Stainless steel thread forming screws and their susceptibility to stress corrosion cracking

Abstract

Thread forming fasteners incorporate both drilling and hole tapping features and are commonly used in the construction industry to fix together steel sheets of different material types. With this practical application in mind, fasteners manufactured from martensitic and austenitic stainless steels have been subjected to alternating corrosion conditions in accordance with test standards DIN 50021-SS and DIN 50018-K WF 2·0. The torque applied to the screws during these tests was controlled to place the fasteners under equal tensile loads, independent of their tensile strength. Thus, the results provided information on their relative susceptibilities to stress corrosion cracking. At the end of the tests, up to 80% of the martensitic stainless steel drilling and tapping screws had failed due to hydrogen induced stress corrosion cracking. The fasteners manufactured from austenitic materials withstood identical test conditions without any evidence of cracking or crack initiation. It is concluded that fasteners manufactured from modified martensitic stainless steel are more susceptible to stress corrosion cracking under the conditions of test than those made from cold worked austenitic stainless steels. This suggests that in practical applications the potential for catastrophic failure due to stress corrosion cracking could be considerably higher in modified martensitic fasteners in comparison with austenitic stainless steel fasteners, including those with hardened carbon steel drill points.     

Corrosion of stainless steels in marine conditions containing sulphate reducing bacteria

Abstract

The corrosion behaviour of a range of austenitic and duplex stainless steels has been studied in sea water containing sulphate reducing bacteria under conditions where the influence of an external aerated steel surface is absent. By means of a combination of anodic polarisation, cathodic polarisation, galvanic coupling experiments, microscopy, and microanalysis, the complex corrosion behaviour has been rationalised. It has been demonstrated that severe corrosion, involving a number of localised and general features, can occur in the low oxygen conditions, and good correlation has been obtained between accelerated electrochemical tests and long term corrosion occurring without the intervention of electrochemical monitoring.     

Potentiostatic Study of the Corrosion Behaviour of Austenitic Stainless Steel Weld Metal

Abstract

The corrosion behaviour of austenitic stainless steel weld metals has been studied potentiostatically with particular reference to marginally and moderately oxidising conditions. Sample welds covering the range of material composition and welding process in industrial use were employed.

Corrosion behaviour has been shown to be primarily determined by weld metal composition and micro-structure, choice of welding process having no significant effect. Susceptibility to a number of different types of preferential attack has been observed, and the practical significance of these has been considered.     

Intergranular corrosion resistance of stainless steel diffusion‐bonded joints with B‐containing amorphous insert metal

Summary

This paper describes an investigation of the intergranular corrosion resistance of austenitic stainless steel joints diffusion‐bonded with an amorphous insert metal containing 1.5 mass% boron, in order to clarify optimum bonding conditions for austenitic stainless steel pipes joined by a high‐speed diffusion bonding system. The sulphuric acid/copper sulphate test is conducted to evaluate the intergranular corrosion of bonded joints caused by a Cr depletion zone. Joints diffusion‐bonded with lower bonding pressure and shorter bonding time are more sensitive to intergranular corrosion. Under such bonding conditions, Cr boride and Cr carboboride precipitates causing selective corrosion are found in the bonding layer. Joints diffusion‐bonded under higher bonding pressure and with longer bonding time, however, show no intergranular corrosion and few precipitates in the bonding layer. With regard to the B‐containing amorphous insert metal used in the present investigation, the optimum bonding conditions for prevention of intergranular corrosion are a bonding temperature of 1473 K, bonding pressure of > 9.8 MPa, and bonding time of > 180 sec.     

Effects of erosion on corrosion of type 430 and 316 stainless steels in aqueous environments

Abstract

The effects of erosion on the corrosion of two types of stainless steel in aqueous 0·5M NaCl and 1M NaCl environments at ambient temperature have been studied using a modified rotating cylinder electrode system. Erosion by silica particles greatly increased the passive current density for both ferritic type 430 and austenitic type 316 stainless steels. It is suggested that two factors were largely responsible for the observed effects. The impact of the eroding silica particles on the specimen surface destroyed the passive film and removed corrosion products from the specimen surface; and impact with the fast moving silica particles generated numerous microplastic deformation sites on the metal surface which were activated owing to the presence of residual stress, dislocations and defects, etc., and the emission of low energy electrons.     

