Anodic protection of 316L stainless steel piping in sulfuric acid service: failure causes and remedial actions

1 Introduction

The anodic protection (AP) system is based on the formation of the passive film on the metals (as an anode) that can decrease the corrosion rate by an applied direct current (DC) anodic current. Several metals and alloys exhibit passive-active behavior in some chemical environments such as stainless steel in phosphoric acid and sulfuric acid, C-steel in sulfuric acid, nickel in nitric acid, nickel alloys in nitrate solutions, and titanium in ammonia solution. When the surface potential of these metals and alloys are in the passive potential range, they are under AP. The principle of AP had been mentioned in US patents in 1945 (Lawrence and Engle 1945a,b). Edeleanu (1954, 1960) as the first researcher presented the feasibility study of the AP in 1954 and tested it on a small-scale stainless steel boiler used for sulfuric acid solutions. In another article, he explained that with adequate laboratory work beforehand and proper instrumentation, the use of AP can make an effective contribution to the life of a chemical plant. Shock et al. (1960) found that with the use of the 18-8 stainless sheets of steel, the AP process could be applied to HNO₃, H₃PO₄, NaOH–Al₃(SO₄)₂, and NH₄NO₃ solutions. Acello and Greene (1962) specified that AP not only decreases general corrosion but also prevents stress-corrosion cracking. The study performed by Hil (1963) describes the use of AP of steels against corrosion in various solutions has been well authenticated by laboratory experiments. This article summarizes the basic principles of the technique and describes the substantial successes obtained in these field trials reported up to 1963. In the article of Foroulis (1965) some theoretical considerations pertinent to the design of large-scale AP cells are discussed, and an equation is developed from electrochemical principles which can be used for design optimization purposes. Neufeld and Williamson (1965) explained that a simple method of determining whether this technique is applicable is described, with a practical application in the case of a storage tank for 78% of sulfuric acid. Walker and Ward (1969) studied the principles, the practical methods, advantages, and disadvantages of reducing the corrosion of immersed metallic surfaces by AP. It has been shown how, from considerations of Pourbaix diagrams in conjunction with experimental potential/current-density diagrams, it may be possible to evaluate the practical conditions for the protection of vessels containing acids and other corrosive liquids. The overview of industrial applications of AP was published by Gräfen et al. (1971). The important parameters in AP are the current density and the potentials required to produce and to maintain passivity on the surface; these are discussed. A combination of galvanic and impressive current protection for an AP system is described by Jones (1976). Paulekat et al. (1982) studied the anodic corrosion protection of sulfuric acid plants concerning the recovery of heat has been considered. According to F. Paulekat et al., AP by the APR system provides safe and low-cost protection for air coolers and bare tube coolers in sulfuric acid plants. Locke (1984) studied the status of AP: 25 years old. Kuzub and Novitskiy (1984) presented a review of the different applications of AP and corrosion control of industrial equipment from the 1980s.

In recent decades, several review articles have been written about AP. Riggs and Locke (1981) published a book about AP. In this book, almost all the theoretical and practical foundations of AP are mentioned. Kuzub et al. (1986, 1991 investigated AP of equipment in the production of ammonium carbonates and studied on the AP of chemical-production equipment. Sathianandham et al. (1991) explained the design of the AP system for stainless steel heat exchangers in the sulfuric acid industry. Munro (1992) studied AP of white and green-liquor tankage with and without the use of protective organic linings. Wensley (2000) investigated corrosion protection measures include AP, application of stainless steel weld overlay, and thermal spray coating. Construction of new digesters from duplex stainless steels offers the best opportunity for long corrosion-free and maintenance-free service.

