The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 pair in aqueous LiBr solutions

The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 stainless steel pair in a concentrated lithium bromide solution was investigated according to the mixed potential theory. A hydraulic circuit was designed to study dynamic corrosion processes in situ. A potential relation between corrosion current density (i_corr) and Reynolds number (Re) was found for copper, showing a mixed control of a chemical step and mass transport through the corrosion products film with the predominance of the former. No dependence of i_corr on Re could be established for AISI 304, showing a chemical step control. Moreover, under stagnant conditions, partial passivation may occur in AISI 304; however, under flowing conditions passivation is not possible. Copper is the anodic element of the pair under all flowing conditions analysed. The galvanic phenomenon is more important as Re increases, but the results show compatibility of both materials at all Re values analysed. Similarly, a potential relation between galvanic current density (i_G) and Re was found, showing a mixed control of a chemical step and mass transport with the predominance of the latter. Copper corrosion resistance decreases more rapidly as Re increases due to the AISI 304 galvanic effect: there is a synergy between the galvanic effect and the hydrodynamic conditions. Under stagnant conditions, the galvanic behaviour of the materials is close to the compatibility limit and an inversion of the anodic element of the galvanic pair takes place.     

On the corrosion mechanism of AISI 204Cu stainless steel in chlorinated alkaline media

Electrochemical techniques (CV, SECM, CPT) and surface analysis techniques (EDX, SEM) have been employed to assess the corrosion behaviour of the AISI 204Cu stainless steel. The behaviour of this steel has been compared with that of AISI 304 and AISI 434 stainless steels in chlorinated alkaline media. All samples performed well at room temperature under potentiodynamic polarisation up to a chloride to hydroxyl ratio of 10. At this ratio the AISI 204Cu and the AISI 434 steels presented pitting potential at +0.47 V vs. SCE and +0.31 V vs. SCE, respectively. Moreover, the critical pitting temperature was higher for the AISI 204Cu steel than for the AISI 434 steel, respectively 58 °C and 28 °C.In terms of corrosion performance of the AISI 204Cu stainless steel can be classified better than the AISI 434 steel and worse than the AISI 304 steel.Local electrochemical and chemical examinations allowed evidencing the local activity of some pits over long period, and to conclude that the improved corrosion performance of the low nickel alloy AISI 204Cu stainless steel should be ascribed to copper cementation at active corrosion sites.     

High temperature corrosion properties of ionic liquids

The corrosion behaviour of several metals and metal alloys (copper, nickel, AISI 1018 steel, brass, Inconel 600) exposed to a typical ionic liquid, the 1-butyl-3-methyl-imidazolium bis-(trifluoromethanesulfonyl) imide, ([C₄mim][Tf₂N]), has been investigated by electrochemical and weight-loss methods. Corrosion current densities have been determined by extrapolation from Tafel plots and by polarization resistance measurements and 48 h immersion tests were performed at 150, 250, 275 and 325 °C. Room temperature results show low corrosion current densities (0.1-1.2 μA/cm²) for all the metals and alloys investigated. At 70 °C, the corrosion current for copper dramatically increases showing a strongly dependence on temperature. At 150 °C copper shows significant weight-loss while nickel, AISI 1018, brass and Inconel do not. At higher temperatures (?275 °C), the copper sample crumbles and localized corrosion occurs for the other metals and alloys.     

Corrosion behaviour of micro-plasma arc welded stainless steels in H3PO4 under flowing conditions at different temperatures

This paper studies the general corrosion behaviour of the micro-plasma arc welded AISI 316L stainless steel in phosphoric acid at different temperatures (25–60 °C) and at a Reynolds number of 1456. Galvanic corrosion has been studied using zero-resistance ammeter (ZRA) measurements and polarization curves (by the mixed potential theory). Results show that the microstructure of the stainless steel is modified due to the micro-plasma arc welding procedure. Coupled current density values obtained from polarization curves increase with temperature. ZRA tests present the highest i_G values at 60 °C; however, the values are very close to zero for all the temperatures studied. This is in agreement with the low value of the compatibility limit and of the parameter which evaluates the importance of the galvanic phenomenon. Both techniques present the most positive potentials at the highest temperature. This study reveals that micro-plasma arc welded AISI 316L stainless steels are appropriated working in the studied H₃PO₄ media from a corrosion point of view for all the temperatures analysed.     