Microstructure and pitting corrosion of similar and dissimilar stainless steel welds

Abstract

Pitting Corrosion behaviour of similar and dissimilar metal welds of three classes of stainless steels, namely, austenitic stainless steel (AISI 304), ferritic stainless steel (AISI 430) and duplex stainless steel (AISI 2205), has been studied. Three regions of the weldment, i.e. fusion zone, heat affected zone and unaffected parent metal, were subjected to corrosion studies. Electron beam and friction welds have been compared. Optical, scanning electron microscopy and electron probe analysis were carried out to determine the mechanism of corrosion behaviour. Dissimilar metal electron beam welds of austenitic–ferritic (A–F), ferritic–duplex (F–D) and austenitic–duplex stainless steel (A–D) welds contained coarse grains which are predominantly equiaxed on austenitic and duplex stainless steel side while they were columnar on the ferritic stainless steel side. Microstructural features in the central region of dissimilar stainless steel friction welds exhibit fine equiaxed grains due to dynamic recrystallisation as a result of thermomechanical working during welding and is confined to ferritic stainless steel side in the case of A–F, D–F welds and duplex stainless steel side in the case of D–A welds. Beside this region bent and elongated grains were observed on ferritic stainless steel side in the case of A–F, D–F welds and duplex stainless steel side in the case of D–A welds. Interdiffusion of elements was significant in electron beam welding and insignificant in friction welds. Pitting corrosion has been observed to be predominantly confined to heat affected zone (HAZ) close to fusion boundary of ferritic stainless steel interface of A–F electron beam and D–F electron beam and friction weldments. The pitting resistance of stainless steel electron beam weldments was found to be lower than that of parent metal as a result of segregation and partitioning of alloying elements. In general, friction weldments exhibited better pitting corrosion resistance due to lower incidence of carbides in the microstructure.     

Behaviour of stainless steel in simulated concrete pore solution

Abstract

The localised corrosion behaviour of austenitic, martensitic, and duplex stainless steels has been studied in several solutions simulating the liquid present in the pores of both alkaline and carbonated concrete in the presence of chloride ions. The work aimed to evaluate the suitability of these types of stainless steel as rebars in reinforced concrete structures exposed to very aggressive environments. Electrochemical tests have been performed in solutions with pH values in the range 7·6-13·9, chloride concentration in the range 0-10%, and temperatures of 20 and 40°C. The adverse effect of a decrese in pH on the critical chloride content is discussed, as a function of stainless steel composition and temperature, and the inhibitive effect of high carbonate/bicarbonate concentrations is also shown.     

Improved Ferritic–Austenitic Stainless Steel

Abstract

For the efficient operation of chemical process plant it is of paramount importance to ensure as far as possible that the periods of closurefor maintenance and repair are minimised.

The arduous conditions encountered in many chemical plants as a result of progressive trends towards. higher yields, etc., have therefore highlighted the need for greater strength and pitting resistance than can be obtained in conventional austenitic stainless steels. This paper describes the development of ferritic/austenitic high chromium duplex cast stainless steels and demonstrates how, by addition of nitrogen and close control of composition, certain inherent drawbacks of duplex alloys can be overcome. The nitro gen-containing steel has an attractive combination of mechanical properties, corrosion resistance, ductility, weldability and castability, and has received application in a wide range of aggressive environments.     

Forthcoming corrosion events

Abstract

Pitting potentials have been measured and some gravimetric testing has been carried out on a series of experimental austenitic stainless steels with varying chromium, molybdenum and nitrogen contents. All three of these elements were found to contribute to the resistance to the initiation of pitting corrosion, and synergistic effects have been noted. The effect of nitrogen is especially potent in a steel with 22% chromium and 3% molybdenum.     