Shim et al. (2000) discussed the use of AP in combating iron contamination, general tank wastage, and hydrogen grooving in carbon steel sulfuric acid storage tanks. It also addresses the use of AP for stainless steel acid piping and coolers, including experiences using AP in acid storage theory of AP, design philosophy, engineering materials used, and remote monitoring equipment. Munro and Shim (2001) in their articles discussed AP, its operation, and applications, especially in sulfuric acid caustic liquor environments. Nakahara et al. (2002) investigated the AP method for stainless steel in contact with noble metals in the concentrated sulfuric acid environment. Xv et al. (2002) studied AP in stainless steel spiral plate cooler for concentrated sulfuric acid. Wensley (2003) in his study reveals explained that AP has been effective in preventing stress corrosion cracking, although it may not stop the propagation of existing stress corrosion cracks. Significant reductions in corrosion rate can be achieved by shifting the corrosion potential from active values to passive values. During the 1980s, numerous AP systems were installed to protect carbon steel continuous digester vessels against both stress corrosion cracking (SCC) and corrosion. Rodda et al. (2005a,b) have investigated about failure analysis of cathode electrodes in the AP system in sulfuric acid. Zhong et al. (2006) application of polyaniline to galvanic AP on stainless steel in H₂SO₄ solutions studied. Mohammadian and Saebnoori (2019) corrosion monitoring of sulfuric acid tank under AP by electrochemical noise have been examined. After about 70 years of field applications, AP has been established as a viable, effective, economical method of corrosion control.

Most designers of the sulfuric acid plant have chosen 316L stainless steel pipelines and heat exchangers based on their corrosion and erosion properties. The corrosion behavior of alloys suitable for equipment used in the manufacture of sulfuric acid was described in the following references (Grubb 2009; International Nickel Company 1983; Li et al. 2004; Louie 2008; NACE Standard 2001; Pardo et al. 2008; SPX Company 2008). In the sulfuric acid manufacturing plant, AP can be used for the concentration greater than 60% at room temperature up to 110 °C. Under AP, the surface of stainless steel 316L due to the formation of the oxide film of Mo and Cr-enriched is passivated (International Nickel Company 1983; Kirchheim et al. 1989; Kish et al. 2003; Loto et al. 2012; Yu et al. 2018). A control signal (monitoring system) provides an accurate amount of required current to maintain the pipelines in the passive region. The cathodes are used to complete the external DC circuit. Any defect in the AP system caused leakage of the acid that results in equipment failure, personnel hazards, and environmental problems. Most researchers have considered the potential criteria for AP and have not noted the importance of passive region current. However, a few articles have been published on the principles, application, and design of the AP, but little research about the failure causes of 316L stainless steel pipelines and electrodes, under AP in sulfuric acid has been taken (Foroulis 1980; Locke 2003; Novák 2010; Riggs and Locke 1981; Rodda et al. 2005a,b; Shock et al. 1960). Specifically, Rodda et al. (2005a,b) have investigated failure analysis of cathode electrodes in the AP system in sulfuric acid. They showed that accelerated corrosion of the cathodes commonly occurs under cathodic polarization. It occurs over a limited range of current density, which is larger than those typically required to establish and maintain protection of the anodic structure during normal operation. They have not explained the cause of rising current densities. In this study, the AP design of pipelines in a sulfuric acid manufacturing plant and failure analysis of 316L stainless steel piping and electrodes has been studied.

