Sulphur-induced corrosion in Duplex stainless steel: a case study in a petroleum refinery

G. Siracusa & A. D. La Rosa Dipartimento di Metodologie Fisiche e Chimiche per l’Ingegneria (DMFCI), Facoltà di Ingegneria, Università di Catania, Italy

Abstract

From the introduction of its first-generation, Duplex (austeno-ferritic) steel has seen a steady increase in popularity [1]. Recently, the production of high-strength, corrosion resistant super-duplex coil has been implemented in several applications. The case study we present in this paper concerns its use in a set of tubes of a heat exchanger affected by corrosion. The tubes material, S31803 Duplex stainless steel, is particularly suited to applications where corrosion is likely due to chlorides and/or sulphides being present. The heat exchanger operates in a chloride environment due to marine water on the inner side while the outer side is in contact with a hydrocarbon mixture rich in sulphur. It also has high resistance to general, pitting, crevice and erosion corrosion and to corrosion fatigue. The aim of our study is to understand the corrosion mechanism occurring and the factors responsible (e.g. the presence of chlorides, sulphides etc.) using several techniques for the experimental characterisation of the materials including optical microscopy and SEM (scanning electron micrography). Keywords: Duplex steel, pitting corrosion, sulphide cracking, SEM.

1 Introduction

The first-generation Duplex stainless steels were developed more than 70 years ago in Sweden for use in the sulfite paper industry. Duplex alloys were originally created to combat corrosion problems caused by chloride-bearing cooling waters and other aggressive chemical process fluids such as sulphides depending on the concentrations. Called Duplex because of its mixed microstructure with about

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equal proportions of ferrite and austenite, Duplex stainless steels are a family of grades, which range in corrosion performance depending on their alloy content [2-10]. The chemical composition based on high contents of chromium, nickel and molybdenum improves intergranular and pitting corrosion resistance. Additions of nitrogen promote structural hardening by interstitial solid solution mechanism, which raises the yield strength and ultimate strength values without impairing toughness. Moreover, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventional stainless steels. Duplex stainless steels have been used in oil refineries around the world due to their interesting combination of high corrosion resistance, good mechanical properties and cost effectiveness. Heat exchangers are typical pieces of equipment where corrosiveness of the process media or cooling water have led to premature failures on lower alloyed carbon steel or Cr-Mo grades, and where duplex stainless steel can offer a cost effective solution in many cases. The nominal chemical composition of three representative duplex stainless steels is given in table 1.

Table 1: Nominal chemical composition of three typical duplex stainless steels, wt%.

Type UNS** C max

Cr Ni Mo N PRE*

Low S32304 0,03 23 4.5 - 0.1 ≥24

Medium S31803/ S32205

0,03 23 5 3.2 0.18 ≥35

Super S32750 0,03 23 7 4 0.3 ≥41

* PRE=Pitting Resistance Equivalent(Cr%+3.3%Mo+16%N); ** (UNS) unified numbering system for metals and alloys

2 Case study

An Italian refinery decided to use a S31803 duplex as construction material of a seawater cooled heat exchanger located in the vacuum unit of the process, in contact with a sulphur-rich fluid. Service conditions are reported below: Service conditions: Tube side: seawater T: 30-34°C P: 5kg/cm2 Shell side: Hydrocarbons + 8%H2 +29%H2S T: 34-43°C P: 3kg/cm2

The pH value of the process fluid is calculated by using the Kane-Wilhelm [11] diagram (figure 1); at the working conditions (Pes 3kg/cm2; Tes: 34÷43,2 °C; pH2S= 0,3; pCO2= 0,01 it is assumed 3,7<pH<4.

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The aim of our study is to analyse the service conditions in order to understand

L’obiettivo è quello di analizzare le condizioni operative ed ambientali in cui le apparecchiature di raffineria lavorano e di studiare gli effetti che il solfuro d’idrogeno in fase acquosa, in presenza o meno di sostanze capaci di inibire laricombinazione dell’idrogeno sulla superficie dell’acciaio, ha sulla propagazione dei difetti nelle strutture dell’acciaio. I danneggiamenti da H2S umido sono causati da una stretta correlazione tra fluido di processo, stato tensionale e caratteristiche del materiale; nelle norme della NACE standard si fa riferimento in particolar modo a sei diversi fattori che influenzano la tipologia di danneggiamento:Temperatura;Tempo.

Figure 1: Kane-Wilhelm diagram for pH evaluations.

With S31803, seawater should always be used on the tube-side, with the tube-to-tube-sheet joints seal welded in order to avoid risk for crevice corrosion between tube and tube-sheet. After few days, the unit presented bad thermal exchange and it was supposed the presence of corrosion. After a preliminary treatment with aqueous vapour of the tubes it was possible to individuate, through hydraulic pressing, the presence of corrosion as reported in figure 1. The experimental investigation allowed to understand the cause of corrosion (e.g. the chlorine or the sulphur-rich fluid) and the possible mechanism.

