Recent advances in the field of corrosion failure evaluation by the extreme value statistics have been reviewed. Historical background of the extreme value statistics, and recent developments in the theory and application to corrosion are described. The extreme value distribution contains three asymptotic distributions which are derived from the initial distributions, but the first type for the maximum value, i.e., the Gumbel distribution and the third type for the minimum value, i.e., the Weibull distribution are mainly used for evaluating the maximum depth of corrosion penetration and the minimum time for corrosion failure, respectively. It is emphasized that the parameter estimation could be accomplished simply by using a minimum variance linear unbiased estimator (MVLUE) method which has been refined recently. Several problems in applying the MVLUE method were discussed with referring experiences in the data analysis.

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An appropriate evaluation technology of corrosion is essential for the development of precoated steels for automotive use. There are various test methods: field surveys, monitor car tests, proving ground tests, vehicle exposure tests, under- and on-vehicle tests, the Volvo test and laboratory tests. They are briefly explained.
A cyclic corrosion test (CCT) which simulates and accelerates the field situations is usually carried out in the laboratory. For the design of a CCT, micro-climate in vehicles, corrosion mechanism and environmental tactors must be clear. The environmental factors taken into account for CCT are salt, temperature, humidity, wet/dry ratio and cyclic frequency. In addition, water immersion, ultraviolet rays and air pollutants are of importance in their own ways.
A perforation test is conducted with hem flange models or lapped panels in the CCT. Cosmetic evaluation consists of paint adhesion, wet adhesion in water and paint degradation by ultraviolet ray.
When a corrosion test is carried out in the CCT, it is recommended to test cold rolled steel and certain zinc coated steel, the behaviors of which in the field are well-known, together with other specimens. The lifespan of these specimens could be estimated in comparison with such steels.

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Anodic segregation of selected alloy constituents is found by ESCA analysis to occur as a result of active dissolution and during the passivation of stainless steel in deaerated acid. The resulting surface composition of the steel is radically different from the bulk and conditions seem to favour the formation of surface phases of intermetallic compounds. The resulting surface phases appear to control the kinetics of active dissolution and the composition of the passive film. In this respect we propose that in the more complex stainless steels passivation may be due to the formation of a duplex kinetic barrier consisting of an outer oxide based passive film and an inner intermetallic surface phase having a higher lattice binding energy than the bulk alloy. This paper will focus on the influence of anodically segregated Ni, Mo and N in a range of austenitic stainless steels exposed to deaerated HCl solutions.

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The pH-dependence of the passivation of Fe, Fe10Cr and Fe20Cr was investigated in borate buffer solution (pH 8.4) and H₂SO₄ (pH 0.3) under potentiostatic conditions. The influence of the pH is due to the pH-dependence of the potential of the oxide electrode and the formation of CrO₄²⁻ which results in additional surface charge. The potential drop within the Helmholtz layer will be enlarged and a higher corrosion rate takes place.
In the passive range 0.2 V≤U≤1.2 V the layer properties are independent of pH. The rise of the potential drop at the phase boundary yields an increase of the passive corrosion current density.
In the range of transpassive dissolution of Cr (1.2 V<U≤1.7 V) the layer dissolution increases with decreasing pH. The start of the oxide growth occurs earlier due to a thinning of the initial layer in acid media. The transpassive Cr-dissolution out of the stationary oxide film depends upon the transport through the Fe-oxide matrix. It increases with decreasing pH due to the higher corrosion rate of the Fe-oxide and the resulting greater amount of Cr-ions at the surface.

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Anodic behavior of amorphous Fe40Ni40B (20-X)PX alloys with X ranging from 0 up 20, in buffered solutions, pH ranging from 4.0 up to 8.4, is reported. Electrochemical measurements give evidence, that P improves ability to passivate and stability of the passive state of these alloys; the effect not attributable to a specific Cr-P interaction in this case. Data concerning chemical state of passive film constituents, in particular of these containing P, are analyzed. A possible mechanism of P action is discussed.

