Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 3: Elektrochemische Untersuchungen und Bewertungskriterien

Testing the corrosion protection of heat transfer fluids Part 3: Electrochemical tests and valuation criteria. The third part of the report demonstrates electrochemical criteria for the assessment of corrosion protection of inhibited antifreezes on glycol base. The measurements were carried out by means of a cylinder of revolution test plant under practice oriented conditions with iron, aluminium and copper materials. The polarisation resistance, the length of passive-range and hysteresis effects at the anodic part of current density-potential curves proved to be the most useful assessment criteria. The most reliable information concerning the inhibitor efficiency is given by a combination of all three criteria. Potentiostatic tests anodic to the rest potential confirm this further. According to this criteria the tested antifreezes offer a good corrosion protection to the metals investigated.     

Corrosion resistance of a new high strength interstitial free steel

Abstract

Potentiostatic polarisation studies have been carried out on a new high strength copper containing interstitial free steel for automotive applications under various industrially important processing conditions in 0·1M H₂SO₄ solution at room temperature. It is revealed that corrosion rate in the new steel is lower than that in the conventional mild and high strength interstitial free steels.     

Electrochemistry of deformed smooth surfaces and short corrosion fatigue crack growth behaviour

Abstract

The polarisation characteristics for a deformed smooth surface of a 0·2% carbon steel in an artificial sea water have been determined under static and cyclic loading conditions. The influence of strain level and loading frequency on anodic and cathodic Tafel constants, corrosion current density, and corrosion potential is described. Based on these data, the conditions for corrosion fatigue testing which corresponded to a maximum synergism between surface deformation and anodic dissolution were determined. Corrosion fatigue tests, which were conducted under both constant potential and constant current density conditions have shown that a process of metal dissolution plays a determining role in the short corrosion fatigue crack growth behaviour. An experimentally based criterion is proposed involving the development of a short corrosion fatigue crack, of characteristic size, which is associated with the spacing between the major microstructural barriers. This criterion is a function of both shear stress and the parameters controlling the anodic dissolution process on a cyclically deformed smooth suiface. An expression predicting the formation of short corrosion fatigue cracks is presented which takes into account the synergistic action of shear stress and the corrosion process.     

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.     

Localised effects of macrofouling species on electrochemical corrosion of corrosion resistant alloys

Abstract

Interactions between macrofouling and corrosion have been studied on two duplex stainless steels (UNS S31803 and UNS S32760), an austenitic stainless steel (UNS S31354), a nickel base alloy (UNS N06625), and a cobalt base alloy (UNS R30006) in long term tests conducted in natural sea water off the west coast of Scotland. After an 18 month exposure period, the specimens were heavily fouled primarily with barnacles and mussels and all the materials exhibited crevice corrosion at the specimen/resin interface, although this was less extensive on the nickel base alloy. Localised corrosion was observed under the base of live barnacles on UNS S31603 stainless steel. Direct current electrochemical anodic polarisation tests undertaken after the 18 month exposure period yielded unusually high currents in the range of potentials between the free corrosion value and the breakdown potential. However, long term exposure and associated macrofouling had little effect on the overall breakdown of general surface passivity as indicated by values of the breakdown potential. The higher currents were associated with the appearance, after the anodic polarisation, of black sulphide corrosion products at the specimen/resin crevices, around barnacles, and around mussel byssus threads.     

Untersuchungen der Spannungsrißkorrosion austenitischer Stähle durch saure Chloridlösungen bei Niedrigen Temperaturen

Investigation of stress corrosion cracking of austenitic steels in acid chloride solutions at low temperatures Tests were carried out on materials 1.4301, 1.4571, 1.4439 and 1.4558 in cold hydrocloric acid (c(Cl^?) = 1.5 mol/L and c(H⁺) = 1 mol/L, a few tests also at 0.1 and 0.01 mol/L). Chronopotentiostatic tests yielded data on active corrosion, passivity and pitting corrosion. CERT tests (10^?6s^?1, a few tests also at 2 · 10^?7s^?1) showed superposition of general corrosion on stress corrosion under free corrosion condition, while the rest potential was relatively negative in the active range. Oxygen purging has only a minor effect. The extent of cracking decreases with decreasing c(H⁺). In the case of cathodic polarisation straining induced surface notches occur which can be attributed to hydrogen induced effects. In the case of anodic polarisation pittings are generated without any crack initiation. Characteristic features of stress corrosion increase with decreasing strain rate. It follows from the results that high acid concentrations are necessary for stress corrosion cracking in the active state to occur. CERT tests cannot be used as an accelerated test for this kind of stress corrosion cracking. Few CERT tests carried out in warm NaCl solution did not show any stress corrosion cracking though these solutions are known to cause stress corrosion cracking after long periods. Furthermore, predamage in the form of pitting does not alter the situation.     

