Petrotech-201031 October-3 November 2010, New Delhi, India Paper ID : 20100265 Wet - H2S Corrosion Mechanism and Control Strategies in Refining Industry B. S. Negi Chief Inspection Manager, Indian Oil Corporation Limited, Refineries Division, Maintenance & Inspection Dept., Indian Oil Bhawan, A-1, Udyog Marg, Sector-1, NOIDA – Uttar Pradesh, INDIA Email :
[email protected] Abstract Carbon steels are the predominant materials for equipment and piping in the refining industry. Refinery operation involves exposure to aqueous environments containing hydrogen sulphide. This wet- H2S environment is mild corrosive to carbon steels. However, hydrogen liberated during corrosion reaction, in presence of sulphide scale, penetrates into steels and causes hydrogen blistering and cracking. As a result, the mechanical properties of the steel are deteriorated and reliability and integrity of equipment is reduced. The welded pressure containing components are more susceptible to wet-H2S corrosion. The extent and types of damages depend upon H2S concentration, pH, contaminants, temperature, steel quality, fabrication practice and corrosion control practices. The damages are surface and sub surface oriented. Visual inspection, wet fluorescent magnetic particle testing and ultrasonic flaw detection are the common non destructive testing (NDT) methods for detection and evaluation of damages. The correct evaluation of these damages and using suitable repair procedures, integrity of equipment can be restored. This paper focuses on the wet-H2S corrosion mechanism, repair procedures and control strategies to be followed for enhancing the reliability and integrity of equipment. Keywords: Wet-H2S, SSC, HIC, SOHIC, WFMT, Blistering, SWC, HAZ, PWHT, UT, NDT Introduction Wet- H2S corrosion is caused by the generation of atomic hydrogen and subsequent diffusion of the atomic hydrogen into the steel during corrosion reaction in presence of sulphide scale. Hydrogen blistering, hydrogen induced cracking (HIC), stress oriented hydrogen induced cracking (SOHIC) and sulphide stress corrosion cracking (SSC) are common to refinery equipment that contains greater 0 than 50 ppm H2S in water, between ambient temperatures and 150 C. The extent and types of damages depend upon H2S concentration, pH, contaminants, temperature, steel quality, fabrication / manufacturing process and corrosion control practices. Visual inspection of welding joints and pressure boundary surface is the best technique for detection of blisters and surface cracks. Wet fluorescent magnetic particle testing (WFMT) and ultrasonic testing (UT) are to be done for surface and sub-surface crack detection and sizing. Evaluation of blisters and cracks are to be done in accordance with API 579 for assessing the equipment integrity for further service. Surface blisters less than 50 mm in diameter can be drilled to relieve the internal pressure. Cracks may be removed by grinding or gouging. Cracks deeper than corrosion allowance can be repaired by weld buildup. The repair area to be re-inspected after welding & post weld heat treatment (PWHT). Hydrogen blistering and HIC can be controlled by using clean steels with minimum inclusion, restricting trace elements, such as sulfur, phosphorus, cyanides and controlling manufacturing variables. PWHT, which reduces residual stresses and tempers the microstructure along with controlling welding parameters to achieve hardness below 200 HBW are the best approaches to mitigating SOHIC and SSC. Injection of good film forming amine, polysulfide inhibitor, wash water and pH can help in reducing wet- H2S corrosion. 1 High strength and high hardness steels are prone to SSC. As a result. However. They diffuse into steel and form hydrogen molecules at inclusion or lamination sites present in steel. pH above 7. such cyanide (HCN). when sulfide scale is present. the surface concentration of atomic hydrogen rises and a corresponding increase occurs in the amount of hydrogen diffusing into the metal. it harmlessly dissipates. molecular H2 forms at the surface of the steel and. the sulfide acts as a negative catalyst and discourages the reaction (2). Atomic hydrogen (H) and molecular hydrogen (H2) are produced in the corrosion reaction of steel with aqueous H2S as illustrated below. Figure-1: Hydrogen Blistering observed in LPG caustic wash vessel 2 . New Delhi. pH. if produced slowly at low corrosion rates.H2S corrosion are free water containing with more than 50 ppm of H2S. temperature and steel quality. contaminants.Petrotech-2010 31 October-3 November 2010. Fe + H2S FeS + 2 H (1) 2H H2 (2) In the conventional corrosion process. accumulating in the crystal structure and affecting the steel’s mechanical properties. India Corrosion Mechanism Hydrogen damage in wet H2S service is caused by the generation of atomic hydrogen during corrosion reaction and the subsequent diffusion of the atomic hydrogen into the steel. In the presence of a catalyst poison. the atomic hydrogen penetrates the steel. which are called recombination poisons or catalyst poisons. exert pressure to cause local deformation and blistering. also interfere with the conversion of atomic hydrogen to molecular