Combating the Effects of Corrosion in Refineries with Duplex Stainless Steels
- Mohan Gawande, Manager, Chemical Group, Sandvik Materials Technology

As worldwide crude oil production levels intensify, duplex stainless steels are proving ever more crucial in withstanding corrosion challenges faced by oil refinery equipment, writes Mohan Gawande, Manager Chemical Group, India, Sandvik Materials Technology.

Heat recovery processes are especially important to critical petroleum refining applications, supplying and reusing energy in order to help achieve more efficient, economic and productive processes. Nevertheless, corrosion aggravated equipment failures in heat exchanger tubes have always been a large and unavoidable issue in critical petroleum refining applications

Traditional metallic tube components like carbon steels are failing to cope in the increasingly harsh corrosion environments, resulting in disruptive maintenance procedures or unplanned shutdowns. Such materials are extremely vulnerable to corrosion and austenitic stainless steels and, widely used in heat exchanger tubing, are susceptible to stress corrosion cracking (SCC) particularly in chloride bearing environments.

According to statistics, such failures are thought to account for the losses of approximately 1-5 per cent of various countries’ gross domestic product (GDP) and around 6 per cent of the overall petrochemical industry’s GDP. The latter figure is twice as much as in other industries.

Aside from the higher productivity demands, these problems are exacerbated by the decline in crude oil quality which is becoming sourer and more corrosive. For these reasons, it is ever more vital to select materials that can both withstand and eliminate failures throughout the plant. A wellconsidered material should possess superior anticorrosion and mechanical properties to help optimise equipment service lifecycles, reduce the need for maintenance, prevent contamination of refinery products caused by corrosion and minimise heat losses caused by fouling of equipment.

Corrosion factors in refineries
The primary corrosion media in oil refineries include sulphides, chlorides, nitrides, hydrochloric acid, polythionic acid, oxygen and heavy metals. These elements tend to react under the dew point temperature, when vapour condenses into liquid water at the same rate at which it evaporates. Either in water or during catalysis, this condensation can cause serious corrosion in equipment.

Dew point corrosion is generally initiated by the formation of hydrochloric acid, and chemical interactions between hydrochloric acid and hydrogen sulphide cause corrosion of the refinery equipment. Deposits in or on heat exchanger tubes emanate from the process side due to tenacious hydrocarbons, process slurries or even ammonium chloride (NH4Cl) deposits in crude overhead condensers. Non-hydrocarbon compounds and additives also build up during the refinery process resulting in further corrosion.

These factors are detrimental to overall process efficiency and pose a threat to the refinery devices’ distillation, hydrotreating and catalytic reforming of crude oil.

Corrosive elements in crude oil
However, it should be noted that the main cause of corrosion in refinery applications from the process side is not the hydrocarbons themselves but the presence of contaminants in the crude oil as it is produced. Generally, the heavier the oil the higher the boiling point. It is corrosion under high temperatures that causes carbon steel equipment to fail and leads to unplanned and costly maintenance, shutdowns or accidents.

Most contaminants in crude oil end up in the refinery tankage along with contaminants picked up during the transportation. Crude oil contaminants that affect corrosion resistance in steels include carbon dioxide (CO2), hydrogen sulphide (H2S), nitrogen compounds, sulphur compounds and inorganic chlorides such as sodium chloride (NaCl), magnesium chloride (MgCl2) or calcium chloride (CaCl2).

Crude oil is normally more than 90 per cent naphthenic acid, of which corrosion is most intense in environments with elevated temperatures and without water, and occurs at its greatest potential at temperatures of 270-2800 C (518-5360 F). The corrosion rate of naphthenic acid declines at temperatures above 2800 C (5360 F), rises rapidly again at 3500°C (6620 F) and stops corroding above 4000°C (7520 F).

One of the most corrosive phenomena that affect stainless steels is the reaction of sulphur with oxygen to create sulphur dioxide (SO2). Sulphur is present in most processes where coal or oil is combusted at high temperature, and Figure 1 presents an overview of trends relating to global crude oil gravity and sulphur content including data by the American Petroleum Institute (API). Different fractions of sulphur levels are shown in Table 1.

SO2 corrosion rates vary under different temperatures. Sulphide does not decompose under temperatures below 1200°C (2480°F), but corrodes and forms into a hydrogen sulfide (H2S-H2O type) under the dew point temperature or when it contains water. H2S is generated under temperatures between 240- 3400°C (464-6440°F) and is aggressive to carbon steel at these temperatures thereby corroding the equipment.

Corrosion worsens as the temperature increases, reaching its highest peak when the temperature falls within the range of 420-4300°C (788-8060°F). A superior replacement grade to carbon steels must therefore be capable of retaining superior anti-corrosion properties at high operating temperatures.

