Corrosion of mild steel in porous media

The corrosion process in large-particle porous media is investigated with particular reference to corrosion in bulk carriers (ships). Examples of large-particle porous media include raw materials such as coal and iron ore which typically have particle sizes ranging up to approximately 50 mm diameter. It is proposed that the corrosion process is dominated by finer particles which co-exist with the larger particles. A mathematical model of the corrosion process is developed and used to estimate corrosion rates. These are shown to compare well with experimental results. The analysis indicates that to minimise corrosion it is desirable to maintain the moisture content of porous media in contact with mild steel either to less than 50%, or to more than 80%, of the maximum water holding capacity of the media.     

Electrochemical studies on interplay of mineralogical variation and particle size on bioleaching low grade complex sulphide ores

The interplay of mineralogical variation and mineral phase distribution within varying particle sizes on the dissolution behaviour of a low grade complex sulphide ore was investigated through bioleaching experiments and electrochemical technique. Investigations were carried out utilizing mineralogical data on the variations in mineral and phase distribution within particle sizes of <53 μm, 53–75 μm, 75–106 μm and 106–150 μm in mixed mesophilic cultures of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans. Electrochemical behaviour was studied using particulate electrodes from the four varying particle sizes and from massive electrodes prepared from the two major sulphide mineral rich phases (sphalerite-rich and galena-rich) and a complex mineralogical phase of the bulk ore. Bioleaching studies reveal the highest recoveries at a particle size of 75 μm, while electrochemical investigations reveal the highest dissolution at particle size of 106 μm. Electrochemical results show that sphalerite rich phase has the highest dissolution rate while galena-rich complex phase has the least. SEM studies confirm the highest bacterial attack at the sphalerite-rich phase. The discrepancies between the dissolutions within particle sizes obtained from bioleaching experiments and electrochemical studies are consistent with and attributed both to the physical and mineralogical influences. Electrochemical behaviour is influenced and controlled by galvanic interaction resulting from mineralogical variation, while bioleaching behaviour is influenced by mineralogical variation as well as physical effect of particle size.     

Corrosion behaviour of Zn–Al–Mg coated steel sheet in sodium chloride-containing environment

Conventional hot-dip galvanised zinc coated (Z) and novel hot-dip galvanised Zn–Al–Mg alloy coated (ZM) steel sheet samples with a coating thickness of 7 μm each were exposed to standardised salt spray test and cross-sections of the corrosion samples were analysed by using SEM and EDS. On Z corrosion proceeds very fast and the steel substrate is attacked even after 100 h of exposure. ZM samples showed a different behaviour. The entire metallic ZM coating is converted into a stable, adherent aluminium-rich oxide layer, which protects the steel substrate against corrosive attacks. This layer is the main reason for the enhanced corrosion resistance of the ZM coating in sodium chloride-containing environment.     

Aeration cell corrosion of carbon steel in soil: In situ monitoring cell current and potential

Aeration cell corrosion in soil has been studied in situ in the soil at a test site in Göteborg, Sweden. The aeration cell consisted of two carbon steel sheets, the anodic, buried in a lump of clay, and the cathodic, buried in the surrounding filling material. Two aeration cells were used, one with a cathodic sheet of the same size as the anodic sheets, and an other with a cathodic sheet 10 times larger. The anode-cathode area ratio of the cells were 1:1 and 1:10. Both corrosion current and potential was measured during the 2.67 y test period. As a reference, the corrosion potential of sheets with no connection to any aeration cells were measured. The corrosion of the anodic sheets in clay can be calculated from the cell current to 31.7 μm/y (2.59 μA/cm²) for the cell with area ratio 10 and 5.0 um/y (0.47 μA/cm²) with area ratio 1. The difference in cell current between the cells increased with time; from about three times larger in the beginning to about eight times larger. This can be explained by deposition of corrosion products on the cathodic sheet, causing a larger part of the anodic dissolution to be transferred to the anodic sheet. The difference in cell current will be 10 with no corrosion of the cathodic sheets and 1.82 with the same corrosion rates on both anodic and cathodic sheets.     