2 System description

This study was conducted in an acid sulfuric plant located in a copper production mine. The acid sulfuric plant with a capacity of 100 tons has been installed in the copper production mine to reduce environmental pollution caused by SO₂ and SO₃. SO₂ and SO₃ gases are unavoidable in the copper production process. The acid sulfuric plant consists of towers, pipelines, coolers, and storage tanks. The pipeline and coolers’ metal is 316L stainless steel. The concentration, temperature, and velocity of acid in the pipelines are 85–98%, 25–110 °C, and 1–10 m/s, respectively. To prevent corrosion and safety considerations, the AP system for pipelines and coolers is considered. The AP system consists of 4 DC power supplies, 118 cathodes (Hastelloy B-2), and 43 references (Pt/Ti) electrodes. Depending on the concentration and temperature of the acid, the potential for protection of the pipes is set at two +100 and +200 mv with respect to the reference electrode (Figure 1) (Movassaghi 2008). Due to the importance of the reference electrodes in the AP, a complete check is performed weekly, quarterly, and annually during the operation. There are several checks the reference electrode; (a) The potential difference between pure Pt and perfect electrode and a saturated calomel electrode (SCE) is +500 and +400 mV respectively in 88% of sulfuric acid at 40 °C, and (b) After installing the reference electrode in the pipeline, the potential difference between the reference electrode and the pipeline should be in the range of −300 in the sulfuric acid of 88% at 40 °C. The reference electrodes should also be pulled out of the pipelines, disassembled, and tip erosion inspection annually. Despite, record potential on the monitor screen at a close set point (SP), after seven years of operation of the AP system in the sulfuric acid plant, a section of the protected pipelines (Figure 1) has severely corroded (Figure 2). After evaluating the corrosion of pipelines, the AP system components were inspected. The study of the system showed that the reference electrodes (RE 38 and RE 39 in Figure 1), which were titanium-electroplated with platinum, lost their titanium-based metal in the acid with the loss of Pt coating (Figure 3). Also, the surface of the cathodes has been shown that fouling, corrosion, and erosion have occurred during operation under AP conditions (Figure 4).

Figure 1:

Schematic connections in the anodic protection system of the corroded pipeline (Movassaghi 2008).

Figure 2:

(a) Images of failed acid transfer pipeline due to inadequate performance of the AP system; corrosion products and perforation; (b) thickness reduction.

Figure 3:

(a) Images of removal of platinum coating from reference electrode; (b) remained platinum coating after titanium-based metal solution in acid.

Figure 4:

(a) Macro images of corrosion product deposits on the cathode electrode surface; (b) EDAX analysis of corrosion product deposits on the cathode electrode surface; (c, d) macro and SEM images of the cavities and the groove on the cathode surface after removing of corrosion product.

3 Evaluation of AP protection potential

At first, it was suggested that the AP design data such as AP protection potential (+100 mV with respect to Pt/Ti) may have been wrong. So according to the acid conditions, the electrochemical studies were carried out in 400 mL Teflon cell, having three electrodes system assembly. The open circuit potential (OCP) variations (versus SCE) of the pure Pt, perfect, and damaged Pt/Ti reference electrode were measured versus time (Figure 5). The results show that Pt/Ti damaged electrode is not suitable as a reference electrode because it does not have potential stability with time. Despite the destruction of the reference electrode, the monitoring system could not detect the destruction of the electrode. One of the design data of AP is to determine the protection potential. The cyclic polarization test is useful in determining the most effective set-point potential E_sp for maintaining passivity. The preferred potential would be the value that occurs at minimum reverse-scan current within the prescribed passive zone (Figure 6). Riggs and Locke (1981) describe how to obtain this potential using the cyclic polarization curve. In all electrochemical tests, Pt/Ti and Hastelloy B-2 were used separately as a reference, and axillary (cathode) electrode, respectively. The working electrode was 316L stainless steel with a surface of about 3.5 cm². The cyclic polarization and potentiostatic tests are shown in Figure 7. One of the most important design data is the SP potential of +100 mV. The potentiostatic test, current variations versus time, at a potential of +100 mV was performed (Figure 7b). Polarization studies and potentiostatic tests were carried out using an EG and G 263-A potentiostat in 88% of sulfuric acid at 40 °C and started about 10 min after the working electrode was immersed in the solution to allow for a stable potential. The chemical composition of acid is given in Table 1.

Figure 5:

(a) OCP variations of pure platinum; (b) undamaged Pt/Ti reference electrode; (c) damaged Pt/Ti reference electrode versus SCE in 88% of sulfuric acid at 40 °C.

Figure 6:

Schematic of cyclic polarization curve for determination of anodic protection potential (E_sp) (Riggs and Locke 1981).

Figure 7:

(a) Cyclic polarization curve of 316L; (b) potentiostatic curve of 316L at +100 mV potential with respect to the perfect Pt/Ti reference electrode in 88% of sulfuric acid at 40 °C in 88% of sulfuric acid.

