Corrosion – Issues and Solutions for Hydrocarbon Refineries


temperature and higher hydrogen pressure ranges, carbon and low-alloy steels may suffer high temperature hydrogen attack (HTHA). This is due to the reaction of atomic hydrogen that diffuses through the steel with the carbides in the steel to form methane.22–24 As methane cannot diffuse through steel, pressure builds up and resultantly forms blisters and/or cracks. In order to improve the


Considering the future evolution of refining, this industry will still have to face new corrosion challenges in order to reduce the incidents rate due to corrosion.


carbide stability of pressure vessel steels, special low-alloy grade steels containing chromium and molybdenum have been developed. Vanadium and tungsten are also added in order to increase the mechanical properties of these steels.


Based on their experiences, end users of these steels have defined a design practice for each grade of steel.25


and as a consequence, the borders for acceptable process conditions or the exclusion of steel grades such as C-0.5Mo, have been published.


Extensive studies have also been and are still being performed in order to evaluate the remaining lifetime of aged equipment under hydrogen at high temperatures.25


Carbonate Stress Corrosion Cracking


Carbonate stress corrosion cracking is becoming active in FCC units.26 Significant process changes in FCC refining technology to enhance unit operation leads to a decrease of sulphur content and an increase


of nitrogen and CO2, resulting in an increase of the pH of the sour water. With these conditions, stress corrosion cracking of carbon steel can appear within only a few months.27


High Temperature Carbon Attack


In processes such as reforming or steam cracking, the high temperature and the high carbon activity lead to more severe conditions of coke formation, plugging, carburisation and metal dusting. Better understanding of the mechanisms of these degradations permitted the development of improved solutions and process requirements.


Coke Deposits


The formation of carbon deposits, which occurs at carbon activities ac >1 in a range of temperatures 450–1,100°C, is a major problem in petrochemical and refinery processes.28–36


The carbon deposition


on reactor walls induces localised disruption in the process such as heat-transfer reduction and pressure drops. An excessive carbon deposition causes deterioration of furnace alloys, such as an important migration of carbon into the alloys and high cleaning cost to remove the coke from the surface. To limit coke deposition and improve process efficiencies, specific alloy metallurgy, coatings (mainly based on chromium, aluminum or silicon), or chemical inhibitors (such as sulphur compounds) have been studied and developed.


78 This is periodically reviewed Carburisation At high temperature, carbon (from hydrocarbons, coke, gases rich in CO


and CO2) can diffuse into an alloy – a solid state carbon saturation occurs and then formation of metallic carbides.37–39


Carburisation


occurs in refinery and steam cracking furnace tubes when too high temperatures are involved (due to the furnace configuration or to the formation of a coke layer acting as thermal insulation), this can result in the loss of high temperature creep ductility, loss of ambient temperature ductility, loss of weldability and corrosion resistance. The main forms of damage that have been encountered are grooves and cracks. Metallurgists have developed new grades of refractory alloys, which have a better resistance to carburisation due to their higher contents of nickel, chromium and other elements (aluminum, silicon, etc).


Metal Dusting


Metal dusting is a form of carburisation resulting in accelerated localised pitting – which occurs mainly between 450 and 850°C – in carburising atmospheres and process streams containing carbon and hydrogen. Metal dusting is preceded by carburisation and is characterised by rapid metal wastage (metallic ‘dust’ particles). Metal dusting has been observed in catalytic reforming furnace tubes, cokers, methanol reforming units and hydrodealkylation furnaces and reactors. To avoid these disruptions, some alloy compositions are more resistant, but the application of a chromium- or aluminium-based diffusion coating or the presence in the carburising atmosphere of a sufficient amount of sulphur are also remedies.39–40


Acid Corrosion in Alkylation Processes


Acid alkylation units process alkylate light olefin fractions (butenes from FCC or visbreakers) with isobutane using sulphuric acid or hydrofluoric acid as a catalyst to produce gasoline. Refiners had to manage the safety and the performance of these processes where very corrosive catalysts are involved.


Hydrofluoric Acid Alkylation


Hydrofluoric (HF) acid can generate high general corrosion rates and may also act as a hydrogen promoter leading to hydrogen-induced cracking and blistering. In order to select more resistant materials, many investigations have been performed, leading to successful use of carbon steel and copper-nickel (alloy 400). Recommendations for specific compositions and mechanical property limitations have been developed. For example, due to the formation of a protective iron fluoride scale, carbon steel, with a low concentration of residual elements ([%Cu+%Ni+%Cr]<2.2%), is resistant in nearly all anhydrous HF conditions.41–45


Sulphuric Acid Alkylation


The limits of corrosion resistance of the main alloys have been investigated for different ranges of acid concentration, temperature and flow velocities. For carbon steel, the conditions under which a protective iron sulphate layer is active have been investigated (concentration >85%, temperature [T]<40°C, velocity [v]<6m/s). For the more severe conditions, nickel-based alloys (such as alloy B2, C276) have been developed.46–48


The Impact of Environmental Requirements on Acid Gas Treatments Units


The removal of acid gas (CO2, H2S) from refinery gas streams using amine-treating units is not a problem-free technology.


HYDROCARBON WORLD – VOLUME 6 ISSUE 1


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