Microbially influenced corrosion on stainless steels in waste water treatment plants: Part 1

Abstract

Field tests on stainless steels have been carried out at five waste water treatment plants for one year. Three grades of stainless steel, i.e. AISI 304 (UNS S30400), AISI 316 (UNS S31600), and duplex 2205 (UNS S31803) were tested in the final settling tank in the plants. The time dependence of the open circuit potential (OCP) was measured for all coupons. Ennoblement of the OCP, similar to that reported from investigations in sea water, was found in one of the plants. Waters from three of the exposure sites, containing dispersed deposits from exposed coupons, were chemically analysed. Pitting corrosion was observed after the field test on steel grade AISI 304 in three of the five plants, and on AISI 316 in one plant. No corrosion was found on 2205 in any of the plants. Laboratory measurements of the OCP were carried out for the AISI 304, AISI 316, and duplex 2205 steels in water collected from one of the plants. Cathodic polarisation curves were recorded in waste water from the same plant. T he cathodic reaction rate increased at the highest OCP. Simulation of the ennoblement was carried out by potentiostatic polarisation in a 600 ppm chloride solution. The current response indicated corrosion of welded AISI 304 material and of AISI 304 and AISI 316 steels in crevice assemblies after a long induction time. Part 2 of this study presents the results of further testing and a risk assessment design.     

Polyaniline coating on 304 stainless steel by electropolymerisation from aqueous NaOH solution for corrosion protection in NaCl medium

Abstract

Polyaniline (PAn) coating was electrochemically synthesised on 304 stainless steel using cyclic voltammetry in 0·25M NaOH solution containing 0·1M aniline. Characterisation of the adhesive and stable PAn coating was carried out by cyclic voltammetry, Fourier transform infrared spectroscopy and scanning electron microscopy. The protective properties of PAn coating on 304 stainless steel were elucidated using linear anodic potentiodynamic polarisation and Tafel test in 3·5%NaCl solution. Linear anodic potentiodynamic polarisation test results proved that the PAn coating improved the degree of protection against pitting corrosion in NaCl solutions. Tafel test results showed that the PAn coating appeared to enhance protection for 304 stainless steel in 3·5%NaCl solution.     

Anodic Dissolution of Copper-Zinc Alloys in Alkaline Solutions

Abstract

Potentiostatic anodic polarisation measurements on brasses (100–60 wt.-% Cu) in 0 ·150 N-NaOH are described and compared with those of the pure components. The inapplicability of current theories on anodic dissolution of homogeneous binary systems is discussed, and a possible mechanism for anodic dissolution is suggested. Dezincification of the brass surface before and during anodic polarisation is described.     

Fatigue strength of welded joints in austenitic Cr-Ni steels

Abstract

High quality corrosion resistant Cr-Ni steels are used in many branches of industry. Constructors are faced with the problem of correctly dimensioning statically and dynamically loaded parts, but have little data available for dynamically loaded welded constructions of high alloy austenitic steels. To close this gap fatigue strength tests were carried out on three different steels and five different welded joints.     

Polarisationsmessungen an niedriglegierten Stählen in konzentrieten Nitratlösungen

Polarisation measurements on low alloy steels in concentrated nitrate solutions Current density potential curves were determined by potentiostatic measurements involving constant potentials and stepwise potential change for six low alloy steels in Ca(NO₃)₂ and NaNO₃ solutions. The instationary curves vary markedly with the polarisation technique and do not characterize the anodic behaviour of the materials. For Ca(NO₃)₂ solutions the polarisation measurements do not show any correlation with stress corrosion cracking behaviour. Similar results were also obtained in NaNO₃, solutions. Contrary to the situation in Ca(NO₃)₂ solutions, a potential range for resistance against intergranular corrosion types exists at about U_H = 0,5 V. This range is characterized by relatively high stationary current densities. For materials without this range of resistance, the stationary current densities are significantly smaller. The electrochemical properties of the system steel/nitrate solution are extremely complicated. The free corrosion potential varies between the potential ranges for passive and transpassive corrosion. For anodic polarisation, the potential ranges of susceptibility as well as resistance against intergranular corrosion types follow each other.     