In fact, the potentiostatic test shows the actual conditions of the AP of the pipeline. The potentiostatic curve (Figure 7b) indicates that by imposing +100 mV potential on the 316L stainless steel, the current decreased from about 200 to 50 µA after 20 min. The decreasing current indicates that the surface of 316L stainless steel has been protected. After the electrochemical test, the sample surface was examined by optical microscopy. No evidence of corrosion was found. The results show that if the perfect Pt/Ti, as a reference electrode, and the Hastelloy B-2, as cathode electrode is used, the surface of 316L, was completely protected at +100 mV. Similar to the above test conditions, the potentiostatic test was carried out at the potential +100 mV with a damaged Pt/Ti electrode at +100 mV (Figure 8). The test results show that the current is not stable and fluctuates continuously. In other words, the current does not decrease. When the current value is high, severe corrosion occurs on the surface. According to the potentiostatic test, the damaged Pt/Ti electrode is not suitable for AP of the pipelines. After 20 min the sample surface was evaluated. Figure 9 shows the specimen surface at different magnifications. Surface analysis shows that the corrosion products contain nickel and sulfides. After washing the surface with desalinated water, the microstructure of 316L stainless steel was evaluated. The anions like SO₄²⁻ penetrated the passive layer and created cavities that cause the shallow pitting and grain boundary corrosion visible on the surface in Figure 10. Due to the loss of platinum coating from the surface, during testing, titanium has been exposed to acid and increased the corrosion potential, which could be shifted into transpassive where corrosion is intense. On the other hand, titanium cannot act as a reference electrode due to instability in sulfuric acid. Therefore, a wrong signal from the reference electrode is sent to the monitoring unit. The monitoring system does not have a current control set-up, as a result, the excessive current through the pipeline and causes severe corrosion. By performing electrochemical tests and reviewing their results, it is better to failure causes and remedial actions of pipelines and electrodes.

Figure 8:

Potentiostatic curve of 316L at +100 mV potential with respect to the damaged Pt/Ti reference electrode in 88% of sulfuric acid at 40 °C.

Figure 9:

(a, b) Macro and SEM images of corrosion product on the 316L surface after potentiostatic test with respect to damaged Pt/Ti reference electrode; (c) EDAX analysis of corrosion products on the 316L surface.

Figure 10:

SEM images at different magnifications of grain boundary corrosion and shallow pitting of 316L stainless steel surface after potentiostatic test with respect to damaged Pt/Ti reference electrode.

4 Failure analysis of pipeline

Austenitic stainless steels such as 316L and 304L are resistant to corrosion in strong sulfuric acid. Unprotected type 316 is somewhat less resistant than type 304 in hot acid above about 93% because of the ease of oxidation of the Mo component (Schillmoller 1990). Despite the higher corrosion resistance of 304L than 316L in hot acid above about 93%, 316L performs better in a wider range of concentration and temperature (International Nickel Company 1983; SPX Company 2008). The high corrosion resistance of 316L is related to the passive film formed on the surface. Although the film is very thin, it will rebuild very rapidly if it is damaged mechanically or chemically attacked (Sanni et al. 2018). It should be noted that 316L stainless steel is immune to attack at concentrations of 80–98% and room temperature in the sulfuric acid. As the temperature increases, however, the surface potential changes from passive to active or active to passive region. This behavior change becomes observable from about 32 °C to higher. As a result, 316L stainless steel will remain active and will be corroded. In this condition, AP will be useful for restricting the fluctuation and passive film stability. By imposing a positive current that maintains the potential in the passive region, the pipeline is protected by AP and its life can be increased. But it should be noted that any defect in the AP system can lead to corrosion and failure of the pipes (Shem et al. 2000). At AP conditions, stainless steel 316L contains about 2% of Mo, which can increase the resistance to pitting corrosion and expands the passive region in sulfuric acid in contrast to 304 stainless steel because it can improve to modify passive film composition and active dissolution by the formation of insoluble oxides (International Nickel Company 1983).