Effects of microplasma arc AISI 316L welds on the corrosion behaviour of pipelines in LiBr cooling systems

The effect of microplasma arc welding (MPAW) on the electrochemical and corrosion behaviour of AISI 316L stainless steel tubes has been studied. Scanning electrochemical measurements were performed in sodium chloride to evaluate the difference in the electrochemical activity of base (non-welded) and welded samples. Oxygen reduction rates increase in AISI 316L due to the heat treatment effect induced by welding, indicating a higher electrochemical activity in the welded samples. Additionally, the use of MPA weldments in lithium bromide (LiBr) absorption machines was also analysed at typical operating temperatures and Reynolds numbers. The welding process increases corrosion rates, hinders passivation and increases the susceptibility to pitting attack in LiBr. However, zero-resistance ammeter and localization index measurements show that the galvanic pair generated between the base and welded alloys is weak, both electrodes being in their passive state. Temperature greatly affects the corrosion process.     

Galvanic corrosion of titanium coupled to welded titanium in LiBr solutions at different temperatures

The galvanic corrosion generated between the titanium-welded titanium pair has been studied in heavy brine LiBr solutions at 25, 50 and 100 °C under open circuit conditions using a zero-resistance ammeter (ZRA). The results showed that welded titanium was the anode of the pair, so that its corrosion resistance decreases due to the galvanic effect. However, the extremely low galvanic current densities registered by the pair reveal the poor severity of the coupling under the studied conditions. Furthermore, it was observed that the electrodes were in the passive state, increasing the probability of localized corrosion with temperature.     

Corrosion properties of plasma nitrided AISI 410 martensitic stainless steel in 3.5% NaCl and 1% HCl aqueous solutions

Samples of an AISI 410 martensitic stainless steel were plasma nitrided at a temperature of 420 °C, 460 °C or 500 °C for 20 h. The composition, microstructure and hardness of the nitrided samples were characterised using a variety of analytical techniques. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarisation tests in 3.5% NaCl solution and immersion tests in 1% HCl acidic water solution. The results showed that plasma nitriding produced a relatively thick nitrided case consisting of a compound layer and a nitrogen diffusion layer on the 410 stainless steel surface. Plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the martensitic stainless steel. In the immersion test, nitrided samples showed lower weight loss and lower corrosion rate than untreated one. In the electrochemical corrosion tests, the nitrided samples showed higher corrosion potentials, higher pitting potentials and greatly reduced current densities. The improved corrosion resistance was believed to be related to the iron nitride compound layer formed on the martensitic stainless steel surface during plasma nitriding, which protected the underlying metal from corrosive attack under the testing conditions.     

Corrosion and impressed current cathodic protection of copper-based materials using a bimetallic rotating cylinder electrode (BRCE)

A bimetallic rotating cylinder electrode (having individual electrode areas of 10 cm² and rotating at 200-1400 rpm) has been used to examine the corrosion and protection characteristics of copper/nickel aluminium bronze and 90-10 copper-nickel/nickel aluminium bronze galvanic couples in filtered seawater at 25 °C. The flow-influenced electrochemistry of the systems was determined using zero resistance ammetry, corrosion potential measurements and a potential step current transient technique. In each case, the galvanic corrosion potential and corrosion rate displayed a Reynolds number dependency where mass transport control of the anodic dissolution reaction partially controlled the reaction rate. Bimetallic impressed current cathodic protection (ICCP) has also been demonstrated for a range of applied protection potentials and Reynolds numbers. A comparison has been made between the directly measured bimetallic ICCP current demand and that determined from independent, single-metal rotating cylinder electrode measurements. In this case, the mixed charge and mass transport controlled rate of oxygen reduction was examined.     

Weight loss studies of fastener materials corrosion in contact with timbers treated with copper azole and alkaline copper quaternary compounds

Corrosion rates of mild steel, AISI 316 stainless steel and hot-dipped galvanised steel in contact with preservative-treated Pinus radiata have been determined using four distinct accelerated (49 ± 1 °C) and non-accelerated (21 ± 2 °C) weight loss methodologies. The data were measured as a function of timber moisture content and copper concentration over periods of exposure ranging from 2 weeks to 14 months. The results show that the corrosion resistance of the stainless steel was not influenced by classification or magnitude of preservative loading. Corrosion rates of this material were multiple orders of magnitude lower than those of the mild and galvanised steels. In most instances, corrosion rates of hot-dipped galvanised layers in contact with alkaline copper quaternary-treated timbers were up to a factor of 10 times, or greater, than those measured for copper-chrome-arsenate treatments. A direct negative influence of copper ion concentration on the corrosion resistance of mild steel was also observed for each preservative type.     

Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques

Extension of sensitized zone (SZ) in welded AISI 304 stainless steel was determined by two non-destructive electrochemical tests: double loop electrochemical potentiokinetic reactivation technique (DLEPR) and local electrochemical impedance spectroscopy (LEIS). Welding was carried out using the shielded metal arc with two selected welding energies: the first one (0.7 kJ mm⁻¹) does not promote the sensitization of the 304 steel and it constitutes the reference sample and the second one (2.2 kJ mm⁻¹) which leads to the precipitation of chromium carbides in the grain boundaries after the welding process. The non-destructive DLEPR and LEIS tests allowed the length of the SZ to be determined and a good agreement between the two techniques and the microstructure of the two welded samples was shown. The presence of an inductive loop on the local impedance diagrams seems to reflect a galvanic coupling between the weld string (anode) and the welded stainless steel plates (cathode) which will be very prejudicial to a good corrosion resistance of the welded system. The results showed that the two electrochemical tests could be applied in practical cases in industrial field.     

Effect of Mo and Mn additions on the corrosion behaviour of AISI 304 and 316 stainless steels in H2SO4

The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30 wt.% H₂SO₄ at 25 and 50 °C. Corrosion mechanism was determined by gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 °C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.     

Corrosion behavior of iron-based alloys in the LiBr + ethylene glycol + H2O mixture

The corrosion resistance of 1018 carbon steel, 304 and 316 type stainless steels in the LiBr (55 wt.%) + ethylene glycol + H₂O mixture at 25, 50 and 80 °C has been studied using electrochemical techniques which included potentiodynamic polarization curves, electrochemical noise and electrochemical impedance spectroscopy techniques. Results showed that, at all tested temperature, the three steels exhibited an active-passive behavior. Carbon steel showed the highest corrosion rate, since both the passive and corrosion current density values were between two and four orders of magnitude higher than those found for both stainless steels. Similarly, the most active pitting potential values was for 1018 carbon steel. For 1018 carbon steel, the corrosion process was under a mixed diffusion and charge transfer at 25 °C, whereas at 50 and 80 °C a pure diffusion controlled process could be observed. For 316 type stainless steel, at 25 and 50 °C a species adsorption controlled process was observed, whereas at 80 °C a diffusion controlled mechanism was present. Additionally, at 25 °C, the three steels were more susceptible to uniform type of corrosion, whereas at 50 and 80 °C they were very susceptible to localized type of corrosion.     

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.     

Influence of Cu and Sn content in the corrosion of AISI 304 and 316 stainless steels in H2SO4

This paper addresses the influence of Cu and Sn addition on the corrosion resistance of AISI 304 and 316 stainless steels in 30 wt% H₂SO₄ at 25 and 50 °C. The corrosion process was evaluated by gravimetric tests, DC measurements and electrochemical impedance spectroscopy (EIS). The corrosion products were analysed by SEM, X-ray mapping and XPS before and after accelerated tests. The behaviour of both AISI 304 and 316 stainless steels in sulphuric acid solution was greatly improved by increasing Cu concentration and the synergic effect of Cu and Sn. Addition of Sn increased corrosion resistance, but less than addition of copper.     

Microstructure and corrosion behavior of the AISI 304 stainless steel after Nd:YAG pulsed laser surface melting

Different laser energy densities were utilized to treat AISI 304 stainless steel via Nd:YAG pulsed laser surface melting (LSM). The surface composition and microstructure of the stainless steel were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). In particular, the corrosion behaviors of the stainless steel surface without and with LSM were evaluated by the electrochemical polarization measurement in 3.5 wt.% NaCl aqueous solution at room temperature. The results showed that the stainless steel surface without LSM suffered severe localized pitting under the testing conditions. A thin surface oxide protective layer was produced on the stainless steel surface with LSM, which considerably improved the corrosion resistance properties of the stainless steel. The height differences of the corrosion regions on the stainless steel surface with LSM were measured to establish more corrosion resistant region, using scanning confocal laser microscopy. The underlying corrosion mechanism of the stainless steel with LSM was revealed.     

Depassivation-repassivation behavior of type-312L stainless steel in NaCl solution investigated by the micro-indentation