hydrogen. Blisters may appear on either or both surfaces of a plate or on top of one another.6 or below 4. depending on the location of the lamination. selenium and arsenate ions. Types Of Damages Hydrogen Blistering The blister results from hydrogen atoms that originate during the sulfide corrosion process at the steel surface. under ordinary acidic conditions. The types of damages depend upon H2S concentration. Figure-1 Illustrates the hydrogen blistering observed in LPG caustic wash vessel. The favourable environmental conditions for promoting the wet. phosphorus.0 and temperature between 0 ambient to 150 C. Increasing blister growth can produce tears in the surface and result in loss of the pressure retaining capability of the equipment. The hydrogen molecules so formed. antimony. Other compounds. They vary in size and appearance from small protrusions to swellings several feet or more in diameter. New Delhi. post weld heat treatment is of little significance. Figure-3 shows the surface crack observed during wet fluorescent magnetic particle inspection (WFMPI) of LPG storage vessel fabricated from ASTM A537 plates. elongated sulfide or oxide inclusions occurring parallel to the plate rolling direction are typically associated with HIC. Susceptibility of steel to SSC is highly dependent on its composition. Interconnecting cracks between the blisters often have a stair step appearance and so HIC is sometimes referred to stepwise cracking (SWC). These inclusions serve as sites for formation of microscopic hydrogen blisters that grow and eventually connect via stepwise cracks. microstructure. Non-homogenous. SSC can initiate in high hardness zones of high strength steels. Sulfide Stress Cracking (SSC) Sulfide stress cracking is defined as cracking of metal under the combined action of tensile stress and corrosion in the presence of moisture and H2S. Figure-2: HIC & SOHIC cracking in LPG caustic wash vessel Stress Oriented Hydrogen Induced Cracking (SOHIC) SOHIC is similar to HIC but have arrays of cracks linked in the through thickness direction by transgranular cleavage cracks. HIC is not stress dependent or associated with hardened microstructures. residual stress and applied stress levels. Since stress is involved in crack development and propagation. Figure-2 shows the crack orientation in HIC. post weld heat treatment is somewhat effective in reducing SOHIC. It is driven by high levels of residual or applied stress and usually appears in the base metal adjacent to the weld heat affected zones (HAZ). India Hydrogen Induced Cracking (HIC) It is manifested as blister cracks oriented parallel to the plate surface. Steel composition and microstructure influence the threshold hardness of steel to SSC. Since. Figure-2 shows the crack orientation in SOHIC. HIC occurs due to the stresses generated by hydrogen gas collected in the planar stringer inclusions in steel. Controlling manufacturing variables and trace elements is also effective.Petrotech-2010 31 October-3 November 2010. strength. 3 . Petrotech-2010 31 October-3 November 2010. If blisters are observed. In addition to this. condensers. Sour water stripper and amine regenerator overhead systems are especially prone to wet H2S damage because of generally high ammonia bisulfide concentrations and cyanides presence. 100 percent wet fluorescent magnetic particle testing of all the weld seams shall be carried out internally. New Delhi. arc strike locations and support location shall also to be tested with WMFT. 400 series SS valve trim. 4 . For detection of surface breaking cracks WFMT is to be done. SOHIC and SSC damages can occur throughout the refinery wherever there is a wet H2S environment present. lamination and blisters. Inspection for SSC is to be considered in the process units and storage facilities using pressure vessels made of high strength steel.H2S service. mapping of blisters and laminations for fitness for service evaluation. India Figure-3: Linear indication observed in WFMT Inspection Methodology Thorough internal visual inspection of weld joints. fractionation towers. SSC is most likely found in hard weld and heat affected zones and in high strength components including bolts.H2S corrosion includes LPG and sour water treating units. fractionator overhead drums. compressor shafts. Area prone to wet. absorber and stripper towers. cleat. compressor interstate separators and knockout drums and various heat exchangers. HAZ and pressure boundary of vessel shall be carried out. It is one of the most commonly used methods for internal pressure vessel inspection in wet. UT is most frequently used method for sizing cracks. Blistering. LPG storage vessel. sleeves and springs. UT is to be used to detect subsurface cracks. A representative number and type of indications are be evaluated to determine their relevance and severity with respect to code requirement. The surface of the weld and about 150 mm base metal on both sides of welding should be cleaned for WFMT using AC yoke. Some indications may be irrelevant. Generally. HIC. and coolers. relief valve springs. About 20-25 percent of weldments including all tee joints should be tested using shear wave ultrasonic testing. However. Cracks