Volume 36, Issue 1 - Special issue: Recent advances in corrosion science / Issue editors: Masahiro Seo, Hideaki Takahashi and Hiroki Habazaki

Microelectrochemical approaches using a microcapillary cell (MCC), scanning electrochemical microscopy (SECM), and in situ ellipsomicroscopy for studying heterogeneous passive or corroding surfaces of materials are reviewed. An MCC can be used to investigate the localized behavior of a site of interest on a material by various electrochemical methods, including electrochemical impedance spectroscopy. SECM has often been used for imaging corroding surfaces or passive films on materials. Moreover, the use of a liquid-phase ion gun, a mode of SECM that forms a local aggressive environment at the solution/material interphase, can reveal the depassivation mechanism and kinetics of the surface of a material. The heterogeneous growth or degradation of a thin passive film on a material has been monitored using in situ ellipsomicroscopy, and a depassivation site has been successfully found before a film breakdown is initiated. These microelectrochemical methods are useful for monitoring the heterogeneous distribution of a passive film and for investigating the heterogeneity of the passivity of materials.

The experimental results reported so far for the inhibition effect of underpotential deposition (UPD) of metallic cations on aqueous corrosion of metals are criticized and discussed based on the prerequisite for UPD. The inhibition effect of Pb 2+ , Cd 2+ , Zn 2+ or Mn 2+ on hydrogen evolution or hydrogen absorption on Armco iron or various steels is classified into three potential regions: bulk deposition, UPD and adsorption of metallic cations without discharging. Moreover, the inhibition effect of Pb 2+ , Tl + or Sn 2+ on anodic dissolution of pure Fe, Ni or Ni-base alloys in acidic solutions is ascribed to the UPD of these metallic cations. Particularly, in situ X-ray absorption spectroscopy (XAS) for Pb-UPD on Ni has indicated that the electro-deposited Pb species on the Ni surface is metallic to form partly surface alloy and the metallic Pb blocks the active dissolution sites of Ni to inhibit the anodic dissolution. In contrast, in situ XAS for Sn-UPD on Ni has indicated that the electro-deposited Sn species is not only bonded with Ni atoms but also bonded with oxygen atoms and that the anodic dissolution of Ni is inhibited by the oxygenated Sn species on Ni.

Anodic oxide films on aluminum are classified into two groups: porous-type anodic oxide films (PAOF) and barrier-type anodic oxide films (BAOF). The present paper is a review of the corrosion of pure aluminum (Al) and Al alloys covered with PAOF and BAOF, focusing on the role of anodic oxide films in the process of corrosion. Several topics are discussed in detail, including (a) changes in the dissolution mode of PAOF in acidic media by pore sealing, (b) hydration and dissolution of BAOF in pure water and neutral solutions containing organic and inorganic electrolytes, (c) pitting corrosion during the cathodic polarization of Al covered with PAOF and BAOF, (d) corrosion of PAOF-covered Al/Bi/Sn alloys in alcohols at 142°C, and (e) synergistic effects of Cl − and Cu 2+ ions in the corrosion of PAOF-covered Al alloys in aqueous solutions.

Two types of self-repairing coatings for the protection of Al and its alloys are reviewed: (1) organic coatings with capsules containing repairing agent and (2) porous anodic oxide films with inhibitor solution stored in the pores of the oxide film. First, polyurethane microcapsules containing liquid surface-repairing agents were synthesized and polyurethane coating with the capsules was painted on Al alloy specimens. After mechanical damage to the coating, self-repairing occurred by the reaction of water vapor in the air with the repairing agents released from the capsules. Second, porous-type anodic oxide films were formed on Al alloys, and the pores of the anodic oxide films were filled with inhibitor solutions, followed by application of a covering polyurethane layer. Inhibitors released from the pores efficiently protected the Al alloy substrate from corrosion arising from induced mechanical damage.

An increasing demand for electricity and preserving the environment has become a great driving force for the development of advanced boilers for coal-fired plants to improve their plant efficiency by raising the operating temperature. To achieve this purpose, the development of materials that possess a high mechanical strength and excellent resistance against corrosion at high temperatures is required. Considering such situation, the high-temperature corrosion behavior of SiO 2 -forming materials such as CrSi 2 -Ni and CoNiCrAlY-Si alloys in atmospheres containing NaCl and Na 2 SO 4 is introduced. The SiO 2 scale plays a significant role in suppressing the inward diffusion of sulfur and chlorine, resulting in the remarkable suppression of the formation of sulfide and chloride at the scale/substrate interface and inside of the substrate. Based on such results, it is emphasized that SiO 2 -forming materials show outstanding corrosion resistance against atmospheres containing NaCl and Na 2 SO 4 .

Cyclic corrosion, consisting of carburization in Ar+4% CH 4 +3% H 2 for 1 h and oxidation in air for 1 h, of Ni-base in a wrought alloy, Hastelloy-X, was investigated at 1000°C to understand the significant metal loss occurring in fuel injection nozzles of gas turbine combustor operated using natural gas. The mass gain of cyclic corrosion was relatively low up to about eight cycles of corrosion by formation of a Cr 2 O 3 scale in each oxidation stage, but it increased rapidly due to formation of a Fe- and Ni-rich oxide scale as the number of corrosion cycles increases. The Cr content in the subsurface region rapidly decreased under the cyclic corrosion condition compared with a continuous single oxidation or carburization, which resulted in the formation of non-protective Fe- and Ni-rich oxide scale in a small number of corrosion cycles. The formation of a Cr 3 C 2 scale from the protective Cr 2 O 3 scale in the carburization stage was detrimental for the oxidation resistance of Hastelloy-X and was considered to have a similar effect to spallation of a Cr 2 O 3 scale; therefore, it accelerated Cr consumption from the alloy subsurface region.

Growth of passive oxides on iron and titanium under the non-stationary state was studied during constant current density (CD) oxidation and potential sweep oxidation. Increase and decrease of the thickness of the passive oxide film were determined from the relation between dissolution and growth CDs. By using rotating Fe disc-Pt ring electrode, the CDs of the dissolution from the oxide to solution and the growth of the passive oxide on iron were simultaneously measured. From the ratio of the growth CD to the dissolution CD and the pH dependence of the growth CD, a possible mechanism was discussed. Next, the growth of passive oxides on iron in pH 8.4 borate solution and on titanium in 0.1 m sulfuric acid solution during the potential sweep oxidation was examined by using three-parameter ellipsometry. Oxide growth was initially delayed when the sweep in the positive direction was started, and then the oxide film linearly grew with the potential increase. It was found that the oxide growth rate influenced the properties of the oxide film. The electric field in the oxide film during the non-stationary growth was discussed, concerned with the CD of the oxide film formation.

The influence of metal cations on the ability of gluconates to inhibit corrosion of mild steel in a model fresh water was investigated by electrochemical impedance spectroscopy (EIS) and immersion tests. The EIS and immersion test results showed that the corrosion inhibition of mild steel in model fresh water by gluconates was improved by metal cations that have a large value of corrosion inhibitory effect of cation, Y. The X-ray photoelectron spectroscopy (XPS) analysis showed that metal cations that have larger Y values are bonded on mild steel. It was thought that metal cations with large Y values and gluconates confer a synergistic inhibition effect on the corrosion of mild steel in model fresh water.