Selective laser melting of Fe-Ni-Cr layer on AISI H 13 tool steel

An attempt to fabricate Fe-Ni-Cr coating on AISI H13 tool steel was performed with selective laser melting. Fe-Ni-Cr coating was produced by experimental facilities consisting of a 200 W fiber laser which can be focused to 80 μm and atmospheric chamber which can control atmospheric pressure with N₂ or Ar. Coating layer was fabricated with various process parameters such as laser power, scan rate and fill spacing. Surface quality and coating thickness were measured and analyzed. Three different surface patterns, such as type I, type II and type III, are shown with various test conditions and smooth regular pattern is obtained under the conditions as 10 μm of fill spacing, 50–350 mm/s of scan rate and 40 μm of fill spacing, 10–150 mm/s of scan rate. The maximum coating thickness is increased with power elevation or scan rate drop, and average thickness of 10 μm fill spacing is lower than that of 40 μm fill spacing.     

Localized corrosion behavior of a zirconium-based bulk metallic glass relative to its crystalline state

To date, few detailed corrosion studies of the new bulk metallic glasses (BMGs) have been presented. In the present work, the aqueous electrochemical corrosion properties of BMG-11, 52.5Zr–17.9Cu–14.6Ni–5.0Ti–10.0Al (atomic percent), were investigated. Cyclic-anodic-polarization tests were conducted on amorphous and crystalline specimens in a 0.6 M NaCl solution (simulated seawater) and on amorphous specimens in a 0.05 M Na₂SO₄ solution (simulated moisture condensation, as related to ongoing fatigue experiments in humid air), all at room temperature. In the NaCl solution, both amorphous and crystalline materials were found to exhibit passive behavior with low corrosion rates (15 μm/year or less). However, susceptibilities to pitting corrosion were observed. The amorphous material was found to be more resistant to the onset of pitting corrosion under natural corrosion conditions. In the 0.05 M Na₂SO₄ solution, the amorphous BMG-11 was found to exhibit passive behavior with a very low corrosion rate (0.4 μm/year), and to be immune to pitting corrosion. Furthermore, when the protective passive film was removed by scratching with a diamond stylus, it was found to quickly reform. This result suggested that a corrosion influence on the fatigue properties of BMG-11 in humid air would be minimal.     

304L在模拟压水堆一回路条件下长期均匀腐蚀性能的研究

在模拟压水堆一回路水条件下,用静态高压釜对304L不锈钢进行了1680 h腐蚀实验,对氧化膜进行了宏观和微观分析,对均匀腐蚀和均匀腐蚀速率进行了定量评估。结果表明：304L不锈钢在很短的时间内（336 h）就形成了氧化物层。氧化物分为两层,最靠近基体的氧化物颗粒直径为50~100 nm,在细小致密的氧化物颗粒表面均匀分布着0.5~1.6 μm的多边形大颗粒。经过1680 h高温高压腐蚀实验,最靠近基体的氧化物颗粒直径增大为80~250 nm,致密细小氧化物颗粒表面分布的多边形颗粒直径增大为0.8~2.5 μm。致密细小氧化物膜具有很强的耐蚀性,腐蚀增重速率先是大幅降低,然后逐渐平缓。经过1680 h后,其均匀腐蚀速率降为2.85×10^-3 mg/(dm²h)。     

The behaviour in SO2 at high temperatures of Fe---B coatings on Fe obtained by ion beam, laser or pack-cementation techniques

