Sulfuric Acid Corrosion

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.

Mechanism

Diluted sulfuric acid (approximately 70% concentration) will react with iron in a simple manner according to Equation 1, producing hydrated ferrous sulfate and gaseous hydrogen.

Iron (II) sulfate (FeSO₄) readily dissolves in diluted acid, resulting in rapid corrosion of carbon steel (as shown in Equation 2). However, as the acid concentration increases to about 60-70%, the solubility of FeSO₄ reaches its minimum, thereby reducing the corrosion rate. The situation becomes more complicated when the acid concentration exceeds 70-80%. At this concentration, the solubility of iron (II) sulfate (FeSO₄) in sulfuric acid begins to increase, leading to an increase in the corrosion rate. However, the corrosion rate falls after reaching approximately 90% concentration (as depicted in Figure 1), consequently reducing the corrosion rate of carbon steel.

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Key Variables

The corrosion of carbon steel by sulfuric acid is primarily determined by a combination of three parameters:

• temperature,
• acid concentration,
• flow. Additionally, presence of oxidizing contaminants may also have detrimental effects on material’s resistance, especially when high-alloy corrosion resistant materials like Alloy B-2 or B-3 are used.

Temperature and Acid Concentration

These two parameters are typically discussed jointly when describing the behavior of metallic materials in sulfuric acid. Carbon steel remains the most common material for concentrated sulfuric acid storage and transportation piping, usually analyzed separately from other CRAs. This distinction stems from the in-depth studies on carbon steel behavior conducted over six decades ago by renowned researchers including Sheldon Dean, Fontana, and others.¹

The behavior of carbon steel in sulfuric acid has been described in detailed mathematical form by Sheldon Dean (Equation 3), which reasonably mimics the corrosion of carbon steel under actual field conditions.^{5 ,7} API RP 581 tables in Annex 2B for determining sulfuric acid base corrosion are mostly based on the Sheldon Dean equation and can be used for relatively accurate prediction of carbon steel corrosion rates in concentrated sulfuric acid (60-100%).⁶ (Figure 3)

Where: CS CR = carbon steel corrosion rate (mm/y); T = temperature (K); d = pipe ID (m), v = velocity (m/s); g = density (kg/m³); μ = viscosity (cP); W = solubility of FeSO₄ in H₂SO₄ (wt%), Wbulk = concentration of FeSO₄ in the bulk H₂SO₄ (wt%).

The situation becomes more complex when dealing with corrosion-resistant alloys. Typically, CRAs exhibit a notable degree of immunity to sulfuric acid as a general principle. However, similar to carbon steel, CRAs may experience some fluctuations in corrosion resistance under varying concentrations of H₂SO₄ and operating temperatures.

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The most typical and well-described example of a CRA with swinging passivity is Alloy 20 (UNS N08020). It has been reported that deaerated sulfuric acid at concentrations of about 60-70%, at elevated temperatures, may cause accelerated corrosion of Alloy 20.^{7 ,8} This phenomenon is usually attributed to the loss of the passive layer and the effect of the swinging properties (reducing/oxidizing) of H₂SO₄ at concentration of 60-80% and elevated temperature. The deficiency of oxygen prevents the reinstatement of the passive layer, eventually leading to high corrosion rates. It is important to highlight that temporary diminishment of corrosion resistance is not exclusive to Alloy 20. Austenitic stainless steel (316L) or alloys like Hastelloy G or Hastelloy C-4 , may experience similar phenomena.⁸

Excluding these relatively incidental cases with sudden loss of passivity, most CRAs tested in H₂SO₄ environments (both diluted and concentrated) demonstrate solid performance, as described by their respective iso-corrosion diagrams (Figure 4). The iso-corrosion diagrams should be treated with some caution because the data used typically comes from static immersion tests. This caution is especially important in the case of new or less popular alloys. Popular CRAs like 316L, 304L, Alloy 20, and alloys from the Hastelloy group have been thoroughly tested in both static and dynamic conditions over the last few decades, resulting in reasonably accurate corrosion resistance data.