Repassivation behavior of type-312L stainless steel containing 6% of molybdenum was examined in NaCl solution using in situ micro-indentation technique, together with type-304 and 316L stainless steels. High stability of the passive film formed on the type-312L stainless steel was also examined by depth profiling analysis of passive films using glow discharge optical emission spectroscopy (GDOES). In 0.9 mol dm⁻³ NaCl solution at 296 K the type-304 and 316L stainless steels are passive only up to 0.3 V (SHE), above which pitting corrosion occurs. In contrast, no pitting corrosion occurs on type-312L stainless steel. Despite the significant difference of the pitting corrosion resistance, the repassivation kinetics of the three stainless steels, examined by micro-indentation at 0.3 V (SHE), is similar. The presence of molybdenum in the stainless steel does not influence the repassivation kinetics. The charge required to repassivate the ruptured type-312L stainless steel surface increases approximately linearly with the potential, even though the passivity-maintaining current increased markedly at potentials close to the transpassive region. Repassivation occurs without accompanying significant dissolution of steel, regardless of the stability of passive state. Depth profiling analyses of the passive films on the type-312L stainless steels formed at several potentials revealed that molybdenum species enrich in the outer layer of the passive film, below which chromium-enriched layer is present. The permeation of chloride ions may be impeded by the outer layer containing molybdate, enhancing the resistance against the localized corrosion of the type-312L stainless steel.     

Electrochemical characteristics of zirconium oxide treated Type 304 stainless steels of different surface oxide structures in high temperature water

Electrochemical polarization analyses were conducted to investigate the impact of different oxide structures on corrosion resistance of ZrO₂-treated Type 304 stainless steel specimens in high temperature water. All specimens were pre-oxidized in high temperature water containing either 300 ppb dissolved oxygen or 50 ppb dissolved hydrogen, followed by a hydrothermal deposition treatment with ZrO₂ nanoparticles. Experimental results revealed that the corrosion potentials and corrosion current densities on the ZrO₂-treated specimens were lower than those on the untreated ones, and the decreases in these parameters were more distinct on the ZrO₂-treated specimens with oxides developed under dissolved oxygen condition.     

Pitting corrosion behaviour of PM austenitic stainless steels sintered in nitrogen-hydrogen atmosphere

PM 304L and 316L stainless steel have been compacted at 400, 600 and 800 MPa and sintered in vacuum and in nitrogen-hydrogen atmosphere. Postsintered heat treatments (annealing solution and ageing at 375, 675 and 875 °C) have been applied. Pitting corrosion resistance has been studied using anodic polarization measurements and the ferric chloride test. Anodic polarization curves reveal that densities and atmospheres are relevant on anodic behaviour. Pitting resistance is higher for PM 316L and for higher densities and vacuum as sintered atmosphere. Ageing heat treatments at medium and high temperatures are detrimental to passivity although susceptibility to pitting corrosion barely changes. But heat treatments at 375 °C even show certain improvement in pitting corrosion resistance. The results were correlated to the presence of precipitates and mainly to the lamellar constituent which appears in some samples sintered in nitrogen-hydrogen atmosphere. The role of nitrogen on the samples sintered under nitrogen-hydrogen atmosphere and its relation to the microstructural features was described.     

Effect of surface nanocrystallization on the corrosion behaviour of AISI 409 stainless steel

The influence of surface mechanical attrition treatment (SMAT) on the corrosion behaviour of AISI 409 grade stainless steel in 0.6 M NaCl was studied. SMAT using 2 mm ∅ 316L stainless steel (SS) balls for 15, 30 and 45 min and 5 mm ∅ balls for 15 min offers a better corrosion protective ability. In contrast, treatment using 5 mm ∅ balls for 30 and 45 min and by using 8 mm ∅ balls for 15, 30 and 45 min, induces microstrain and defect density that results in a decrease in corrosion resistance.     

Corrosion resistance properties of glow-discharge nitrided AISI 316L austenitic stainless steel in NaCl solutions

Glow-discharge nitriding treatments can modify the hardness and the corrosion resistance properties of austenitic stainless steels. The modified layer characteristics mainly depend on the treatment temperature. In the present paper the results relative to glow-discharge nitriding treatments carried out on AISI 316L austenitic stainless steel samples at temperatures ranging from 673 to 773 K are reported. Treated and untreated samples were characterized by means of microstructural and morphological analysis, surface microhardness measurements and corrosion tests in NaCl solutions. The electrochemical characterization was carried out by means of linear polarizations, free corrosion potential-time curves and prolonged crevice corrosion tests. Nitriding treatments performed at higher temperatures (>723 K) can largely increase the surface hardness of AISI 316L stainless steel samples, but decrease the corrosion resistance properties due to the CrN precipitation. Nevertheless nitriding treatments performed at lower temperatures (?723 K) avoid a large CrN precipitation and allow to produce modified layers essentially composed by a nitrogen super-saturated austenitic metastable phase (S-phase) that shows high hardness and very high pitting and crevice corrosion resistance; at the same polarization potentials the anodic current density values are reduced up to three orders of magnitude in comparison with untreated samples and no crevice corrosion event can be detected after 60 days of immersion in 10% NaCl solution at 328 K.