Showing papers in "Corrosion Engineering Science and Technology in 2003"

Holistic model for atmospheric corrosion Part 1 - Theoretical framework for production, transportation and deposition of marine salts

Abstract: This paper discusses the limitations of parametric modelling of corrosion and presents the case that accurate and flexible models of atmospheric corrosion require an 'holistic' approach. In such an approach, the processes controlling corrosion across a wide range of physical scales are modelled. These models are based as much as possible on the processes controlling the phenomena under consideration. Being fundamentally based, these models can be extended outside the data sets from which they are derived. This is the first of a series of papers outlining the use of this approach to predict corrosion in marine environments. It will consider the theoretical formulations required to model the production, transport and deposition of marine salts. It will present some predictions from these formulations and it will discuss the implications to corrosion of this theoretical understanding. Later papers will present the incorporation of these models into an information system and the validation of these mo...

Holistic model for atmospheric corrosion: Part 2 - Experimental measurement of deposition of marine salts in a number of long range studies

Abstract: This paper is the second in a series looking at understanding the factors controlling and predicting marine aerosol concentration on land. It looks at results from three transects across the Australian continent. In each transect, the airborne salinity was measured, using the wet candle method at distances from 10 m to 40-300 km from the coast. The positions of the transects were selected to give a significant variation in the factors controlling salt production and transport. For example, one transect in South Australia was established where both high whitecap activity is likely to promote salt production and flat terrain and prevailing winds are likely to favour transport. Another, in Queensland, was established where calm seas will limit salt production and very seasonal winds and high relative humidity and rainfall will limit transport. On the basis of this experimental study, the general validity of the fundamental concepts put forward in Part 1 is assessed. Further, the feasibility of buildi...

Review: Corrosion and cracking of weldable 13 wt-%Cr martensitic stainless steels for application in the oil and gas industry

Abstract: A critical review of the literature has been undertaken to establish the current state of knowledge and understanding of the corrosion and stress corrosion cracking (SCC) of 13 wt-%Cr martensitic stainless steels for applications in the oil and gas industry. Corrosion data for 13%Cr martensitic stainless steels are critically evaluated to establish the conditions in which the various steels are in the active state, pitting or passive state. There is evidence that as welded specimens have poorer pitting resistance at ambient temperatures than at elevated temperatures owing to the formation of an oxidised layer, perhaps depleted in chromium. Data for hydrogen uptake and diffusivity in 13%Cr steels are reviewed. The results suggest that incorporation of Mo in the steel reduces the subsurface hydrogen content when the steel is in the active state, suggesting repressed dissolution kinetics. The data in the literature on SCC of modified 13%Cr steels indicate that increasing the temperature at pH values ...

Corrosion assessment of type 304L stainless steel in nitric acid

Abstract: AISI type 304L stainless steel (SS) was assessed for its corrosion resistance in nitric acid solution. The effect of variation in nitric acid concentration (1 to 10N) and temperature (298 K, 313 K, 333 K, 348 K) on the corrosion resistance of 304L SS was investigated using potentiodynamic polarisation studies. The corrosion resistance of the alloy was also studied in 1N nitric acid containing different concentrations of chloride ion (1000, 5000, and 10 000 ppm) in order to understand the tendency to pitting corrosion. The results show that the corrosion resistance of the material deteriorated for acid concentrations greater than 4N. The temperature of the acid was found to have a profound effect on the corrosion resistance, with corrosion becoming more prominent at temperatures of 333 K and above. Chloride ions up to a concentration of 10 000 ppm were found to have no effect on the pitting resistance of the material studied.

Inhibitor pretreatment synergies demonstrated using a scanning Kelvin probe technique

Abstract: A scanning Kelvin probe technique has been used to study the influence of pretreatment and pigment interaction on the kinetics and mechanism of corrosion driven delamination processes affecting model polyvinyl butyral coatings adherent to the intact zinc surface of hot dip galvanised steel. The effects of incorporating dispersions of strontium chromate (SrCrO4), silica and bentonite based Cerium (III) cation exchange pigments within the polyvinyl butyral coating are investigated on galvanised steel specimens in the absence and presence of a mixed oxide/chromate rinse surface pretreatment. Placing aqueous sodium chloride electrolyte onto a penetrative coating defect establishes an electrochemical delamination cell in which cathodic O2 reduction at the delamination front is coupled to anodic zinc dissolution at the coating defect by a thin layer of electrolyte which ingresses beneath the delaminated polyvinyl butyral film. The SrCrO4 pigmented polyvinyl butyral films allow CrO42- diffusion directly ...