Corrosion behaviour of NiTi endodontic instruments

Abstract

The present study investigates the influence of the sterilisation treatment conducted at 50°C in 5% sodium hypochlorite solution (NaClO) on the corrosion behaviour of endodontic instruments made of NiTi (GT Rotary and K3) and stainless steel (K file) by measuring potentiodynamic polarisation curves in the same environment at 37°C. For comparison, the electrochemical characterisation was also carried out on endodontic instruments that were not sterilised. The characterisation of the sample surface after the electrochemical tests were carried out by means of scanning electron microscopy coupled with X-ray energy dispersive spectroscopy. There was no significant influence of the sterilisation treatment on the corrosion behaviour of K file and GT Rotary endodontic instruments. However, a negative influence of the sterilisation treatment on the corrosion resistance of K3 endodontic instruments was observed, and the effect appears to be more dramatic for longer sterlisation treatment periods.     

Stress corrosion cracking of 13% Cr martensitic steels in sodium chloride solutions in the presence of thiosulphate

The stress corrosion cracking (SCC) susceptibility of 13% Cr martensitic (UNS S42000) and supermartensitic (UNS S41125) steels in sodium chloride solutions in the presence of thiosulphate was evaluated by slow strain rate tests (SSRT). The tests were performed in 5% sodium chloride solutions buffered at pH 2.7, 3.5, 4.5 and 6.0 in the absence and presence of thiosulphate in a concentration range between 10⁻⁶ and 10⁻³ M, at 25 ± 0.1 °C. The electrochemical behaviour of the two steels in the different solutions was determined by recording the anodic and cathodic polarisation curves. 13% Cr martensitic steel showed SCC in 5% sodium chloride solutions with pH ≤ 4.5 in the presence of 3 × 10⁻⁶ M thiosulphate. Decreasing the chloride ion concentration from 50 to 10 g/l, the critical concentration of thiosulphate to provoke SCC susceptibility increased from 3 × 10⁻⁶ to 1 × 10⁻⁵ M. The resistance to SCC of the supermartensitic steel was higher than that of the martensitic steel. The critical concentration of thiosulphate to induce SCC on the supermartensitic steel were 1 × 10⁻⁵ M at pH 2.7 and 1 × 10⁻⁴ M at pH 3.5. At pH ≥ 4.5 the supermartensitic steel did not crack. The anodic and cathodic polarisation curves evidenced the influence of the thiosulphate on the corrosion and the activation effect on the steels. The SCC of the two steels was attributed to hydrogen embrittlement produced by sulphur and hydrogen sulphide formed by dismutation and reduction of thiosulphate.     

Untersuchung über die Schwingungsrißkorrosion rost- und säurebeständiger Stähle in Schwefelsäure hoher Konzentration

Investigation into corrosion fatigue of stainless steels in high concentrated sulfuric acid Four commercial stainless steels (mat. Nos. 1.4539, 1.4586, 1.4571 and 1.4462) were tested in 96% and 90% sulfuric acid of 80 °C as to their corrosion fatigue strength by subjecting notched round bars to the rotary bending test. At the same time, the efficiency of anodic protection was examined. With all four steels, the alternating bending strength obtained in the air is essentially reduced. The semiaustenitic steel Nr. 1.4462 shows a lower corrosion fatigue strength than the three austenitic steels. Although making the corrosion fatigue strength of steels better calculable, anodic protection proves insufficient to completely prevent notched round bars from corrosion fatigue in concentrated sulfuric acid.     

Microbiologically influenced corrosion of steels by thermophilic and mesophilic bacteria

Bacterial influenced corrosion of AISI 316 stainless steels (SS) and ASTM A36 carbon steel by two strains of sulfate reducing bacteria (SRB) were analyzed. Thermophilic and mesophilic bacteria were isolated from the condensate fluid of “Los Azufres”, a geothermal electric field located in the State of Michoacan at Central Mexico. Anaerobic corrosion tests were carried out for 15, 30 and 60 days in lactate‐containing media at 50 °C and 40 °C. Scanning electron microscopy (SEM) was used to determine corrosion morphology. Pitting density was determined with an optical microscope. Corrosion potential, anodic potentiodynamic polarization curves and pH values were measured under anaerobic conditions. Results show that the microbial activity influenced the overall corrosion process, whereas, pitting corrosion and localized attack corrosion (LAC) were found. The anodic polarization curves show that passivation and activation processes should take place on the steel surface of the sample and pH decreases as the exposure time increases.     