Microstructure,hardness and residual stress distribution in maraging steel gas tungsten arc weldments

Abstract

The distribution of residual stresses due to welding has been studied in maraging steel welds. Gas tungsten arc welding process was used and the effect of filler metal composition on the nature of residual stress distribution has been investigated using X-ray diffraction technique with Cr K_α radiation. Three types of filler materials were used, they include: maraging filler, austenitic stainless steel and medium alloy medium carbon steel filler metal. In the case of maraging steel weld, medium alloy medium carbon filler, the residual stress at the centre of the weld zone was more compressive while, less compressive stresses have been identified in the heat affected zone of the parent metal adjacent to the weld metal. But, in the case of austenitic stainless steel filler the residual stresses at the centre of the weld and heat affected zone were tensile. Post-weld aging treatment reduced the magnitude of stresses. The observed residual stress distribution across the weldments has been correlated with microstructure and hardness distribution across the weld.     

Studies on Stress Corrosion Cracking of Super 304H Austenitic Stainless Steel

Stress corrosion cracking (SCC) is a common mode of failure encountered in boiler components especially in austenitic stainless steel tubes at high temperature and in chloride-rich water environment. Recently, a new type of austenitic stainless steels called Super304H stainless steel, containing 3% copper is being adopted for super critical boiler applications. The SCC behavior of this Super 304H stainless steel has not been widely reported in the literature. Many researchers have studied the SCC behavior of steels as per various standards. Among them, the ASTM standard G36 has been widely used for evaluation of SCC behavior of stainless steels. In this present work, the SCC behavior of austenitic Fe-Cr-Mn-Cu-N stainless steel, subjected to chloride environments at varying strain conditions as per ASTM standard G36 has been studied. The environments employed boiling solution of 45 wt.% of MgCl₂ at 155 °C, for various strain conditions. The study reveals that the crack width increases with increase in strain level in Super 304H stainless steels.     

Electrochemical behaviour of Type 316 austenitic stainless steel weld metals with and without microfissures

Abstract

The corrosion performance of austenitic stainless weld deposits with and without microfissures has been evaluated by cyclic polarisation testing in 3·5% sodium chloride solution at room temperature to define the electrochemical behaviour. The corrosion test results revealed that the presence of microfissures in the weld deposit significantly degrades corrosion resistance. Corrosion attack preferentially occurred at the tips of microfissures in fissure-containing samples and at the interpass boundaries for fissure-free samples.     

Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO₃ + 25%H₂O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel.     

Corrosion performance of selected stainless steels in brine from reverse osmosis desalination plants

Abstract

The corrosion performances of some commercial stainless steel alloys in the brine reject solution from a reverse osmosis sea water desalination plant was studied in terms of their pitting susceptibilities (investigated under aerated conditions at ambient temperature using a cyclic polarisation technique) and crevice corrosion resistances (evaluated in the plant over a 3 month exposure period using multiple crevice test assemblies). The alloys used were four austenitic steels, UNS S31603, UNSS 31703, UNS N08904, UNS S31254, a ferritic steel UNS S44635, and a duplex steel UNS S32550. Cyclic polarisation studies show that the pitting or breakdown potentials for S31603 and S31703 occurred at more active values than for N08904, S31254, S44635, and S32550 alloys, and indicated a reduced resistance to pitting corrosion. The multiple crevice tests show that the alloys S31603, S31703, and N08904 do suffer crevice corrosion in the brine reject solution at ambient temperature, while the S44635 S32550, and S31254 alloys showed considerably higher crevice corrosion resistance.     

Spannungsrißkorrosion nichtrostender austenitischer Chrom-Nickel-Stähle bei Raumtemperatur

Stress corrosion cracking of austenitic chromium-nickel stainless steels at ambient temperature For the chloride-induced SCC with transgranular crack path in austenitic 18Cr10Ni stainless steel, a critical temperature between 45 and 50°C exists. This critical temperature, however, is valid only for the passive state of the steel in nearly neutral, chloride-containing aqueous environments. In the active state, SCC with transgranular crack mode can occur at temperatures down to ambient temperature. The active state is caused by highly acidic, high-chloride containing aqueous corrosive media. Adherent aqueous films with these properties can grow on the surface of structural components inside swimming-pools when the water is disinfected by addition of chlorine. Under these conditions, failure of austenitic CrNi and CrNiMo stainless steels by SCC with transgranular crack path at ambient temperature is possible and actually occurred. SCC with preferentially intergranular crack path can also occur at ambient temperature when austenitic stainless steels with a sensitized microstructure are used. Under these conditions, the corrosion attack is caused by non-specific aggressive environments, e.g., adherent aqueous films not containing chloride ions. The crack mode, intergranular or mixed, depends on the stress level.     