Relationship between microstructure and corrosion resistance in Zn-Al alloy coated galvanised steels

Abstract: The effect of cooling rate on the coating microstructure and corrosion resistance of Zn - 4.5 wt-%Al alloy galvanising coatings applied to 0.7 mm gauge strip steel substrates is presented. Increasing the cooling rate by the use of a high powered air cooler does not significantly alter the volume fraction of primary zinc in the alloy coating layer, which remains at ~ 14 vol.-%. There is, however, a dramatic change in the number and size of dendrites with far greater numbers (changing from 270 to 1500 mm2) of smaller dendrites as the cooling rate is increased using cooler outputs of 55 and 100%, respectively. This corresponds to a change in the surface appearance of the material with the surface spangle (i.e. eutectic cell) size being reduced from 7.5 to 2.1 mm as the cooling rate is increased. Since the volume fraction of the primary zinc remains roughly constant and the dramatic change is in the size and numbers of dendrites, it appears that this phenomenon is related to increased nucleation of p...

Holistic model for atmospheric corrosion: Part 3 - Effect of natural and man made landforms on deposition of marine salts in Australia and south-east Asia

Abstract: This paper present results from a large number of airborne salinity measurements in Australia and south-east Asia (SEA). Salinity measurements were made in the open on clear ground, in the open in urban regions, and on the facades of buildings. The data were used to assess the validity of the concepts regarding saline sources and transport presented in previous papers. In particular, the salinity measurements in SEA are used to assess the validity of the models for ocean produced salt aerosol. A classification scheme for defining sheltering by man made and natural landforms and types of water bodies was developed. The data were then used to assess the sheltering effect of urban landforms and of building facades. Lastly, the data were used to assess the possible contribution of narrow and wide bays to airborne salinity.

Corrosion of thermally sprayed zinc and aluminium coatings in simulated splash and tidal zone conditions

Abstract: The corrosion rate in the splash and tidal zones of marine steel structures is very severe. Thermally sprayed coatings of zinc or aluminium and their alloys are among the most important technologies to eliminate this problem. This experimental study on the protective properties of thermally sprayed zinc and aluminium coatings on steel piles was conducted in an apparatus for simulating the marine corrosion environment. Particular attention was given to the anticorrosion performance of thermally sprayed zinc and aluminium coatings in the splash and tidal zones in combination with the cathodic protection of bare steel in the immersed zone. The experimental results showed that top to bottom thermal spraying of zinc or aluminium coatings on steel piles before their installation was effective, but that under cathodic protection conditions, protection of thermally sprayed zinc or aluminium coatings is effective up to the average middle tidal zone.

Overview Steam turbines: Part 1 – Operating conditions and impurities in condensates, liquid films and deposits

Abstract: A review of the literature and discussions with plant operators have provided the basis for this overview of present knowledge and understanding of the chemistry of the condensates formed on steam turbines and the link to system operation. This has been supplemented by Part 2 of this study: a review of the effect of test variables on environment assisted cracking, which is to be published in Corrosion Engineering, Science and Technology. Steam turbine operating conditions are described including the steam cycle, chemical control of the steam/water circuit and the distribution of temperature, pressure and steam wetness in the turbine. Service experience indicates that cracking occurs mainly in regions where condensates form. The saturation temperature, at which condensation occurs, varies from station to station, depending on the turbine design and operating conditions. Calculations based on thermodynamic data under steady flow conditions show that the concentration of non-volatile species (e.g. ch...

Corrosion of aluminium in perchloric acid in presence of various inorganic additives

Abstract: The corrosion of aluminium in 1M HClO4 at 40° C has been investigated using the potentiodynamic polarisation technique. The effect of various inorganic additives has been investigated. It was found that NO3-, NO2- and SCN- anions exhibit an inhibiting action on acid corrosion of aluminium by reducing the general corrosion rate as well as raising the pitting potential Ep . It was found that CrO42- ions were unique in demonstrating a remarkably high inhibiting action but their influence on the pitting potential was less pronounced than that of the group above. On the other hand, added halides induced an accelerating effect on the corrosion rate and lowered the pitting potential. However, SO42- ions do not show any significant effect on the corrosion behaviour of Al in 1M HClO4. The results have been interpreted on the basis of the adsorption mechanism followed by incorporation into the native film.

