Looking at martensitic stainless steels (MSS) NACE MR0175 / ISO 15156 lists six tables with different H2S acceptance levels. Generic Table A.18 lists several MSS with a maximum allowable partial pressure H2S of 0.1 bar whilst equipment specific Table A.23 (Wellhead and tree components and valve and choke components) shows no restrictions when it comes to partial pressure H2S.To substantiate the applicability of Table A.23 a study was performed to evaluate environmentally-assisted cracking resistance of cast alloy CA6NM (UNS J91540) in highly sour environments (Level VII) and the implications of the findings on the usage of CA6NM as pressure containing valve bodies in wellheads.After that a ballot (no. 2013-03) was written to clarify the scope of Table A.23 and limit the use of cast alloy CA6NM (UNS J91540) based on applied in situ stress. An additional note was incorporated for UNS J91540; "Low-carbon martensitic stainless steel J91540; the maximum design tensile stress shall not exceed 2/3 specified minimum yield strength or 345 MPa (50 ksi) whichever is less." The presentation will include field history a historic perspective of NACE MR0175 results of NACE level VII tests the ballot process including an FEA study to simulate stress distribution in valves for wellhead equipment.
Synthesis of materials for immobilization of well-known pH indicators, added to coatings for corrosion sensing: Layered double hydroxides, silica nanocapsules and polymeric microcapsules (chitosan). Characterized by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopies.
The use of traditional corrosion inhibitors in paints and coatings continues to be challenged from both an environmental and performance aspect. End users are demanding better corrosion performance and in many cases this cannot be achieved with traditional zinc or chromate type inhibitors. The use of VCIs(vapor corrosion inhibitors) in coating formulations has shown that in many systems they can replace the older technology or significantly improve the performance of the system by working in synergy with the existing inhibitors.
To restrain the failure of plate heat exchanger in customer boiler working fluid, the effect of crevice former type on the corrosion behavior of Type 316L (UNS S31603) stainless steel plate was investigated using electrochemical methods and surface analysis in chloride-containing synthetic tap water.
Corrosion protection of large structures such us wind turbines or offshore platforms operating in corrosive seawater environment is usually provided by cathodic protection (CP) and/or protective coatings. However those methods have some limitations. Organic coatings without CP can provide protection to steel substrate only when they remain intact whereas sacrificial anodes can considerably increase the overall mass of the protected structure and have to be replaced periodically. Moreover sacrificial anodes are only effective under submerged conditions and don’t protect the structure under alternating wetting and drying condition so-called “splash zone” which is particularly corrosive environment due to constant splashing of highly aerated seawater UV radiation and increased concentration of seawater constituents during drying. Furthermore confined volume of electrolyte easy access to oxygen and atmospheric pollutant deposited on the metals’ surface lead to more severe corrosion in this region than in the submerged zone.An alternative corrosion mitigation method is application of thermally sprayed metallic coatings such as thermally sprayed aluminium (TSA). TSA affords long-term and maintenance-free protection to steel substrate in two ways. Firstly when intact it acts as a barrier to the corrosive environment and secondly it provides sacrificial protection by working as an evenly distributed anode which preserves steel in case of a damage of a coating. Moreover large operating temperature range high resistance to mechanical damage and low corrosion rate in ocean water make it a perfect corrosion prevention method for offshore applications.One of the characteristic features of thermally sprayed coatings is porosity which is filled with corrosion products when the corrosion progresses. To delay the self-corrosion of the protective coating application of sealers is recommended.In this work the behaviour of several arc-sprayed metal coatings is investigated under full artificial seawater (ASTM D1141) immersion and compared with simulated splash zone conditions under droplets of artificial seawater. Effectiveness of TSA coatings is evaluated using electrochemical techniques and corrosion products are examined. The effect of novel sealers containing nanomaterials is also assessed.
Thiourea and its derivatives have been studied for several decades for their corrosion inhibition characteristics with steels. However their interaction mechanisms with steel are not well understood. Such an understanding may be vital for design of novel thiourea derivatives as corrosion inhibitors for steels. We have carried out state-of-the-art density functional theory (DFT) computations and experiments (weightloss polarization and electrochemical impedance) to understand the effect of thioureas on corrosion of steel. The interactions of several thiourea derivatives viz. thiourea dibutyl thiourea acetyl thiourea diphenyl thiourea tetramethyl thiourea and ethylene thiourea have been investigated. DFT indicates strong interactions between Fe atoms of the surface and C S - atoms of the molecule while the N-atoms do not show any significant proclivity towards interactions with the Fe-surface. Among the hollow on-top and bridge sites on the Fe-surface S-atom was found to preferentially occupy the bridge site which facilitates formation of two strong Fe-S bonds which possibly explains the strong corrosion inhibition by thioureas on steels. Besides DFT computations provided insights for design of much more promising molecules for corrosion inhibition. Experimentally dibutyl thiourea was found to have an extraordinary inhibition effect for steel in 1N HCl in accordance with DFT computations.Keywords: Corrosion inhibitors thiourea derivatives DFT experiments