Effect of nitrogen addition to shielding gas on residual stress of stainless steel weldments

Abstract

The objective of the present study was to investigate the effect of nitrogen additions to the shielding gas on the ferrite content and residual stress in austenitic stainless steels. Autogenous gas tungsten arc (GTA) welding was applied on austenitic stainless steels 304 and 310 to produce a bead on plate weld. The delta ferrite content of the weld metals was measured using a Ferritscope. The residual stress in the weldments was determined using the hole drilling strain gauge method. The present results indicated that the retained delta ferrite content in type 304 stainless steel weld metals decreased rapidly as nitrogen addition to the argon shielding gas was increased. The welding residual stress increased with increasing quantity of added nitrogen in the shielding gas. It was also found that the tensile residual stress zone in austenitic stainless steel weldments was extended as the quantity of added nitrogen gas in the argon shielding gas was increased.     

Corrosion of stainless steels under heat transfer conditions in nitric acid

Abstract

Novel test rigs are described for the study of the corrosion of metal specimens under controlled heat fluxes. In the corrosion of stainless steels in nitric acid, tests at various heat fluxes with steel surface temperature kept constant have shown that the cooler acid present at the surface under higher heat fluxes leads to slightly smaller corrosion rates than under isothermal conditions. Crevice corrosion can develop under the gasket sealing the stainless steel specimen to the test cell. This crevice corrosion can produce enhanced corrosion rates (by factors up to 100), not only on surfaces within the crevice, but also on those external to the crevice. The factors influencing the development of crevice corrosion are discussed.     

Application limits of stainless steels in the petroleum industry

Stainless steels have a great variety of potential applications in the petroleum industry, mainly as an alternative to carbon steel in corrosive environments. Within a number of media that can cause corrosion problems with these materials, only chloride solutions and hydrogen sulfide are of importance in oilfield service. A reliable tool that permits the proper selection of stainless steels has yet been missing. In order to provide engineering diagrams for this purpose, pitting and stress corrosion cracking (SCC) tests were performed. Specimens were exposed to NaCl solutions containing from 3 to 100,000 ppm Cl^? at temperatures from 40 to 200 °C. This test configuration was chosen to give a better representation of actual service conditions than accelerated standard test procedures do. Tested materials were the austenitic stainless steel grades 321, 316Ti (API LC30‐1812) and 254 SMO, and 22Cr duplex (austenitic‐ferritic) steel (API LC65‐2205). Based on an optical examination of the specimens, no‐risk regions of chloride concentration vs. temperature have been identified. Subsequently, service temperature limits have been deduced for each tested material. Thus, material failures by pitting and SCC can be prevented without overdesigning. The results of the testing series are applicable to all chloride environments without presence of H₂S, as they have to be handled by primary production equipment, as well as transportation and gas processing facilities.     

Localised Corrosion of Stainless Steels During Food Processing

Abstract

The austenitic stainless steels are widely used in the food industry as constructional materials for processing plant. One of their drawbacks is a susceptibility to localised attack, particularly in the presence of the chloride anion. The various forms of localised attack which stainless steels can suffer, including pitting, crevice attack, deposit attack, stress corrosion cracking and corrosion fatigue, are illustrated with case histories drawn from the food processing industry.     

Effect of temperature on the galvanic corrosion of a high alloyed austenitic stainless steel in its welded and non-welded condition in LiBr solutions

Galvanic corrosion generated by the coupling between the austenitic stainless steel Alloy 926 (UNS N08926) and the welded Alloy 926 has been studied by means of electrochemical methods. The materials have been tested in highly concentrated LiBr solutions at different temperatures. The effect of Li₂CrO₄ as corrosion inhibitor has also been evaluated. Galvanic corrosion has been studied under open circuit conditions using a zero-resistance ammeter (ZRA). Results have demonstrated the poor severity of the coupling between the Alloy 926 and the welded metal in the studied conditions. The probability of localized corrosion increased with temperature and concentration, as the galvanic current density and galvanic potential data demonstrated.