Mitigating plant shutdowns
Utilisation of corrosion resistant materials not only eliminates unscheduled plant shut downs, but also reduces the risk of costly lost production and expensive emergency maintenance and repair. Consideration should also be given to the formation of crevice corrosion beneath such deposits at temperatures below the critical pitting temperature (CPT) of the material.

Carbon steel is extremely vulnerable to corrosion and austenitic stainless steels, widely used in heat exchanger tubing, become susceptible to SCC particularly in chloride bearing environments. Research has shown that these materials are highly susceptible to corrosion at the elevated operating temperatures found in refineries.

Gradually, the industry has recognized the advantages of duplex stainless steels that offer the optimum combination of corrosion resistance, mechanical properties and excellent fabrication capabilities which all lead to genuine cost advantages.

Development of duplex stainless steels
Since the development of duplex stainless steels began in the 1930s, they have emerged as a favoured alternative to traditional carbon steel tube in corrosive heat exchanger applications.

The first generation grades contained chromium (Cr), molybdenum (Mo) and a high content of carbon (C) with low weldability. Further developments into the 1970s reduced the content of C, and alloy elements that improved the material’s corrosion resistance such as Mo, copper (Cu), silicon (Si) and especially nitrogen (N) were added.

These innovations led to the formation of a new ‘second generation’ of N-containing duplex stainless steels that included the 18Cr-type, 22Cr-type and 25Cr-type, and a further three generations of duplex stainless steels which emerged later in the 1980s. These had low C content but high amounts of Mo and N with content of ferrite that is about 50 per cent or slightly lower.

Some of these grades were called ‘superduplex’ and characterised by a Pitting Resistant Equivalent (PRE) number of at least 40. The PRE number (=%Cr + 3.3x%Mo + 16x%N) is a measurement for ranking the resistance of stainless steels to pitting and crevice corrosion. Exact testing procedures to determine the PRE number are specified in the ASTM G48 standard, one of the most severe pitting and crevice corrosion tests applied to stainless steel which exposes test specimens to 6 per cent iron(II) chloride (FeCl) solution, also known as ferrous chloride, with and without crevices.

In general, the higher the PRE value the more corrosion resistant the steel. The pitting resistance value of a stainless steel is of great importance when assessing its suitability for heat exchanger applications, and also fabrication practices like welding which are of vital importance for performance in service.

As a comparison, typical grades like AISI 316L and AISI 317L have insufficient PRE values to withstand many corrosive heat exchanger environments – even when working at the upper limits of their standards.

Improved corrosion resistance Figure 2 illustrates a comparison of several commonly used refinery materials’ resistance to SCC. They include duplex Sandvik SAF 2304TM (UNS S32304) and super-duplex Sandvik SAF 2507TM (UNS S32750).

Both Sandvik grades are well suited to heat exchanger applications, and each possesses a combination of good corrosion resistance and high mechanical properties for advantages like reduced wall thicknesses and lower processing costs. The materials have proven especially useful in corrosive heat exchanger environments and are helping to solve many of the problems faced by today’s oil refining industry.

Key attributes of Sandvik SAF 2304 include low Ni content, and a two-phase microstructure with approximately 50 per cent ferrite which imbues the grade with a more stable metallurgy than comparably instable high-nickel alloys. A high 23 per cent Cr content compensates for an absence of the vital yet costly anticorrosion element Mo, and N content further increases the material’s strength while improving weldability and resistance to pitting corrosion.

The nominal chemical composition of Sandvik SAF 2304 is shown alongside the standard materials ASTM 304L and ASTM 316L in Table 2 with, for comparison, the minimum PRE numbers of each material.

The duplex stainless steel has a PRE number of 24 which suits the material for used in the harshest corrosive environments at a temperature range of -50 to 3000°C (58 to 5720°F). As Table 2 shows, the PRE number for Sandvik SAF 2304 is considerably higher than the number for AISI 304L and comparable to the number for AISI 316L. In practical terms, the grade demonstrates better resistance to SCC compared to austenitic steels of AISI 304 and AISI 316 type, and is demonstrably better than AISI 316L in most acid environments, with resulting cost advantages, see Table 3.

For example, lean duplex stainless steels, such as Sandvik SAF 2304, offer high strength with a yield strength twice that of AISI 304L and AISI 316L austenitic stainless steels, low thermal expansion, very good weldability, physical properties that provide design advantages, as well as ease of fabrication and toughness.

When looking at super-duplex grades, such as Sandvik SAF 2507, based on the established PRE values of AISI 316L and its variants like AISI 317L, the minimum standard PRE value of 42.5 for Sandvik SAF 2507 identifies the super-duplex grade as superior. The performance levels are comparable to 6.0 per cent Mo austenitic stainless steels like 254 SMOTM and AL-6XN. Yet Sandvik SAF 2507 has distinct advantages over these 6.0 per cent Mo steels: it is more readily available and therefore offers lower initial costs. Figure 3 compares the material’s critical pitting and crevice temperature alongside standard steel grades according to ASTM G48.