Corrosion behaviour of duplex stainless steel weld metals with nitrogen additions

Abstract: The addition of nitrogen to duplex stainless steel weld metal has been achieved and the resultant weld metals have been assessed for their microstructural features and corrosion behaviour. It was found that, although autogenous welding of duplex stainless steels is not recommended, the addition of nitrogen through the shielding gas mixture can help to obtain quality welds with the desired phase balance and corrosion resistance.

Localised coating failure of epoxy coated aluminium alloy 2024–T3 in 0.5M NaCl solutions: comparison of conventional electrochemical techniques and microelectrochemical methods

Abstract: The formation of red blisters is an important mode of coating failure on epoxy coated aluminium alloy (AA) 2024–T3 exposed to chloride solutions. In order to achieve a better understanding of the mechanisms behind blister formation and growth and to understand the influence of the environment, impedance spectroscopy with and without microelectrodes was applied to the localised study of coating defects on AA 2024–T3. The results allowed features observed in regular impedance spectroscopy to be related to local phenomena and thereby allowed validation of the equivalent circuits used for data interpretation. In addition the electrochemical properties of smaller blisters and of uncorroded substrate areas could be determined separately on the same specimen. Such data are not accessible via regular electrochemical impedance spectroscopy experiments if a large blister is present. The data proved that the two semicircles observed at low frequencies in a regular impedance spectrum solely reflect the corros...

Technical note: Anticorrosive behaviour of octadecylamine for protection of boiler surfaces

Abstract: The anticorrosive behaviour of octadecylamine (ODA) films has been investigated via weight loss and electrochemical methods. Tests were conducted on specimens of carbon steel, brass, and austenitic...

Overview Steam turbines Part 2 - Stress corrosion cracking of turbine disc steels

Abstract: Stress corrosion cracking data for steam turbine disc steels are reviewed critically. The effect of material properties (steel cleanliness, yield strength), environment (oxygen, carbon dioxide, chloride level), temperature, and stress conditions, on the initiation and growth of stress corrosion cracks is assessed. For long cracks, growth rates are perceived to be independent of oxygen and CO2 concentration, and steel cleanliness, but the data are not wholly consistent. Unquestionably, the variables having the most dramatic effect are strength level and temperature. For medium strength disc steels, dissolved gases and steel cleanliness influence the 'initiation' of stress corrosion cracks, certainly in relation to pit development and growth, but the impact on short crack growth is less well established.

Effect of cyclic stresses on stress corrosion cracking of Cu–Ni alloy

Abstract: The author has studied several failures in the field of bent and branched copper-nickel pipelines exposed to marine environments contaminated by ammoniacal byproducts. The cause of the failures was investigated by laboratory tests on the Cu-5.37 wt-%Ni alloy used for the failed pipes. The tests were conducted under slow strain rate testing (SSRT) conditions, in aqueous ammonia and ammoniacal sea water environments. These studies revealed that the presence of ammonia in sea water impairs the load bearing capacity of the alloy, and the aqueous ammonia environments tend to cause brittle stress corrosion cracking failures, which are often premature and/or catastrophic. However, in addition to induced residual stresses from manufacturing/processing, the operating conditions in marine environments subject these pipes to external forces and widely varying pressures and fluid flow rates, which can cause both static and cyclic stresses. Experiments conducted under SSRT conditions, however, could not predic...

Corrosion behaviour of Ta-10 wt-%W alloy in sodium hydroxide solutions

Abstract: The corrosion behaviour of the alloy Ta–10W has been investigated in sodium hydroxide solutions at different temperatures, using open circuit potential measurements together with potentiodynamic polarisation and polarisation resistance studies and electrochemical impedance spectroscopy. The Ta–10W alloy showed passive behaviour in 5, 10 and 15 wt-%NaOH at 25 ° C, in 5 and 10 wt-%NaOH at 50 ° C, and in 5 wt-%NaOH at 75 ° C. In 10 wt-%NaOH at 75 ° C, 15 wt-%NaOH at 50 and 75 ° C and 30 wt-%NaOH at all temperatures, the Ta–10W alloy remained in the active state after the spontaneous dissolution of its superficial air formed oxide. In all cases, the corrosion rates increased with increasing NaOH concentration and temperature.