In this study, one of the pipelines (Figure 1) in the sulfuric acid plant that suffered from severe corrosion was evaluated (Figure 2). In this pipeline, the set-point potential of the AP of the pipeline is +100 mV with respect to the Pt/Ti reference electrode. The concentration, temperature, and acid velocity in the pipelines are about 88%, 40 °C, and 1 m/s, respectively. Due to AP, pipelines should not be corroded, but the thinning and pitting corrosion of protected pipelines have occurred. Even in conditions without AP, referring to the iso-corrosion curves (International Nickel Company 1983), corrosion rates below 3 mpy are meaningful for forming a passive film of alloys. After the seventh years’ inspection of the pipelines and electrodes, the pipeline is suddenly corroded due to the destruction of the reference electrode in a specific area of the tubes. Reducing the thickness of the pipe shows that after a few months of the seventh year inspection, the corrosion rate of pipelines is more than 20 mpy. According to electrochemical studies, the reason for the increase in corrosion rate is the increase in the protection current in the transverse region. This increased current led to the formation of deposits that have caused changes in the thickness of the internal pipes. To determine the composition of these corrosion deposits, the X-ray fluorescence (XRF) analysis was carried out (Table 2). In the deposits, the elements of iron, chrome, and nickel are visible. The presence of Fe₂O₃ over other oxides (Cr₂O₃, NiO, and MoO₃) showed that the surface reaction occurred in the transpassive region. It is noticed that transpassive film on the stainless steels in the transpassive region is assumed to be covered by a mixed Cr(vi) and Fe(iii) oxide film and some Mo-containing species. In other words, the surface of the stainless steel is not completely passivated and protected. At higher potentials, passivity is observed to break down, and the dissolution rate of the substrate increases dramatically as the system transitions into the transpassive state. This process is postulated to be due to the dissolution of Cr₂O₃. Based on the chemical composition of corrosion products, it is assumed that the following reactions occurred on the steel surface:

The point defect model assumes the segregation of Cr in the film as high valency cations (Fattah-Alhosseini and Attarzadeh 2011). These cations are assumed to reduce the concentration and diffusivity of cation vacancies in the film using forming complexes with them. Researchers assume that there is an accumulation of oxygen vacancies at the metal/film interface and a corresponding accumulation of cation vacancies at the film/solution interface (Krishnamurthy et al. 2002; Lin et al. 1981; Sikora et al. 1996). XRF analysis is a widely used technique for corrosion product analysis. In the XRF analysis (Table 2), SO₃ is reported instead of total S (sulfates + sulfides of metals such as Fe, Ni, and Cr). For example, the reaction of Fe base alloy with H₂SO₄ can produce FeS, FeSO₄, and or Fe₂(SO₄)₃.

Metal sulfides and sulfates compounds are decomposed to metal oxides and SO₃ during XRF analysis (Jenkins 1999).

5 Failure analysis of platinized titanium

The reference electrode used for AP must be mechanically rigid, insoluble, and the potential stability concerning time and acid composition changes. In this regard, platinized titanium, with a few microns of Pt (about 5 µm) on the commercially pure titanium Gr-2 has been widely used as a reference electrode in the AP of stainless steel pipes in sulfuric acid. The reference electrode is used to measure the potential of the piping internal surface. Each AP system operates as a single loop controller on structure potential. The installation position (Figure 5b) of the reference electrode is suitable. But the possibility of foreign object (particles) damage (FOD) is not unexpected. When the platinum coating is removed from the surface of the reference electrode (due to FOD), the titanium base dissolves in the acid and the coating shell remains. In short, the platinum coating is removed from the surface by a/the mechanical factor (FOD), but the titanium substrate is chemically dissolved in concentrated sulfuric acid. When the platinum coating is consumed or separated from the titanium substrate (Figure 3), the potential stability of the reference electrode is not achieved. Despite the degradation of the platinum coating, the control system still adjusts the pipeline protection voltage to +100 mV relative to the damaged electrode (or titanium). While the protection potential of +100 mV is set relative to platinum, not titanium. Though Gr-2 titanium as a substrate is highly resistant to corrosion in many aggressive environments, it has a significant corrosion rate with sulfuric acid. Corrosion of titanium in sulfuric acid causes problems in recording potential. Setting the pipeline potential at +100 mV relative to titanium puts the pipeline in a trans-passive state. In these conditions, both the pipeline and the titanium-based will corrode. Due to corrosion of titanium and instability in the acid environment, it cannot be used as a reference electrode. The anodic behavior of titanium in acid media has been studied by several investigators (Baillon et al. 2008; Krỳsa et al. 1997; Sinigaglia et al. 1973; Vaughan and Alfantazi 2006).