MMore detailed information about the performance of CRAs in sulfuric acid, as well as simple corrosion rate calculators, is presented in the Materials & Tools sections.

Flow

Carbon steel exposed to concentrated acid is highly sensitive to flow. Figure 5 illustrates the behavior of carbon steel in the presence of 98% sulfuric acid, which is the most common acid used in bulk storage and distribution systems within process plants.⁶ Given carbon steel’s inability to handle an acid flow rate exceeding 1m/s, its usage is primarily restricted to storage tanks or, in certain cases, to pipelines with an inner diameter greater than 2 inches, where the flow rate of concentrated sulfuric acid remains below 1 m/s.¹¹ The velocity boundary will be significantly reduced for elevated temperatures, for example, when the temperature increases above approximately 25°C (77°F). Moreover, it is recommended to use more precise flow modeling with Wall Shear Stress (WSS) as the controlling factor for flow. It is a known phenomenon that at the same velocities, the actual shear forces near the wall may vary significantly (e.g., plain wall vs. weld protrusion), leading to “unexpected” failures of carbon steel pipelines.¹²

The first-choice Corrosion Resistant Alloys, such as 316L and Alloy 20, offer far better resistance to fluid turbulences than carbon steel. Figure 6 illustrates the impact of sulfuric acid (95% and 98%) flow velocities on 316/316L corrosion within a temperature range of approximately 30-71°C.^{6 ,8} It is evident that operating of concentrated sulfuric acid (95-98%) with 316/316L material, the relatively safe flow velocities are below 1.8 m/s at 30°C.

For more detailed information about the behavior of carbon steel and CRAs in concentrated sulfuric acid at different temperatures and flow velocities, refer to the Tools section.

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Contaminants and other factors impacting material’s behavior in H₂SO₄

The role of acid contaminants, particularly in concentrated acid, as well as other elements such as post-welding treatment, should not be overlooked when assessing the risk of corrosion in sulfuric acid handling systems. Generally, the presence of dissolved oxygen promotes faster passivation of CRA surfaces and consequently improves corrosion resistance.⁸ Additionally, the presence of iron ions also positively affects the corrosion resistance of CRAs.^{8 ,14} Chloride ions, due to their destructive role in passive layer formation, typically cause an increase in the corrosion rate of CRAs even those considered highly immune to sulfuric acid attack, such as Alloy B-3 (see Figure 7).¹³

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Another interesting fact is the role of surface condition in determining the corrosion resistance of 316 and Alloy 20 materials. The effect of surface treatment (dry/wet grinding) has been shown to have a positive effect on the behavior of 316 stainless steel.⁸ It is believed that the welded surface of either 316 or Alloy 20, if not post-weld treated (grinding/pickling/passivation), may not have a chance to be properly re-passivated by interaction with oxygen from the air, eventually leading to accelerated corrosion. Inspection of welded CRA surfaces should focus on unexpected surface irregularities such as arc strikes and slag, which, if left untreated, may serve as initiation points for corrosion.

Materials

Carbon steel remains the primary construction material for systems handling concentrated sulfuric acid (97-98%) in many industries. The behavior of carbon steel in concentrated acid is well-known and well-defined, as discussed in earlier chapters. It is relatively easy and accurate to predict its behavior under stagnant and flowing acid conditions. Normative documents such as AMPP (formerly NACE) SP0391 and SP0294 provide reasonable guidance for handling concentrated sulfuric acid in carbon steel systems.^{11 ,15} Guidelines incorporated in these standards represent a conservative approach in setting boundaries for acid velocities, concentrations, material allowances, and operating temperatures. The only critical comment to be added for these standards is the need for a more careful approach to velocity limits, emphasizing the role of Wall Shear Stress (WSS).¹²

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Tools

Below is a user-friendly calculator to estimate the sulfuric acid corrosion of carbon steel and popular CRAs.

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References

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