The super-duplex stainless steel’s antisulphide and chloride SSC resistance and crevice corrosion resistance capabilities have been further enhanced compared to austenitic grades., see Figure 4.

The data shows that Sandvik SAF 2507 is a competitive alternative to high alloyed austenitics and nickel alloys in applications where standard austenitic steels corrode at a high rate; it is demonstrably the best choice material for use in high-temperature heat exchangers containing chlorinated or nonchlorinated water.

Cost advantages
On many projects cost is of primary importance. However, the ability of a material to fully meet the application requirements has to be a major consideration for plant efficiency.

Strength of the material is a significant factor. For example, selecting a duplex grade such as Sandvik SAF 2304, despite a higher price per kilogram, can prove to be the most economical solution. This is because the wall thickness of the tubes as they are subjected to internal pressure or tensile loads is directly related to the material strength. As thinner wall tubes can be specified, the cost of the duplex material can be around 35 per cent lower. This should be compared to the cost of tubes of other material grades which would require a thicker wall in order to achieve the same strength, see Table 3. There are also associated savings to be achieved on transport, installation, welding etc when specifying the lighter thinner walled duplex grade tubes.

Successful application in heat exchangers
The Sandvik duplex stainless steel grade Sandvik SAF 2304 and the super-duplex material Sandvik SAF 2507 are among the manufacturer’s advanced materials that have become success stories in oil refineries around the world, and each has exhibited high performance cost advantages.

In the USA, a plant was experiencing inadequate performances in the AISI 316 tube in its heat exchangers. The tube, used to process maleic anhydride which is an organic compound for applications in coatings and polymers, had failed due to SCC after only 2-3 years’ service.

Sandvik SAF 2304 was chosen as the replacement grade, of which 7350 m of 19.05 x 1.65 mm tube was installed in the secondary cooler after the reactor and would be required to operate at temperatures of 204-2320°C (400-4500°F). The material exhibited superior mechanical and physical properties, along with excellent resistance to SCC and other forms of corrosion.

In another installation, a refinery was experiencing corrosion problems in its overhead condensers. The system comprised four heat exchangers which each contained 951 steel tubes of length 19.05 x 1.65 x 6096 mm. Used to process crude oil, the tube was subjected to many of the main media sources that cause corrosion in refinery equipment including H2S, dew point corrosion, chloride pitting, NH4Cl and under-deposit corrosion.

The existing tubes, made of carbon steel, were again exhibiting a maximum service life of 2 to 3 years when subjected to condensation of hydrocarbons and H2O in small amounts (30-100 ppm Cl and 200- 1000 ppm H2S) generally at pH levels of 6-7, 45-550°C (113-131 °F) inlet temperatures, inlet pressures of 0.078 – 0.12 MPa (0.8- 1.2 kg/cm2) and outlet temperatures of 60- 750°C (140-1670°F).

An inspection carried out in April 2000 revealed that the tubes were still in good condition, and a visual inspection found a good connection of the pipe’s tube plate. The tube’s interior had not been subject to corrosion; and examination of the exterior found no under-deposit corrosion. In contrast, a phenomena of general corrosion on the old carbon steel tube plate was observed. Between the years 2000 and 2011, a total of four new heat exchangers were subsequently put into use at the refinery all using the duplex material.

Similarly, a refinery in Italy required replacement tube in six overhead condenser system heat exchangers. Each heat exchanger comprised 1,298 tubes made from AISI A179 steel. After two years of use, multiple tubes in the bundle experienced an early onset of failure. Sandvik SAF 2507 was installed in the heat exchangers in 2003. An inspection in 2006 revealed the heat exchangers to be in good condition, despite serious decomposition seen in the tube plates and shell passes. It was not until 2008 that, with moderate concentrations of hydro-cleaning, some signs of corrosion were found.

Conclusions
As heat recovery processes remain at the forefront of economical and environmentally compliant processes, refineries must implement the most reliable materials to fully-realise these objectives and also remain cost-effective. Duplex and super-duplex stainless steels by Sandvik can realise these goals, while addressing the ongoing decline in crude oil quality and the failure due to corrosion of a range of materials, such as copper based alloys as well as different types of austenitic stainless steels.

In such environments, duplex stainless steels provide excellent resistance to corrosion attack as recorded in extensive laboratory testing, as well as in successful documented installations in process plants and refineries worldwide. Ease of fabrication and the durability of duplex stainless steels means significant advantages, not only for new equipment but also when retubing existing heat exchangers. Further to these advantages, duplex stainless steel Sandvik grades are today included in numerous refinery and equipment manufacturers’ preferred procurement lists.