Hiroo Nagano

The long term growth of the protective rust layer formed on weathering steel by atmospheric corrosion during a quarter of a century

Abstract The rust layers formed on weathering and mild steels by atmospheric corrosion in an industrial region for a quarter of a century have been characterized using various analytical techniques. In particular, analysis of a local portion of the rust layers by means of Raman spectroscopy gives important information on the structure of the layers. It is elucidated that the inner stable and protective rust layer which covered the surface of weathering steel mainly consists of nano-particles of α-FeOOH containing a considerable amount of Cr. The relative change of the amount of rust constituents for low alloy steels supports well a newly proposed schematic progress of long term alteration in stable and protective rust layer formed on a weathering steel in an industrial environment, i.e. the γ-FeOOH, as an initial rust layer, is transformed into a final stable rust layer of α-FeOOH, probably via an amorphous oxyhydroxide substance, during the long term atmospheric corrosion of a weathering steel.

Dependence of corrosion potential and corrosion rate of a low-alloy steel upon depth of aqueous solution

Abstract The Kelvin vibrating-capacitor technique has been used to measure the corrosion potential and the oxygen-consumption method has been applied to measure the corrosion rate of a low-alloy steel under very thin layers of electrolyte in air. The layer thickness dependencies of the corrosion rate and of corrosion potential are due to kinetic control by oxygen diffusion, and the latter dependence permits the Tafel slope of the anodic dissolution reaction to be evaluated. The anomalously large value of the Tafel parameter may be understood in terms of partial passivation of the metal surface by the intermediate reaction product, FeOH. This signifies a distinct difference from corrosion in the same system under bulk solutions.

Effect of Heat Treatment on the Susceptibility to Stress Corrosion Cracking of Alloy 600

It was shown that a modified Huey test, i.e., a 24-h immersion in boiling 40% HNO3, is able to exactly detect the susceptibility to intergranular corrosion of Alloy 600, which is correlated with intergranular stress corrosion cracking (IGSCC) behavior in high temperature water containing small amounts of chloride ions. Chromium content at the regions adjacent to grain boundaries in sensitized Alloy 600 was analyzed as ~3% by the measurement of magnetic susceptibility.Heat treating Alloy 600 at low temperatures, e.g., at 700°C for 15 h, improves the IGSCC resistance to both high temperature water and caustic solutions by constructing suitable microstructures resistant to sensitization, in which chromium carbides precipitate predominantly in the matrix instead of grain boundaries.

The stress corrosion cracking performance of pure zirconium and zirconium alloys in highly oxidizing nitric acid

Abstract Zr has been used for many plants dealing with nitric acid due to its high corrosion resistance. However, stress corrosion cracking (SCC) occurrence has recently been reported on Zr immersed in hot nitric acid. Here corrosion performance and countermeasures against SCC of Zr, Zr-Ti and Zr-Ti-Ta alloys in HNO 3 were studied. Ti has good general corrosion and SCC resistance in highly oxidizing HNO 3 , although it is severely attacked in pure HNO 3 at elevated temperatures. Ta is an excellent corrosion resistant material over all concentrations of nitric acid. Alloying Ti and/or Ta into Zr has synergistic effects on improving both the general corrosion and SCC resistance. The optimization of corrosion resistance has been studied in Zr alloys.

Corrosion resistance of zirconium and zirconium-titanium alloy in hot nitric acid

Abstract Since zirconium (Zr) has superior corrosion resistance to concentrated nitric acid (HNO 3 ) at elevated temperature, it has been successfully used as a material for chemical plants to produce HNO 3 or spent nuclear fuel reprocessing plants. However, the occurrence of stress corrosion cracking (SCC) has recently been reported in hot nitric acid conditions. The necessary conditions for SCC in Zr, and the preventive method against SCC by alloying were investigated. The slow strain rate technique test, electrochemical measurements, strain electrode tests, and analysis of corrosion products were carried out in this work. SCC does not occur in Zr in boiling HNO 3 at concentrations less than 70%, whereas SCC is initiated in 6–94% HNO 3 when a potential is applied above the critical potential for SCC. A new invention of Zr-15%Ti alloy is immune to SCC under applied potential conditions in hot HNO 3 ; this alloy forms a stable passive film composed of ZrO 2 and TiO 2 .

Study on Water Adsorption-Desorpton on Metal Surfaces and the Early Stage of Atmospheric Corrosion in Steels

Atmospheric corrosion of steel is comparatively different from the corrosion in bulky water or seawater in the sense of smaller corrosion loss. In general, steel exposed to the air forms thin rust layers under wet and dry alternating atmospheric conditions, reaching a stabilized lower corrosion rate with time. This paper was aimed at studying the early stage of rust formation on steel for clarifying the mechanism of high corrosion resistance of weathering steel. First, water adsorption-desorption phenomena were measured with an ellipsometer and Kelvin microprobe for pure Fe, Ni, Cr, and weathering steel. Second, corrosion potential of weathering steel was measured with a Kelvin microprobe as a function of the thickness of water layers on the metal surface. All specimens of as a wet polished surface condition increase water adsorption with relative humidity (RH), whereas the specimens pre-oxidized at 523K hardly adsorb water irrespective of RH. Work function o calculated from Kelvin potential responds well to the amount of water adsorbed on metal surfaces or trapped within oxides. Specimens as polished show decrease in o with the amount of adsorbed water, on the other hand, specimens with pre-oxides on their surfaces indicate a little decrease in o for pure Fe, Ni, and almost no change for pure Cr. The corrosion potential for the weathering steel under the 200 and above 200 μm thickness of 1M sodium sulfate solution was around -600mV (SHE), meaning that this steel corroded at an active dissolution potential under the control of oxygen diffusion through the solution layer. When the thickness of the solution layer was reduced down to between 0.2 and 10 μm, the corrosion potential was shifted upward to -100mV, meaning that the steel was in a passive potential region where ferric oxyhydroxide could be formed.