Boron was incorporated in the near surface region of iron samples by three different methods: ion implantation, laser alloying and pack cementation. The behaviour of these different surface alloys was studied thermogravimetrically between 400 and 600°C in static SO₂ atmospheres at the pressure of 133 hPa. Implanted samples were obtained near room temperature at a dose of 10¹⁷ B⁺ cm⁻². The boron amount ( 2 μg cm⁻²) and the thickness involved ( 300 nm) although small, induced a noticeable reduction of the corrosion rate. At low temperatures (T ≤ 450°C), the rate law was paralinear and the only product formed was iron (II) sulphate in good agreement with thermochemical provisions. In this temperature range, boron was thought to have a catalytic effect as, when absent, iron (II) sulphate did not appear. At higher temperatures a duplex layer containing iron oxide and sulphide grew parabolically. Surface alloys obtained by laser alloying were 50–100 μm in thickness and exhibited compositions lying in the Fe---Fe₂B region of the phase diagram. Different microstructures were observed, depending on the irradiation conditions. The kinetic laws were parabolic or cubic, showing a greater protection when compared to implanted samples. Eutectic surface alloys exhibited the best behaviour for high temperatures or long times of oxidation. Pack-boronization at 950°C for 2 h in a powder mixture containing boron, ammonium fluoride and alumina led to a layer consisting mainly of Fe₂B. Kinetic experiments showed that such a coating exhibited the best protection at the lowest temperatures.     

碳钢在煤水两相介质中的腐蚀行为

采用失重及电化学方法研究了碳钢在煤水两相介质中 的腐蚀行为.结果表明,在煤水两相介质中,钢的腐蚀既不同于钢在均相水中的腐蚀,也不 同于与煤整体接触形成的腐蚀,腐蚀的初始形态为坑状,表象为均匀腐蚀.具有一定腐蚀性 的固相(高煤化度的煤)明显加速钢的腐蚀.     

Initiation events for pitting corrosion of stainless steel?

Events which could conceivably trigger off pitting corrosion of 18Cr-8Ni stainless steel in chloride-containing solution have been observed upon potentiostatic polarization of microelectrodes (10 μm diameter) of the steel. These events were undetectable on larger electrodes because they could not be distinguished above the background current. The events had amplitude 30–80 pA, frequency up to about 0.05 s⁻¹, and lifetime around 50 s. The average charge carried by an event was 100 pC, rather less than the charge required to dissolve one monolayer of the steel over the entire micro-electrode surface (600 pC). Most of the events were characterised by a sharp rise and slow fall of the current, in contrast to the current transients (slow rise, sharp fall) associated with the nucleation and temporary propagation of micropits on larger electrodes. It is inferred that the transients were associated with a microstructural feature in the microwire used to fabricate the electrodes. The results are rationalized with a model of very slow dissolution of one component of a binary alloy, and rapid dissolution of the other, with a different rate for different types of surface site.     

Dynamic variable depth of cut machining using piezoelectric actuators

This paper describes the design of a piezoelectric actuated cutting tool and implementation of digital servo controls for machining surfaces with dynamically varying depth of cut. Through a flexure hinge, the tool holder could generate 50 μm travel at the tip of the cutting insert. Tool motion errors of less than 0.5 μm were achieved in tracking cyclic waveforms by employing a digital repetitive servo control. When applied to turning aluminum and steel workpieces with variable depth of cut using carbide tools, less than 5 μm machined surface errors were measured.     

The importance of surface roughness for implant incorporation

318 threaded implants with a diameter of 3.7 mm, length 6 to 7 mm and a pitch-height of 0.6 mm were inserted in 56 New Zealand white rabbits. Implants of varying surface roughness were produced by a blasting procedure using 25m 75m and 250 μm sized particles. As- machined, (i.e turned) implants served as controls. The implant surface roughness was measured with the confocal laser scanner (TopScan 3D), and appropriate software was used for visual and numerical characterization. After implantation time of 4 weeks, 12 weeks or 1 year, the animals were euthanized. The implants were evaluated with respect to the peak removal torque, the percentage of bone-to-implant contact.The results from the animal experiments demonstrated, generally, higher removal torques and higher percentages of bone-to-implant contact for implants blasted with 25 and 75 μm sized blasting particles than with as-turned or 250 μm blasted implants. The corresponding average height deviation for the 4 surfaces was 1 μm 1.5 μm 0.7 μm and 2.0 μm When comparing the 25 and the 75 μm blasted implants it was found that 75 μm blasted screws showed the strongest bone fixation.     