Ma and Peres (1951) have studied the Ti–H₂SO₄ system and have observed an increase in the rate of corrosion to a maximum at 40% and a fall to a minimum at 65% which is then followed by a rapid increase. The increase in the rate of corrosion at concentrations greater than 60% is explained by the formation of another complex [TiO₂ (SO₄)_x]^−2x. In this study, accelerated corrosion also occurs when pieces of the platinum coating disappear and the titanium substrate is coupled to platinum coating. The dissolution reaction of titanium in sulfuric acid can be guessed as follows:

The OCP variations (versus saturated calomel electrode; SCE) of the perfect and damaged Pt/Ti reference electrodes were measured versus time (Figure 7) since the stability of the passive film is determined by measuring its surface potential relative to a reference electrode. So, the variation of the OCP is important and gives the occasion to choose the best reference electrode. The results show that Pt/Ti damaged electrode is not suitable as a reference electrode because it does not have potential stability with time and the difference of its potential respect to SCE is the same as the difference Ti with respect to SCE. Due to the sensitivity of the pipeline to corrosion and the possibility of repeated destruction of the Pt/Ti reference electrodes, a pure Pt electrode is recommended.

6 Failure analysis of cathode electrode

The cathode electrodes are necessary to complete the circuit in the AP system. The cathodes should be stable and resist attack by impressed cathodic currents. The cathodes are installed at specific intervals along the length of the pipeline (Figure 1). Hastelloy B-2 was used as the cathode in the AP system. B-2 alloy is a nickel-base wrought alloy with excellent resistance to sulfuric acid at all concentrations and temperatures. B-2 alloy is not recommended in the presence of ferric or cupric salts as these salts may cause rapid corrosion failure. Many problems have been reported including corrosion, erosion, and fouling during operation under AP conditions. Initial observations show that there are deposition and groove effects on the electrodes. Among them, an electrode was selected for evaluation. Chemical analysis shows that the deposits have sulfide and chloride composition (Figure 4). After washing the electrode with water, the surface of the electrode was evaluated. The cavities were caused by corrosion and probably the groove was caused by the release of hydrogen on the surface. The reason for the cathode electrode degradation can be related to sulfuric acid reduction reactions. Depending on the potential value of the electrode surface, the reduction reactions can be different. In the reduction reactions, H₂SO₄ molecules are sequentially reduced to SO₂, S, H₂S, and H₂. H₂ is produced at the high cathodic overpotentials (Rodda et al. 2005a,b). Predictable reduction reactions can include:

7 Conclusion

In this paper, after seven years of AP system operation in the sulfuric acid plant, a section of the protected pipelines that has severely corroded was studied. The set-point potential of the AP of the pipeline is +100 mV with respect to the platinized titanium reference electrode. At the SP potential of AP, the surface of the stainless steel tube 316L pipe places in the passive region. The field study in a manufacturing H₂SO₄ plant showed that failure and damages of reference electrode coating cause stainless steel to be located in a trans-passive state. In this condition, the anodic current has increased sharply and caused leakage of pipes. Increasing currents also cause the cavities and grooves on the surface of cathode electrodes by the release of hydrogen. The experimental results showed that without any decay of the platinum coating, the stability of the electrode is impaired and cannot be employed as a reference electrode. If the electrode with the damaged coating is used, severe pitting and intergranular corrosion will occur in the stainless steel specimens. By replacing the defective platinized titanium electrodes with perfect Pt/Ti electrodes or preferably pure Pt electrodes in the sulfuric acid manufacturing problems were resolved.