage tanks. However, at high concentrations of sulfuric acid and exposure to sunlight, the resistance of plastic materials may change, leading to unpredictable degradation of pipes or vessels.

Given the wide range of applications for sulfuric acid, it is challenging to compile an exhaustive list of problematic areas. Typical locations for H2SO4 corrosion are listed in Table 1.

Table 1 Examples of typical locations of H2SO4 corrosion.

Process UnitH2SO4 corrosion impacted areas
Condensate Polishing Unit (CPU)
Wastewater Treatment Plant (WTP)
Concentrated acid injection skid, as a whole system with particular focus on:
• Acid injection quill
• Pipe segment from injection quill to first check valve
• Small bore piping in injection skid (high velocities)
Utility & Storage
• Concentrated acid storage tank
• Intermediate storage tanks
• Outlet from acid pumps
• Elbows and tees in concentrated acid transportation pipelines
H2SO4 Alkylation Unit
• Contactor (mixing section)
• Spent acid lines from acid settlers and spent acid tank

Sulfuric Acid Corrosion is governed by a combination of several factors like chemical species and concentration, temperature, materials and/or flow regimes.

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References

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