Electrochemical characterization of the different surface states formed in the corrosion of carbon steel in alkaline sour medium

In this study the different surface states that manifest in the corrosion process of 1018 carbon steel in alkaline sour environment, solution prepared specifically to mimic the sour waters occurring in the catalytic oil refinery plants of the Mexican Oil Company (PEMEX) (0.1 M (NH₄)₂S and 10 ppm NaCN at pH 9.2) were prepared and characterized. The surface states of the carbon steel were formed by treating the surface with cyclic voltammetry at different switching potentials (E_λ+), commencing at the corrosion potential (E_corr=−0.890 V vs sulfate saturated electrode, SSE). The surface states thus obtained were characterized using electrochemical impedance spectroscopy and scanning electron microscopy techniques. It was found that for E_λ+=−0.7 and −0.6 V vs SSE a first product of corrosion formed, characterized by a high passivity. Moreover, it was very compact (with a thickness of 0.047 μm). However, at more anodic potentials (E_λ+>−0.5 V vs SSE) a second corrosion product with non-protective properties (porous with a thickness of 0.4 μm and very active) was observed. The diffusion of atomic hydrogen (H⁰) was identified as the slowest step in the carbon steel corrosion process in the alkaline sour media. The H⁰ diffusion coefficients in the first and second products that formed at the carbon steel–sour medium interface were of the order of 10⁻¹⁵ and 10⁻¹² cm²/s respectively.     

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.     

某湿气海底管道内腐蚀防护措施探讨

本文对某湿气海底管道的内腐蚀防护措施进行探讨,运用理论分析、预测软件模拟计算和实验室实验等方法,对包括材料选择为碳钢、合金钢内衬和合金钢内衬加碳钢组合的三种方式进行探讨。     

Effect of heat treatment on H2S corrosion of a micro-alloyed C–Mn steel

The H₂S corrosion resistance of a C–Mn pipeline steel with three different microstructures has been evaluated using electrochemical techniques with a 3% wt. NaCl solution at 50 °C. Microstructures included martensite, ferrite, and ferrite + bainite. Electrochemical techniques included potenthiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), and electrochemical noise (EN) measurements. Most of the tests lasted 24 h. All techniques showed that the highest corrosion rate corresponded to the steel with a martensitic microstructure; up to one order of magnitude higher than the corrosion rate for steels with a ferritic + bainitiic microstructure, whereas the steel with the ferritic microstructure showed the lowest corrosion rate. EIS tests showed that the corrosion process was under charge transfer control, whereas EN results indicated that the three steels exhibited a clear tendency towards a localized type of corrosion. However, for longer immersion times, the steel with a martensitic microstructure tended to exhibit a mixture of uniform and localized attack. Results were discussed in terms of grain size, grain boundary energy, amount and distribution of particles found in each steel.     

