The synergistic effect of seawater's parameters on the corrosion rate of the austenitic stainless steel 304

In this article the corrosion rate of the austenitic stainless steel type 304, under the synergistic effect of environmental factors has been assessed via potentiodynamic polarization scan. Salinity, velocity, pH and temperature are the factors which induce effects on the corrosion rate of the considered metal. Quantitative analysis is applied using the full two-level factorial experimental design method, which results in the contribution value of each parameter in changing the quantity of average corrosion rate in both individual and synergistic cases. Besides this quantitative analysis, qualitative analysis has been used to predict the variation direction. These two analyses show the important role of the synergistic action, which changes the variation direction of corrosion rate and the contribution percentage of each parameter in varying the value of corrosion rate. Applying both these analyses can show the magnitude and quality of each parameter’s effect, which is remarkably valuable in marine applications.     

AE response of type 304 stainless steel during stress corrosion crack propagation

Acoustic emission (AE) signals generated by transgranular stress corrosion cracking (TGSCC) and intergranular stress corrosion cracking (IGSCC) have been compared by means of slow strain rate tensile tests (SSRT) performed using both solubilised and sensitised type AISI 304 stainless steel in a 1 M NaCl + 1 M HCl solution. Results show that the AE activity during the propagation of TGSCC is much higher than the AE activity during the IGSCC propagation. However, no significant difference was found between the mean amplitude and rise-time of the AE signals registered during the propagation of TGSCC and those measured for IGSCC propagation.     

Influence of superficial coating of CeO2 on the oxidation behavior of AISI 304 stainless steel

The effect of a superficially-applied, cerium-oxide coating on the non-isothermal oxidation behavior of AISI 304 stainless steel in dry air has been investigated. The heating rate employed was 3 K/min up to a final temperature of 1423 K. The reactive oxide coating not only reduced the reaction rate but also facilitated scale adhesion to the alloy substrate. Post-oxidation analyses of the alloy/scale combination using optical microscopy, SEM, EDAX, and XRD provide evidence for a changeover in the mechanism of oxide growth from the scale/gas interface to the alloy/scale interface for the coated steel.     

Enhancement of intergranular corrosion resistance of type 304 stainless steel through laser shock peening

The present experimental study evaluates laser peening for suppressing intergranular corrosion (IGC) susceptibility of sensitised 304 stainless steel. Under the given experimental conditions, an increase in the number of laser peening treatments of sensitised 304 stainless steel specimen brought about progressive reduction in its susceptibility to IGC. Triple laser peening of sensitised stainless steel specimen introduced remarkable reduction in its IGC susceptibility. The results of the study suggest that breakage of inter-granular network of chromium carbides/chromium-depleted regions through plastic deformation, imposed by multiple laser peening treatment, was responsible for causing significant suppression in IGC susceptibility of sensitised 304 stainless steel specimens.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

The effects of electropolishing (EP) process parameters on corrosion resistance of 316L stainless steel

Improvement of the corrosion resistance capability of a workpiece being processed by the electropolishing (EP) is one of the most importance process characteristics. In this paper, the effects of the EP process parameters on the corrosion resistance performance of the SS 316L stainless steel were studied based on the uniform and localized intergranular corrosion (IGC) analyses. The IGC is the prominent characteristics of localized corrosion in stainless steel.

The workpiece (anode) material was the SS 316L stainless steel. The cathode material under study was platinum. The electrolyte was composed of sulfuric acid, phosphoric acid, glycerin and DI water. The test specimens were all polished before experiments in order to reduce the effect of the bailby layer. Variables of the EP process parameters gap between the electrodes and the process time.

The specimens were analyzed, first, by the surface roughness measurement. Secondly, they were observed under the optical microscope for surface marks and defects. It was followed by the linear polarization analysis for the uniform corrosion performance. The electrochemical potentiokinetic repassivation (EPR) test was employed to study the localized, IGC. Finally, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses were conducted to analyze the metallurgical composition and the thickness of the passive film. The results showed that the EP process greatly improved the corrosion resistance of the SS 316L stainless steel.     

Optical reflection spectroscopy of thick corrosion layers on 304 stainless steel

Corrosion resistant structural materials of both iron and nickel based alloys are used in the electric power industry for the construction of the coolant loops of both conventional and nuclear power generating stations. These materials, in the presence of high temperature (e.g. 287 °C), high pH (e.g. 10.0 at 20 °C) water with dissolved hydrogen will oxidize and form corrosion films that are double metal oxides (or spinels) of the form AB₂O₄. This work describes optical reflectivity techniques that have been developed to study the growth of these films in situ. The optical technique uses a dual-beam specular reflection spectrometer to measure the spectrum of reflected light in small angle (i.e. <15°) scatter. The reflection spectra are then calibrated using a set of corrosion coupons with corrosion films that are well known. Results are compared with models based on multilayer reflection and Mie scattering from a particle size distribution. Surface roughness is found to be the dominant cause of reduced reflection as the films grow.     