3 Experimental analysis

3.1 Pitting analysis

Pitting corrosion is a form of localized corrosion as it does not spread laterally across an exposed surface rapidly but penetrates into the metal very quickly, usually at an angle of 90o to the surface as shown in figure 3. Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the material. Pitting can be initiated by several factors [12,13]:

3,5

3,75

4

4,25

pH

pH S [MPa]

2

2

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a. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)

b. Localized damage to, or poor application of, a protective coating. c. The presence of non-uniformities in the metal structure of the

component, e.g. non-metallic inclusions.

Figure 2: Photograph of the tube section showing pitting corrosion on the external side.

Figure 3: Optical micrography of the pitting corrosion under study.

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The aim of our study is to understand the effective causes of the pitting corrosion analysed and to give an interpretation of the possible mechanism.

Figure 4: SEM micrography of the corrosion products.

3.2 SEM micrography and elemental analysis

Electro-scanning analysis of the sample was carried out. Figures 4 and 5 show micrographs together with the elemental composition of both the corroded and non-corroded areas. It is very clear from figure 5 that the corrosion products (zone 1) are very rich in sulphur (11,2 weight %) while the non degraded area is poor in sulphur (1,13 weight %). Furthermore, elemental analysis of the supplied

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duplex tube (before use) reported in table 1, shows that before exposure to the sulphidic environment there was totally absence of sulphur in the material. After use, especially in correspondence with the corrosion, it seems that sulphur gathering occurs. This result is very interesting because gives the evidence of the sulphur-induced corrosion.

Table 2: Chemical composition of the S31803 duplex as supplied (before use) expressed in weight %.

Figure 5: SEM micrography of the tube sample. The crater (zone 1) corresponds to the corrosion.

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4 Discussion

4.1 Sulphur-induced corrosion mechanism

The corrosion resistance of a stainless steel is dependent on the presence of a protective oxide layer on its surface, but it is possible under certain conditions for this oxide layer to break down, for example in reducing acids, or in some types of combustion where the atmosphere is reducing. To explain the mechanism of this case of wet H2S cracking we suppose that, above a critical concentration, sulphur can enhance the dissolution, block or retard the growth of the protective passive film and cause passivity breakdown and pitting corrosion. Chromium and sulphur play antagonistic effects on the corrosion resistance of Ni-Cr-Fe alloys because chromium reacts selectively with oxygen to produce Cr oxides (the passive film) whereas nickel and iron react with sulphur. This leads to the coalescence of sulphur in small nickel and iron sulphide islands which can be covered by the lateral growth of the chromium oxide islands. A schematic representation of the proposed mechanism of the sulphur-induced breakdown of the passive film is shown in figure 6.

Figure 6: Mechanism of the breakdown of the passive film induced by enrichment of sulphur at the metal-passive film interface.

References

[1] Peckner, Bernstein, (1977), Handbook of Stainless Steel, McGraw-Hill, New York

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[2] Gunn, (1997), Duplex Stainless Steels-Microstructure, properties and applications, Abington Publishing, Cambridge.

[3] Lippold, Varol, Baeslack, (1991), Proc. 3rd Int. Conf. Duplex Stainless ‘91, Les èditiones de Physique, Beaune.

[4] Lacombe, Baroux, Beranger, (1990), Les aciers inoxydables, Les èditiones de Physique.

[5] Pleva, (1992), Proc. Int. Conf. Applications of Stainless Steels ’92, The metal Society, Stockolm.

[6] Bernhardsson, (1991), Duplex stainless steel, Les èditiones de Physique, Beaune.

[7] Costes, Desestret, (1998), HNS ’88 Symposium, Lille. [8] Josefsson, Nilsson, Wilson, (1991), Proc. 3rd Int. Conf. Duplex Stainless

‘91, Les èditiones de Physique, Beaune. [9] Nordberg, (1993), Proc. Int. Conf. Innovation Stainless Steel,

Associazione Italiana di Metallurgia, Firenze. [10] Marrow, King, (1994), Fatigue Fract. Engng. Mater. Struct. [11] R.D. Kane and S.M. Wilhelm, "Slow Strain Rate Testing for Materials

Evaluation in High Pressure H2S Environments," D.R. McIntyre, CORROSION/86, (Houston, TX: NACE International, 1986). Also published in Corrosion Journal 44, 12(1988): pp. 920-926.Kane, Wilhelm, (1986), Journal of Petroleum Technology.

[12] Pedeferri, (1986), Corrosione e protezione dei materiali metallici, Milano. [13] Roberge, (2000), Handbook of Corrosion Engineering, McGraw-Hill,

USA.

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