Relationship between corrosion resistance, microstructure and cobalt content in a Zn–Co alloy electroplated from alkaline electrolyte

Abstract: Various protective methods may be used to improve the corrosion resistance of steel, and the application of coatings of zinc and its alloys is one of them. The efficiency of zinc in corrosion protection is due to its behaviour as a sacrificial anode. To enhance the corrosion protection, zinc has been alloyed with more noble metals such as cobalt, nickel and iron. In this work zinc-cobalt alloys were electrodeposited onto steel from an alkaline electrolyte. The investigation was carried out on electrodeposits with low and high cobalt contents. An in situ study was performed using a TEM equipped with a hot stage to relate the heat treatment with changes in structure and, consequently, corrosion behaviour. The images of the specimens as deposited and after heat treatment showed the presence of a phase that was identified as Co5Zn21. However only the diffraction pattern of the heat treated specimen fitted the strongest lines for this compound. The corrosion tests showed differences in the corrosion re...

Ethanolamine morpholine oleate as corrosion inhibitor for mild steel in acid solutions

Abstract: Ethanolamine morpholine oleate was prepared by direct esterfication and ring closure of triethanolamine monooleate. The structure was characterised by elemental microanalysis and Fourier transform infrared spectroscopy. The product was evaluated as a corrosion inhibitor for mild steel in 10 vol.-%HCl and 10 vol.-%H2SO4 solutions. The behaviour of the product was studied in various concentrations (50-250 ppm) at different temperatures (25, 45, and 60°C) using a potentiodynamic technique. The inhibitor proved its effectiveness in both HCl and H2SO4 solutions, with its efficiency in HCl exceeding that in H2SO4 solution. NaCl in the solution exhibited a synergistic effect on the inhibition of mild steel in H2SO4 solution. The inhibition efficiency increased with increasing concentration and decreased with increasing temperature.

Effect of aluminium on electrochemical corrosion behaviour of Fe-0.5C and Fe-1C alloys in 0.25M H2SO4

Abstract: Electroslag remelted iron – aluminium alloys, with carbon contents of 0.5 and 1 wt-%, were used to investigate the effect of Al on the electrochemical corrosion behaviour in 0.25M sulphuric acid solution. The alloy compositions were (wt-%): (1) Fe – 16Al – 0.5C, (2) Fe – 10Al – 0.5C, (3) Fe – 8Al – 0.5C, (4) Fe – 16Al – 1C, (5) Fe – 10Al – 1C, and (6) Fe – 8Al – 1C. Lowering the Al content of the alloys resulted in a deterioration of the passivation behaviour during potentiodynamic polarisation testing. This deterioration was attributed to the reduced Al content of the matrix phase on lowering the Al content of the alloy. Examination of the polarised surfaces via SEM revealed that in high Al content alloys carbides are preferentially attacked, while in low Al content alloys the attack is more general. Alloys with a carbon content of 1 wt-% displayed somewhat inferior passivation tendencies compared with those of alloys containing 0.5 wt-% carbon.

Damage to gold surfaces induced by cathodic polarisation

Abstract: Prolonged cathodic polarisation of gold and gold alloy electrodes in the hydrogen evolution potential range is a typical operating condition in electroplating applications. Electrodic hydrogen evolution gives rise to superficial failures such as blistering and cracking. These types of damage can eventually cause electroformed objects to fail by macroscopic cracking. The development of these defects is connected with the evolution of the morphology and of the crystallographic and electronic properties of the electrode. In particular, progressive roughening can be detected, giving rise to unexpected and irreproducible behaviour of cathodic systems based on gold and its alloys. In this paper the time dependent behaviour of cathode modification brought about by hydrogen evolution, giving rise to morphological and related electrocatalytic effects, is investigated. The study is based on in situ and ex situ optical microscopy and electrochemical measurements (cyclic voltammetry and impedance spectroscopy...

In situ study of phosphate diffusion in epoxy coatings using attenuated total reflection infrared spectroscopy

Abstract: Attenuated total reflection infrared spectroscopy (ATRIR) has been used to monitor the ingress of water and other low molecular weight, infrared active, species into an epoxy polymer coated onto zinc selenide (a high refractive index, infrared transparent substrate). Such a situation effectively models a coated metal and allows the probing of water migration to the coating/substrate interface entirely in situ. Results of diffusion transients for water were validated with gravimetric data and show two stage diffusion at lower temperatures and single stage Fickian diffusion at higher temperatures. The diffusion coefficients (2×10-9-6×10-9cm2s-1) for water migration obtained in this way are consistent with previous literature values generally obtained using gravimetric methods. The ATRIR method was then applied to study the diffusion of infrared active inhibitor species in epoxy. Although, many such species have a poor or obscured response between 4000 and 400 cm-1, phosphate absorbs sufficiently, an...