may be removed by grinding or gouging. API 579 suggests three level of assessment for determining the acceptability of blister for reliability of equipment. Post weld heat treatment. porosity and inclusion. thick epoxy coating or gunnite lining on vessel internal surface helps in the mitigation of wet H2S corrosion. Local post weld heat treatment of welded joints shall be carried out as per original code of construction. phosphorus and controlling manufacturing variables for steel provide HIC resistance. India Repair Procedures Blisters and cracks are to be evaluated in accordance with API 579. replacement in full or part should be considered. Control Strategies Material selection • • • Selection of correct material manufacturing process to produce material free of blow holes. Calcium treated steels offer better resistance due to inclusion morphology control. Level-3 analysis needs detail fracture mechanics approach if the blister result in crack like defects in thickness direction. Hydrogen bake-out treatment (HBT) shall be carried out to flush out hydrogen and also to facilitate welding. Material chemistry control also play important role in mitigation of wet-H2S corrosion. Application of fiber reinforced plastic (FRP) lining. welding procedure specification shall be developed and qualified for each application. Use of low strength steel containing low sulfur and low phosphor for fabrication of vessel and seam pipes and adherence to NACE MR 0103 to be considered. The cavities formed by removing the cracks are to be made smooth with boat shape contour and to be inspected with WFMT to ensure that all cracks are removed.RP 0472 practice to be followed. Repair welds hardness control to be in-accordance with the NACE RP 0472. Linings and Coatings • Wet H2S corrosion can be controlled and mitigated by reducing or eliminating the hydrogen activity. grinding off sharp edges and smoothening of weld contour reduces the chances of crack initiation. Sharp edges of welding and arc strikes are to be removed by blend grinding. This can be accomplished by using alloy or alloy clad materials of more corrosion resistant metal. The tip of the cracks is to be made blunt to arrest its propagation during grinding. Killed and normalized steel with grain refinement microstructure possess superior resistance to brittle fracture and crack propagation. The repaired area is to be re-inspected after welding & post weld heat treatment. Post weld heat treatment (PWHT) and controlling weld and HAZ hardness below 200 BHN and adherence to NACE. along with controlling welding parameters are the best approaches to mitigating SSC. Controlled-deposition welding method can be adopted to avoid PWHT. Level-1 & level-2 considers the acceptability on various criteria related to size density and distribution of blister. such as sulfur. which reduces residual stresses and tempers the microstructure. Fabrication Practices • • • Welding of fabrication cleats with preheat . the remaining net sections of metal to be capable withstanding the internal design pressure. avoiding frequent arc strike on the metal surface. laminations.Petrotech-2010 31 October-3 November 2010. Restricting trace elements. Silicon killed steels are preferable than rimmed and semiskilled steels. localized thinning assessment. 5 . The weld repair can be carried out in accordance with industry recommended practice such as API 510. Surface blisters less than 50 mm in diameter may be drilled to relieve the internal pressure. For in situ part replacement. Cracks deeper than corrosion allowance can be repaired by weld buildup. . New Delhi. In case integrity of the pressure vessel is found to be unreliable. and Mitigation of Cracking of Existing Petroleum Refinery Pressure Vessel in Wet H2S Environments.5 will result in minimum corrosion There are varieties of approaches like water washing. NACE Standard MR0103-2005. fabrication practice & operational monitoring. Repair. Item No. 5. 2. Monitoring pH within 5. First Edition. Item No. 4.21006 “Methods and Controls to Prevent In-service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments". India Operation Monitoring • • Corrosion at the lower pH is caused by higher concentrations of dissolved H2S in water and at the higher pH is caused by higher concentrations of the bisulfide ion in the water.5 to 7.21078 “Guidelines for Detection. the vessel serviceability and reliability can be increased. Item No.” Jan’2000”. API Recommended Practice 579 “Fitness for Service.5-41.21305 “Material Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments". References 1. NACE Standard RP 0296-2004. polysulfide addition.” 6 .Petrotech-2010 31 October-3 November 2010. It forms a protective layer at the surface and controls corrosion. December 2003. 3. Periodic internal inspection and proper damage assessment and adopting repair procedure. Conclusion The integrity of pressure vessel in Wet H2S service can be enhanced by proper material selection. New Delhi. p. NACE Standard RP 0472-2005. API RP 571 “Damage Mechanisms Affecting Fixed Equipment in the Refining Industry”. as well as the use of organic filming inhibitors for prevention of both corrosion and hydrogen blistering.