Tribology of thermal sprayed WC–Co coatings

This paper looks at the tribology of thermal sprayed WC–Co based coatings and covers the high energy air–sand erosion resistance and slurry jet impingement erosion performance, dry and wet sliding tribology of thermal spray WC–Co based coatings as well as the abrasion and abrasion–corrosion of these coatings. The tribological and tribo-corrosion performance of the coatings will be related to their mechanical and corrosion properties as well as deposition parameters, microstructure and actual composition. For example, the anisotropic microstructure of thermally sprayed WC–Co–Cr coatings, in particular the low fracture toughness in a direction parallel to the substrate, has been observed to affect the nature of crack formation under 200 μJ air–solid particle erosion conditions. Voids and occasionally other microstructural features (i.e., cobalt lakes, splat boundaries, interfacial inclusions) in the coating act as crack initiation sites. The erosion rate was dominated by cracks within 5 μm of the surface and was relatively insensitive to total length of cracks, showing a near-surface damage front controls the erosion rate and this region is coincident with the region of maximum shear stress induced by erodent impacts. Optimisation of the deposition parameters of HVOF 86WC–10Co–4Cr coatings show an improvement in erosion resistance of more than 50% over the conventional D-gun applied coating of identical nominal composition. The variation in the slurry erosion performance of the thermally sprayed coatings is also linked to directional fracture toughness and crack propagation paths which are influenced by the presence of pores, inhomogeneous carbide distributions and substrate grit blast remnants. The influence of slurry jet angle is more pronounced under 0.4 μJ energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to 7 μJ slurry erosion rates which were independent of jet angle. This reflects the lower levels of fluctuating stresses imparted to the coating during low energy slurry impacts leading to the impact angle having a greater effect on sub critical crack growth rate than for higher energy conditions.The abrasion resistance of these coatings was found comparable to sintered cermets of the same composition. The synergistic effects between micro and macro abrasion and corrosion for detonation gun (D-gun) sprayed WC–10Co–4Cr coatings are shown to be significant and depend on the environment. The size effect of the abradant relative to the microstructure and splat size is important as well as the propensity for the various phases to passivate to control corrosion levels. Comparisons between exposed and freshly polished coating surfaces in strong NaOH solutions (pH 11) show that significantly lower wear rates were seen for the exposed surface due to a negative wear–corrosion synergy due to selective phase removal and the effects of localised passivation.Dry and wet sliding wear resistance of these coatings is shown to be high (wear rates of 10⁻¹⁶–10⁻¹⁸ m³/Nm) with modest coefficient of friction levels between 0.2 and 0.5. The presence of oxides on the binder phases appears to influence the friction and wear levels. Wear appears to be by carbide ejection and/or by tribo-chemical processes.     

Corrosion testing of phosphatized steel samples

A method for corrosion testing of phosphatized steel samples has been described. The specimens, treated with different phosphatizing, solutions and covered by a chemically inert paint film, have been exposed in a sulphur-dioxide-polluted atmosphere with 90% relative humidity at 22.0°C. The corrosion rate has been measured as an increase in weight. The corrosion resistance shows a good correlation with the crystal coverage of the zinc phosphate layers. Filiform corrosion can be inhibited by zinc phosphate as well as iron phosphate coatings. However, underfilm corrosion is more frequent on the iron phosphatized samples.     

Fracture toughness of in situ TiCp–AlNp/Al composite

The effect of particle size, particle volume fraction, and matrix microstructure on the fracture initiation toughness of in situ TiCp–AlNp/Al composite was examined. The composites were Al matrix reinforced with 7.8–19.6 vol.% of TiC and AlN particles produced in situ by S-V-L reaction synthetics. The average particle diameters of TiC and AlN were 3.5 and 0.9 μm, respectively, which were distributed in the Al in a matrix dispersedly manner. The room-temperature plane-strain toughness measured using three-point bending specimens ranged from 12.7 to 37.5 MPa . Toughness was adversely affected by an increase in the TiCp–AlNp volume fraction. Fractography revealed that these composites failed in a ductile manner, with voids initiating at the in situ reinforcing TiC and AlN particles. The experimentally measured plane-strain toughness properties of in situ TiCp–AlNp/Al composite agrees with the Rice and Johnson model.     

Investigation of Cr, Cu and Zn sites in corroded/ uncorroded aluminum by X-ray absorption spectroscopy (XAS)

X-ray absorption spectroscopy (XAS) was used to characterize the Cr, Cu and Zn sites in 7175 T736 aluminum which had exhibited corrosion at the bond between the Al and ethylene propylene diene monomer (EPDM) during 9.8 years of storage. The characterization was concerned with the Cr, Cu and Zn coordination chemistry (chemical valence, coordination number, bondlength, disorder, and species of surrounding atoms) of corroded and uncorroded samples. The results show that only the Cu site in the Al alloy significantly changed during the process of corrosion. After corrosion no chemical shift was observed in the Cu K-edge (8979 eV) nor was the significant presence of Cu oxide, sulfide, chloride or hydroxide found. Analysis of the data using XAS methods indicated that ˜20% ± 10% of the Cu sampled (to a depth of ˜ 104 μm using fluorescent yield) was present as disordered, metallic Cu after corrosion. The remainder of the Cu was present in the alloy as it would be found in uncorroded 7175 T736. Very little Cu was found in the top ˜ .235 μm of the corrosion layer as sampled by electron yield methods.