Quantisation of hydrogen gas formed by aqueous corrosion of metallic uranium

Abstract: Three unirradiated experimental breeder reactor II blanket fuel samples containing depleted uranium metal (238U) were corrosion tested in simulated J13 well water at 90°C. The purpose was to study the products of the aqueous corrosion of U metal, i.e. UO2, UO2 + x, UH3, and H2. The corrosion products were weighed for mass balance and analysed by X-ray powder diffraction. The data showed that UO2 powder, as well as higher oxides, i.e. UO2 + x had formed. However, no UH3 was detected. The corrosion rate of uranium metal in water at 90°C was inferred by collecting and quantitatively measuring the hydrogen gas evolved.

Stress corrosion crack initiation and propagation in longitudinally welded 304 austenitic stainless steel

Abstract: The stress corrosion cracking (SCC) initiation and propagation behaviour of a longitudinally welded 304 austenitic stainless steel has been investigated in an aqueous solution of 1M NaCl + 05M HCl The SCC in the base metal and heat affected zone (HAZ) initiates and propagates perpendicular to the applied tensile stress and the cracks initiate and propagate, initially, in the transgranular mode The crack growth mechanism changes into the intergranular mode when the crack size attains a particular value The cracks in the fusion zone are likely to initiate and to propagate in planes inclined at an angle of ~30° to the maximum principal tensile stress Optical microscopy and SEM observations show that the cracks propagate along the boundary of dendrites where δ ferrite exists, indicating that δ ferrite tends to dissolve preferentially In spite of the difference in microstructure, no remarkable difference is found in the kinetics of crack initiation and propagation among the three zones investigat

Corrosion inhibition of steel in neutral chloride solutions by mixtures of N-phosphono-methylglycine with zinc ions

Abstract: Steady state current–voltage curves were combined with electrochemical impedance measurements in order to investigate the corrosion inhibition of a carbon steel rotating disc electrode by N-phosphono-methyl-glycine (NPMG)/Zn2+ mixtures with a concentration ratio of 1:1 between NPMG and zinc cations, in chloride solutions. The NPMG/Zn2+ mixture retarded both the anodic and cathodic partial reactions. Its efficiency was better for a concentration of 40 ppm than with concentrations of either 10 or 100 ppm. At the corrosion potential, electrochemical impedance diagrams exhibited a high frequency loop, which was ascribed to the charge transfer process, and a low frequency loop that was related to diffusion across the protective film. Analysis of impedance spectra in terms of an equivalent circuit model showed that the corrosion behaviour of the metal–layer–solution system was strongly dependent on the physico-chemical properties of the corrosion products–corrosion inhibitor porous layer.

Electrochemical and spectroscopic investigation of synergistic effects in corrosion inhibition of aluminium bronze Part 2 – In acidified 4 wt-%NaCl solution

Abstract: Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy have been used to investigate changes of surface composition when 4-phenyl semicarbazide hydrochloride (PSC) is used as an inhibitor, with and without additions of KI, to suppress the corrosion of aluminium bronze in acidified 4 wt-%NaCl solution of pH 1.8 – 2 at 60 ° C. Anodic polarisation studies in the presence of the PSC inhibitor alone have shown oscillations of the corrosion current at elevated potentials owing to localised attack. The results of AES have shown that in the case of inhibition by PSC some depressions had formed on the surface, within which chloride species were present due either to localised attack by Cl- at weak points in the inhibitor film or to the adsorption of Cl- at the depressions, preventing the formation of a Cu – PSC protective film. In contrast, a synergistic effect between PSC and KI suppressed the occurrence of current oscillations and the development of surface depressions during anodic pola...

Influence of water radiolysis products on passive film formation and reduction in a mixed radiation environment

Abstract: Spallation neutron sources generate a mixed radiation environment when a beam of high energy particles (e.g. protons or deuterons) hits a heavy metal target. Radiolysis results when these primary and secondary (spallation) particles lose energy by Coulomb interaction with the electrons in the hydrogen and oxygen atoms of water. The results of cyclic voltammetry experiments on gold in sulphuric acid (pH 1.6) and exposed to a spallation environment were consistent with the presence of water radiolysis products. However, no changes in the Au OA1 - OA4 oxidation peaks or the OCIII reduction peak were observed in irradiated specimens. These findings indicate that proton irradiation and the resulting water radiolysis products do not influence passive film formation and reduction. Cyclic voltammetry experiments have also shown that a modest quantity of hydrogen bubbled into the solution had little influence on the concentration of radiolysis products that are produced during spallation near the electroch...