Monte Carlo simulation of stress corrosion cracking on a smooth surface of sensitized stainless steel type 304

Stress corrosion cracking (SCC) on a smooth surface of structural metal materials occurs by initiation and coalescence of micro cracks, subcritical crack propagation and multiple large crack formation or final failure under combination of material, stress and corrosive environment. In this paper, a Monte Carlo simulation of the SCC process is proposed based on stochastic properties for micro crack initiation and concepts in fracture mechanics for crack coalescence and propagation. The procedure is as follows: The possible number of grain-sized micro cracks which can be initiated is set for a given space and initiation times for all cracks are assigned by random numbers based on exponential distribution. Sites and sizes of cracks are assigned by uniform random numbers and normal random numbers, respectively. Coalescence and propagation of cracks are determined based on fracture mechanics. The emphasis in the model is put on the influence of semi-elliptical surface cracks. Numerical simulations are carried out based on the results of creviced-bent-beam tests for sensitized stainless steel type 304 under high-temperature and high-purity water containing dissolved oxygen and the influence of micro crack initiation rate and coalescence condition on the simulation results is discussed.     

Influence of tungstate ion dopants in corrosion protection behavior of polyaniline coating on mild steel

Electropolymerization of polyaniline (PANI) and polyaniline‐tungstate (PANIW) coatings on mild steel were successfully performed using cyclic voltammetry technique. Processes were carried out in aqueous electrolyte solutions of 0.3 M oxalic acid + 0.1 M aniline and 0.3 M oxalic acid + 0.1 M aniline + 0.001 M sodium tungstate dehydrate. Corrosion protection of PANI and PANIW coatings was evaluated with the help of open circuit potential (E_ocp) monitoring and electrochemical impedance spectroscopy (EIS) methods. All the results reveal the influence of additional doping agent (i.e., tungstate) in corrosion protection behavior of PANI coating.     

The characterization and corrosion resistance of cerium chemical conversion coatings for 304 stainless steel

A golden yellow-colored cerium conversion coating was obtained on 304 stainless steel surface by immersing the steel into a solution containing cerium (III), KMnO₄ and sulfuric acid. The corrosion resistance of the coatings was evaluated by electrochemical methods, potentiodynamic polarization experiments and electrochemical impedance spectrum. The experimental results indicated that the corrosion resistance for the conversion coated 304SS in 3.5% NaCl solution increased markedly. The corrosion potential of the treated steel increased to a more noble level, the pitting corrosion potential increased also, the passive potential range was enlarged markedly and the passive current density decreased about one order compared to that of the untreated steel. The cathodic and anodic reaction were both inhibited to some extent. The chemical state of the elements in the coatings was investigated by XPS. The cerium element was in the form of tetravalent state. And AES depth profile analysis suggested that the thickness of the conversion coatings was less than 66 nm. The mechanisms of coatings formation and corrosion resistance are discussed.     

Microstructure and corrosion resistance of 304 stainless steel electroslag strip cladding

The 304 stainless steel strips were deposited one layer on carbon steel base metal by electroslag strip cladding(ESC) and submerged arc cladding(SAC),respectively.The solidification microstructure of ESC metal was analyzed by the optical microscopy,scanning electron microscope and energy dispersive spectroscopy.The corrosion resistance studies of strip cladding metals were carried out in 10%oxalic acid electrolytic etching test.The results showed that the cladding metal obtained by ESC presented low content of C,high content of Cr and enough alloying element of Ni in the chemical composition.The transition zone of ESC with small width was almost parallel with the base metal,leading to a lower dilution.There are three types of solidification modes(A→AF→FA) occurred in the ESC metal due to the decrease of cooling rate and degree of dilution from the transition zone to the top of ESC metal.As a result,the microstructure of ESC metal exhibited mainly austenite with a small amount offerrite,contributing to achievement of better corrosion resistance.     

Effect of cold work on erosion–corrosion of 304 stainless steel

Slurry and single particle impingement experiments were performed to study the effects of cold work on erosion–corrosion properties of 304 stainless steel. The specific energies of crater formation on samples were calculated and it was shown that these energies are constant for samples with specific amount of cold work at various impact velocities. The role of frictional force and its effective path of action were investigated during the depassivation process. It was found that the coefficient of friction between the particle and the surface remains invariant regardless of the amount of cold work that is applied to the sample.     

