Volume 41, Issue 5

The review describes an electrode kinetic model of aluminum pitting nucleation taking into account the charge of the metal surface, the adsorption of chloride ions on the oxide surface, their penetration through the oxide film with the help of oxygen vacancies, and the dissolution of the underlying substrate with the initiation of pitting corrosion at the metal/oxide interface. It is shown that the critical pitting potential is a function of the potential of a thin layer of aluminum surface covered by oxide and the value of the pitting potential of a binary surface alloying is related to the isoelectric point of the alloying element oxide in the binary alloy. An electrode kinetic model of pitting initiation is described, which was used to explain the effect of surface alloying on the pitting initiation on binary alloys. A method for changing the surface charge by forming foreign surface organosilicon nanolayers bearing both negatively and positively charged groups was proposed. It has been established that four characteristics ( q , Ψ 1 -potential, E pit and the tendency of aluminum to depassivate) depend on the nature of ion-exchange groups, the degree of their acid dissociation, and the ion-chemical interaction with activator ions.

Hydrogen permeation technique is a widely used testing method for the determination of hydrogen diffusion coefficient ( D ), which is an important parameter considering hydrogen embrittlement. A palladium (Pd) or nickel (Ni) coating is often utilised on the hydrogen detection side of the test specimens. Here, we investigate the effect of Pd and Ni coatings on hydrogen diffusion in a martensitic 500 HBW hardness low-alloy steel in the thickness range of 0.5 – 0.8 mm using a refined successive transient method and compare against an uncoated reference specimen. Both coatings yield similar average D values (6 – 6.6 × 10 −7  cm 2 /s), but the best repeatability is achieved with Pd coating. With Ni coating, D values decrease with the increasing specimen thickness, which is partly caused by a slower hydrogen diffusion in Ni, and therefore a concentration gradient at the specimen-coating interface. The uncoated specimen has a poor transient fit, and significantly lower D (2.1 × 10 −7  cm 2 /s) due to surface oxidation. With both coatings, the steepness of the last decay transient was highly affected by specimen thickness, and therefore the density of reversible hydrogen traps is only comparable for similar thicknesses.

The main objective of the EU-funded project mitigating environmentally-assisted cracking through optimisation of surface condition (MEACTOS) was to gain knowledge on the ability of different surface machining procedures to mitigate environmentally-assisted cracking (EAC) in typical light water reactor structural materials and environments. Surfaces of cold-worked (CW) type 316L austenitic stainless steel and nickel-based weld metal Alloy 182 flat tapered tensile specimens were machined using different processes. EAC initiation susceptibility of these specimens was evaluated using constant extension rate tensile (CERT) tests under simulated boiling water reactor (BWR) and pressurized water reactor (PWR) conditions and assessed using constant load experiments. More than a hundred tests were performed covering about 10 years of autoclave testing time. Only minor or no measurable improvements in EAC initiation susceptibility as a function of surface treatments (grinding or advanced machining) compared to the standard industrial face milling were demonstrated. In most cases, the stress thresholds for EAC initiation determined in constant load tests confirmed the trend obtained from CERT tests. This paper summarises the most important results and conclusions concerning the EAC initiation behaviour for the CW 316L and Alloy 182 under reducing PWR and oxidizing BWR conditions.

The microbiologically influenced corrosion (MIC) behavior of 304 stainless steel by sulfate-reducing bacteria (SRB) in ZnO nanofluids was studied by electrochemical measurements, antibacterial activity tests and surface analysis. The results showed that ZnO nanoparticles (NPs) had little effect on the corrosion resistance of 304 stainless steel, but could effectively inhibit the MIC of stainless steel and reduce the adhesion of biofilm on its surface, which was related to the antibacterial effect of ZnO NPs. The antibacterial effect of 100 mg/L ZnO NPs reached 89.5 %. Electrochemical tests showed that with the increase of the concentration of ZnO NPs, the passive current density of the stainless steel electrode decreased, the film resistance increased in ZnO nanofluids containing SRB, and the MIC was inhibited. The antibacterial effect of ZnO NPs is mainly due to the formation of strongly oxidising ·OH in water. The presence of ZnO NPs inhibited the formation of biofilm and deposition of sulfide on stainless steel surface, thus slowing down the corrosion damage of SRB to stainless steel.

The inhibition of the corrosion of L245N steel in the CO 2 -saturated oilfield produced water by the extract of hairy bamboo leaf extract (BLE) has been investigated using electrochemical experiment (EIS), weight loss test, Fourier transform infrared spectroscopy (FTIR), molecular dynamics simulation (MD), and scanning electron microscopy (SEM). The results showed adsorption of BLE on the surface of L245N steel conforms to the Langmuir isothermal adsorption. Electrochemical measurements indicated the maximum inhibition efficiency of 98.76 % at 45 °C and 4 (%v/v). FTIR analysis showed that the BLE contains functional moieties such as O–H, N–H, C=O, C=C, C–N, C–O, and orientin as the effective compound. SEM illustrated that with the addition of BLE, the corrosion of L245N metal is significantly reduced. MD simulation showed that BLE molecules tend to adsorb on the Fe surface, forming an inhibitor film.

The corrosion behaviour of 6061Al-15 % (v) SiC (P) composite was investigated in a 0.5 M HCl medium using electrochemical techniques. A water-soluble inhibitor, N-(1-morpholinobenzyl) semicarbazide (NMSc), was synthesised to test its corrosion inhibition activity on 6061Al-15 % (v) SiC (P) composite. The inhibitor showed cathodic-type behaviour and 95.42 % inhibition efficiency at 2.56 mM concentration and 303 K temperature. The thermodynamic results revealed the inhibitor’s physisorption, which followed Langmuir’s isotherm model. A drastic reduction in corrosion current density in the inhibited medium indicates that the inhibitor effectively controls the deterioration of the composite in the HCl medium. A significant rise in polarisation resistance on increasing inhibitor concentration offers greater resistance for the charge transfer process, ensuring better control over the composite corrosion. The surface analysis by scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirms the inhibitor film formation on the composite. The quantum chemical theoretical calculations supported the electrochemical results.

The corrosion behavior of GH3535 alloys in KCl–MgCl 2 eutectic salts with or without magnesium (Mg) was investigated using the static immersion tests. The effect of purification treatment on the corrosion of GH3535 specimens was also evaluated by the different analysis. The KCl–MgCl 2 salts purified with Mg and reducing service temperature can slow down the corrosion rate of GH3535 specimens. Infrared spectra results showed that purification treatment reduced the content of crystal water in the KCl–MgCl 2 salts. The difference of hydrate content in KCl–MgCl 2 salts and the diffusion rate of Cr in alloy are the main reasons for the change of GH3535 specimens corrosion rate.