Electrochemical methods in corrosion research

Abstract: This was the 8th International Symposium in a series that began in Manchester during 1982 with a meeting organised by Dr Bob Cottis of UMIST, UK. Your correspondent was fortunate enough to attend that ® rst meeting, together with subsequent meetings in Trento (1997), in which Professor Bonora had a hand (this year`s winner of the European Corrosion Medal), and in Budapest in 2000. The current symposium was held in the delightful seaside town of Nieuwpoort and the chief organiser was Professor Jean Vereecken from Vrije Universiteit Brussel, ably assisted by Lieve Van Den Bossche. Within Nieuwpoort, Ysermonde is a holiday complex with a lake. There was a small exhibition held in what at ® rst sight looked like a `big top’. There was also a nice social programme; your correspondent will just mention the beer tasting evening, which was very pleasant. The `Electrochemical methods in corrosion research’ meetings normally adopt a fairly relaxed style, and this one was no exception with 81 oral presentations given in two parallel sessions spread over four days. There were also two poster sessions to allow study of the 29 posters. The majority of the papers appeared on the CD ROM as extended abstracts. This was a slight drawback compared with the previous symposium in Budapest where the full papers were published on the CD ROM, and the one before that in Trento, where the proceedings were published in a special issue of J. Electrochem. Soc. However, a large number of interesting papers were presented and your correspondent is sure that the authors would be able to provide reprints of related papers, if not the actual ones, on request. To assist in such requests, the email addresses of various authors who provided them on the CD ROM are provided in the addendum to this report. The list of topics, in alphabetical order, was as follows: atmospheric corrosion; conversion treatments; corrosion education; corrosion in concrete; corrosion inhibitors; electrochemical noise; inorganic coatings; local techniques; models and mechanisms; new techniques; organic coatings and passivation. In this report, about 30 eye-catching papers have been selected for comment. These include examples from each session with the exception of those on models and mechanisms and corrosion education.

Mathematical analysis of long term fouling corrosion in sea water

Abstract: The long term creature corrosion behaviour of carbon steel fouled by marine creatures has been investigated at three corrosion test stations located off China coast at Qingdao, Xiamen and Yulin station, in the Yellow Sea, East China Sea and South China Sea, respectively. Fouling by sea creatures can accelerate localised corrosion, but reduce the rate of general corrosion to great extent. The new concept of an environmental corrosion coefficient has been developed based on the analysis of fouling corrosion behaviour, and the revised equation of corrosion depth versus exposure time for carbon steel in sea water is in agreement with the field results. The new: equation allows prediction of the final corrosion rate after prolonged exposure.

Inhibiting effect of silicates on corrosion of low silicon alloyed steels in neutral non-oxidising conditions at 90° C

Abstract: Unalloyed and low alloyed steels undergo predictable uniform corrosion and are potential candidates for high level nuclear waste disposal containers. In this study, low alloyed silicon steel (< 5 wt-%Si) has been tested in a silicate containing corrosive solution at 90 ° C in order to establish whether the addition of silicon to iron would improve the resistance of the surface film to corrosion in an argillaceous (i.e. clayey) soil. Electrochemical tests were conducted for short term immersion of the steel coupons and showed that the addition of silicate to the corrosive solution decreased the corrosion rate, whereas the addition of silicon to the steel increased the critical passivation current peak. A comparison with an unalloyed carbon steel showed that the latter material was more easily passivated than the Si alloyed steel. Weight loss tests were used to study the long term behaviour of the steels in the corrosive medium at 90° C. The inhibiting effect of sodium silicate was significantly inc...

Technical note Corrosion and cracking under insulation of type 304 stainless steel at ambient temperature

Abstract: Corrosion pitting was found under insulation on type 304 austenitic stainless steel pipes used in an ethylene plant since 1980 to transport hydrocarbon products at – 170 ° C. Preliminary investigations established that the corrosion was confined to areas adjacent to the circumferential weld seams. Chlorides were present on the external surface of the pipes owing to ingress through defective insulation. Detailed examination of specimens from the affected pipes revealed that stress corrosion cracks had initiated in the corroded areas near the welds where defects and high residual stresses can be encountered. The cracks had initiated intergranularly before changing to a transgranular mode of crack propagation, typical of stress corrosion cracking of austenitic stainless steels in chloride environments. The evidence suggests that the cracks developed while the plant was not operating when the pipework would have been at ambient temperature.