Pitting corrosion mechanism of Type 304 stainless steel under a droplet of chloride solutions

Pitting corrosion of Type 304 stainless steel under drops of MgCl₂ solution has been investigated to clarify the rusting mechanism in marine atmospheres. A pitting corrosion test was performed under the droplets with various combinations of the diameter and thickness (height) by exposure to a constant relative humidity. Probability of occurrence of pitting corrosion decreased with decreasing the diameter and thickness. Pitting corrosion progressed only when the [Cl⁻] exceeded 6 M (RH < 65%). In almost cases, there was a small hole (∼10 μm diameter) in the center of a single pit, which may be the trace of an inclusion particle like MnS dissolved out. The pitting corrosion mechanism of Type 304 under droplets containing chloride ions has been proposed.     

表面喷丸压力对304不锈钢应力腐蚀敏感性的影响

采用3种表面喷丸压力对304不锈钢板状试样进行强化处理。通过扫描电子显微镜分析了强化层的厚度和晶粒细化程度,采用显微硬度计检测了表层显微硬度分布。通过5%NaCl溶液中慢应变速率拉伸试验,比较了0.25、0.30和0.40 MPa喷丸压力处理后试样的应力腐蚀敏感性。结果表明：表面喷丸强化处理可以细化试样表层晶粒,提高硬度;随着喷丸压力的升高,试样表层晶粒层厚度增大,表面硬度提高,应力腐蚀敏感性指数降低。     

Improvement on the pitting corrosion resistance of 304 stainless steel via duplex passivation treatment

A remarkable improvement in the pitting corrosion resistance of 304 stainless steel was attempted using a novel duplex passivation treatment method. First, chemical passivation in nitric acid followed electrochemical passivation via potential polarization of step cycling in sodium nitrate electrolyte. Compared with traditional chemical passivation, breakdown potential was increased from 0.31 V_SCE to positive than 0.9 V_SCE at 70°C in a solution bearing 0.6 M [Cl^?] concentration. The critical pitting temperature was enhanced from 21.5°C to above 70°C in a solution with 6 M [Cl^?] concentration. Impedance analysis and X‐ray photoelectron spectroscopy results show that a more compact passive film with a higher ratio of chromium oxide on iron oxide was achieved by electrochemical passivation compared with chemical passivation. Morphology observation suggested that the potential polarization of step cycling slightly increased the dissolution of inclusions after being subjected to chemical passivation. The probable reason for the improvement on pitting resistance is discussed in detail based on inclusion dissolution and the protectiveness in passive film.     

光亮退火处理304不锈钢在NaCl溶液中的耐蚀性

采用稳态阳极极化曲线、恒电位开路衰减响应曲线和电化学噪声等多种电化学测试技术研究了光亮退火处理的304不锈钢在0.01-0.6mol/L NaCl溶液中的耐蚀性能.研究结果表明,不锈钢的点蚀击穿电位Eb值与Cl-浓度的对数呈线性关系,在同一浓度下,光亮不锈钢的Eb值比普通不锈钢大约正400-600mV;在0.6mol/L NaCl溶液中不锈钢恒电位开路衰减及电流噪声的测量结果也表明,光亮退火处理使不锈钢在NaCl溶液中钝化膜的稳定性明显改善,耐点蚀能力显著提高.     

Influence of ultrasound on pitting corrosion and crevice corrosion of SUS304 stainless steel in chloride sodium aqueous solution

The change of polarization curves and surface morphologies of SUS304 stainless steel was investigated in 3.5 mass% NaCl solution with or without the application of ultrasound (US). As the result, both the pitting corrosion and the crevice corrosion were largely suppressed by the application of US. The reason is attributed to the decrease in the concentration of hydrogen and chloride ions in pits or in the crevice by removing the corrosion product and stirring the liquid there.     

Super304H钢焊接接头的晶间腐蚀敏感性

利用电化学动电位再活化法(EPR)测试了Super 304H钢焊接接头焊缝和母材的晶间腐蚀敏感性.结果表明,Super 304H钢焊缝和母材均呈现出较低的晶间腐蚀倾向.进一步用扫描电子显微镜、X射线衍射仪和透射电镜研究了它们的微观组织结构,焊缝和母材均为单一的奥氏体组织+少量析出相,二者都未探测到明显的Cr23C6析出相,所以未出现明显的基体贫铬现象,但是焊缝金属因与母材合金元素等存在差异,导致其在H2SO4和KSCN溶液中的耐晶间腐蚀性能稍好于母材.