Fatigue behaviour of 8090-Al-Li alloy in chloride containing environments

The effect of an environment containing chloride on the fatigue behaviour of a 8090 Al Li Mg Cu alloy was studied. Results were compared with those obtained on a traditional 2024 Al Cu Mg alloy. Fatigue and corrosion-fatigue tests were carried out both on smooth specimens and riveted samples. In order to assess the corrosion behaviour of the materials and their susceptibility to stress corrosion cracking, potentiodynamic polarization and slow strain rate tests were carried out. Results showed a remarkable effect of the aggressive environment on the fatigue behaviour both of the innovative 8090 alloy (in T8 ageing conditions) and on the traditional clad 2024 alloy (in T3 natural ageing conditions), though the former showed slightly better behaviour. Nevertheless, in the presence of rivets the reduction in fatigue strength in the aggressive environment was negligible. Slow strain rate tests showed premature fractures under anodic polarization above the pitting potential and with a strain rate of 10⁻⁶ s⁻¹, only for the 8090 T8 alloy.     

Effect of phosphoric acid concentration on corrosion of Cr–Mn–N and Cr–Ni stainless steels

Phosphoric acid concentration (5–85%) effects on the corrosion behaviour of austenitic Fe–18Cr–12Mn–N steel have been studied by potentiodynamic polarisation measurements. After the anodic polarisation, both the film composition and the electronic structure have been investigated by X-ray photoelectron spectroscopy. The specimen surface examinations have been carried out by scanning electron microscopy. The results of the corrosion behaviour of the steel at issue have been compared to those relevant to two trademark materials [austenitic stainless steels AISI 304 (Fe–18Cr–9Ni) and X14AΓ15 (Fe–14Cr–15Mn–N)] and developed under the same test conditions.     

Electrochemical study of corrosion behaviour of carbon steel A106 and stainless steel 304 in aqueous monoethanolamine

Abstract

Corrosion behaviour of carbon steel A106 and stainless steel 304 (SS304) in aqueous monoethanolamine was studied by performing electrochemical polarisation experiments. Potentiodynamic curves were studied and compared under conditions with different temperatures, carbon loading and O₂ percentage in purging gases. It was found that corrosion of A106 and SS304 was promoted under conditions with higher temperature. While the presence of O₂ speeds the corrosion of A106, it has a negligible impact on SS304 at 80°C and lowers the corrosion rate at 40°C. Corrosion rates and other important parameters were calculated based on the electrochemical curves for A106. Sample surfaces after tests were examined by scanning electron microscopy and energy dispersive spectroscopy. Mechanisms involved in iron dissolution and passivation from oxide films were discussed.     

Localised corrosion behaviour of alloys 33 and 24 in simulated flue gas desulphurisation environment

Abstract

Potentiodynamic anodic polarisation measurements have been carried out on type 316L stainless steel (as a reference material) and on alloys 33 and 24 in a simulated flue gas desulphurisation environment in order to assess the localised corrosion resistance. The results showed that the pitting corrosion resistance was higher in the case of alloys 33 and 24 than in the reference material owing to the higher contents of nitrogen, chromium, and molybdenum. An accelerated leaching study conducted on the alloys 33 and 24 showed only minor tendencies for the leaching of metal ions at various impressed potentials. Observations by SEM confirmed the lower tendency towards pitting of the alloys 33 and 24.     

Development of high strength low alloy steels for marine applications: Part 2: Simulation of marine exposure of steel by means of laboratory tests in flowing sea water

Abstract

Sixteen experimental high strength low alloy steels were exposed to natural flowing sea water in the laboratory and in the harbour of den Helder at a depth of 8 m for a period of 2 years. Differences in the corrosion rates of the various steels were less pronounced in the laboratory tests than in the harbour. The effects of the alloying elements aluminium, chromium: and molybdenum on general corrosion and pitting were found to be in general agreement with those obtained from earlier exposure tests at depths of 45 and 90 m in the North Sea. A fairly good correlation was found between the general corrosion rates measured from weight losses and the results obtained from electrochemical techniques based on determination of polarisation resistance.