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Aug-2016

Corrosion resistance in tropical environments

Tests at NASA demonstrate how super-duplex tubes provide corrosion resistance at lower cost in a challenging environment

BARINDER GHAI
Sandvik Materials Technology

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Article Summary

Corrosion problems are creating serious issues for refineries and petrochemical suppliers that are up against fierce competition in growing economies like Asia. Because they are among the most prized industrial assets in the world, refineries and petrochemical plants must be equipped to meet changing industry demands. Suppliers are turning to innovation and collaboration to address the most pressing issues, including the latest economic developments and requirements for greater cost efficiency.

New technologies are at the forefront of this, particularly in China and India as refining and petrochemicals applications move increasingly from West to East in what some are calling the ‘Asian Petrochemical Boom’. Major consumption centres are shifting to Asia mainly because of growing demand for compressed natural gas (CNG), market pressures and fiercer competition. These factors are driving companies into deeper wells with higher pressure environments.

The challenges make it ever more necessary for operators to combat problems of corrosion on the process side. Corrosion phenomena have caused numerous failures to occur in conventional steels, including in hydraulic and instrumentation tubing and particularly chemical injection lines, which are today used by most of the key oil and gas producers.

Such problems typically arise due to the presence of hydrogen sulphides (H2S) and chlorides in process fluids, issues that are becoming more pronounced as refineries must process sourer and more corrosive ‘poor quality’ crude oils.

To address these issues, it is necessary that refinery materials evolve to yield superior performances, improved corrosion prevention and better overall production economy. The answer therefore lies in upgrading the metallurgy of key components.
 
Metallurgy and anti-corrosion
Indian refineries have traditionally opted for conventional materials such as carbon steels and 304L and 316L grade stainless steels. This is despite the fact that carbon steels are prone to general corrosion while AISI 304 and AISI 316 can succumb to stress corrosion cracking (SCC) in chloride bearing environments. This is more so the case with the lower carbon equivalents, 304L and 316L, when they are used at high temperatures.

The normal life cycle of AISI 316L steels can exceed 100 years in non-corrosive environments. Yet the same grade has exhibited a service life of less than five years in corrosive chloride-containing environments, or less than one year in some cases as confirmed both in extensive tests and operational experience. These kinds of corrosion failures cannot be afforded by refineries and petrochemical suppliers as they face fiercer competition and increasing performance demands.

Higher content of alloying elements in general, or the introduction of nickel or ferrite into the metallic structure, can greatly improve the resistance of a steel to SCC. Steels with an austenitic-ferritic metallurgy are found to be superior to conventional austenitic grades like 316 because they have higher mechanical strength and greater stability.

By examining the Pitting Resistance Equivalents (PRE) values of stainless steels, it is possible to gain a better understanding of how resistance to corrosion phenomena, like SCC or pitting, is effected by the chemical composition. The PRE number is widely accepted as the best method to rank the pitting resistance of stainless steels (see Table 1).

PRE values are measured in accordance with exact testing procedures specified in the ASTM G48 standard. Generally, the higher the PRE number, the more corrosion resistant the steel. The formula used to calculate PRE takes into account the content of chromium (Cr), molybdenum (Mo) and nitrogen (N) within the steel’s metallurgy defined in weight as: %PRE = % Cr + 3.3 %x Mo + 16 x % N.

The figures in Table 1 show that AISI 304L and AISI 316L have lower resistance to pitting. Austenitic ASTM 904L (UNS S08904) has a comparably higher PRE number because it contains more molybdenum and nickel. However, the limited market availability of these elements makes the grade more expensive; this is the case with 6% molybdenum or ‘6Mo’ steels, which are frequently chosen for H2S processes by operators in the Middle East and Asia.

Tropical marine tests at NASA
Hydraulic and instrumentation tubes used to process oil or gas either in offshore or shoreline installations are typically subjected to high levels of salt, humidity, ultraviolet (UV) light and temperature.

This is particularly the case in tropical marine installations, where seawater evaporates, leaving a higher concentration of chlorides on the tubing. Hydraulic and instrumentation tubing made from the super-duplex material Sandvik SAF 2507 was exposed to such conditions in a test conducted in the demanding tropical marine atmosphere at the NASA Research Center located in Florida, USA.

The study saw stainless steel seamless tube samples in 316L, 904L and 2507 positioned 150ft (45m) from the mean high water mark on the east coast of Florida, directly facing the Atlantic Ocean, for a period of one year. Each tube sample was bent at 90 degrees and the ends were terminated with fittings and mounted at 30 degrees on a test rig facing the ocean.

Proximity to the ocean was typical of a shoreline petrochemical installation and close enough to ensure that the test samples would be affected by salt spray and mist throughout the 12 months. The conditions on the Florida coastline with its prevailing tropical climate provided an ideally aggressive and corrosive environment for the test samples.

After conclusion of one year’s exposure, the test samples were returned to the laboratory for detailed inspection and analysis. Visual inspection revealed that the most corroded samples were the 316L tubes, more so on the section that had faced seawards. Corrosion damage, discoloration and pitting were found.

Some discoloration and pitting were also observed on the 904L samples, although to a lesser extent. The 2507 super-duplex material, however, exhibited no pitting whatsoever. Some slight discoloration was observed, but this easily washed off after cleaning in water. No signs of SCC were present on any of the tube samples.

When the fittings and ferrules were dismantled, corrosion products were found on the treading of the fitting and the tube showed red corrosion products where they had been in contact with the treading. Once the fittings had been removed, close examination of the tubes with an optical microscope revealed crevice corrosion on the 316L and 904L test samples, but not on the super-duplex 2507 tubes.

Gulf of Mexico tests
A more in-depth understanding of the material properties exhibited at NASA can be gained from the results of other tests performed in the Gulf of Mexico. Sandvik SAF 2507 was installed alongside AISI 316L and the corrosion behaviour of the grades was then assessed after 18 months (see Figure 1).


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