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Passive films on high purity Fe-Cr alloys (13-24 mass% Cr), which were prepared by an Ar plasma melting method using high purity Fe produced by a method combining anion exchange and zone-refining processes and high purity Cr produced by an iodide process, have been investigated in-situ in pH 6.0, 1 kmol·m^–3 Na₂SO₄ using ellipsometry and potential-modulated reflection spectroscopy. The thickness, d, optical constants, N₂, and cationic mass fractons of Cr³⁺ ions, X_Cr, of the films were determined as a function of potential and Cr content of the alloy. The same analyses were performed on 400 series ferritic stainless steels (13-24 mass% Cr) to elucidate the changes in the film properties with the change in the purity of alloy. It was found that for the passive films on high purity Fe-Cr alloys the value of d increased and, at the same time, that of X_Cr decreased with increasing potential. At a given potential, the values of d and N₂ decreased with increasing Cr content of the alloy and those of X_Cr increased correspondingly. Similar results were obtained on commercial ferritic stainless steels. The values of d, N₂ and X_Cr changed in dependence on the purity of alloy. That is, the values of d and N₂ decreased and those of X_Cr increased with increasing purity. The increase in X_Cr of the films should contribute the increase in the corrosion resistance of the high purity alloys.

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Our previous works have demonstrated that the adherence of Al₂O₃ coating layer to alloys is decreased by interfacial segregation of S but increased by interfacial precipitation of TiC. In this work we tried to improve the adherence of the Al₂O₃ coating layer by simultaneous suppression of S segregation and promotion of TiC precipitation. For this purpose surface analysis by AES was made at 1 100 K on Ti containing alloys doped with Ce or Y₂O₃ for suppressing S segregation, and other reference alloys. The adherence was evaluated through cyclic oxidation test at 1 100 K in pure O₂. In accord with the expectation, the Al₂O₃ coating layer was the most protective on MA956-0.52Y₂O₃ alloy which generates TiC surface precipitation and suppresses S surface segregation very well. In order to elucidate the Al₂O₃/alloy interfacial phenomena, some alloys partially coated with an extremely thin Al₂O₃ layer were heated in an apparatus of XPS. Chemical shift of the peaks of O 1s and Al 2p by heating was observed on an alloy on which the surface segregation of S takes place extensively, but not on alloys on which S segregation is slight. This result, along with the oxidation result, affords an important evidence of our previous conclusion that Al₂O₃/alloy bonding is largely weakened by S segregated at the interface.

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Slurry erosion-corrosion tests were conducted on type 304 stainless steel in order to investigate whether the passivating film has any effect of decreasing the damage to the material. Two environments which contrasted with each other were chosen: a slurry consisting of silica sand particles and a 1 % sulfuric acid solution where the passivating film is stable, and a slurry of the same particles and a 5 % hydrochloric acid solution where it is not.
As a result, the material was damaged at higher erosion-corrosion rates in the environment where the film was stable than in that in which it was not. Thus, the ability of the film to decrease the damage appeared to be denied. Nevertheless, the anodic polarization curve of the material in the 1 % sulfuric acid solution slurry revealed that the passivating film was mechanically broken by the impact of the particles but it was repaired rapidly enough to inhibit corrosion. Further it was found that in the 5 % hydrochloric acid solution slurry, erosion as well as corrosion was inhibited by chloride ions adsorbed on the surface.
Consequently it was concluded that the passivating film has an ability to inhibit erosion-corrosion damage to a certain extent through inhibiting corrosion so long as it is chemically stable in the environment.

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The layer structure and composition of passive films formed on iron in borate and phosphate solutions at pH 8.42 and 11.50 have been investigated by means of cathodic reduction combined with ellipsometry and chemical analysis. Breakdown of the passive films has also been examined in 0.5 kmol/m³ NaCl solution at a constant potential higher than a critical pitting potential in the same solution. It is found that the composition and breakdown of the passive films change with the potential of film formation, anion species and solution pH, while the film is always of two-layer structure consisting of a barrier layer in contact with the metal and a hydrated deposit layer next to the solution. The depedence of anion species and pH on the composition and breakdown of the films may be systematically explained in terms of an ion selective property of the passive films. Therefore it is suggested that the ion transport in the layers plays an important role in determining the passive film composition and the breakdown.

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A ceramic sensor for monitoring the hydrogen permeated through steels at high temperatures using an electrochemical method has been developed. A new proton conductive solid electrolyte (5 mol%Yb₂O₃-SrCeO₃) was used as an electrolyte in this sensor. It was found possible to measure the diffusivity and the content of hydrogen permeated through a plain carbon steel at 773 K and 0.101 MPa hydrogen gas by joining the sensor to the steel with a platinum past. However, the detectability of hydrogen was poor, because joining of ceramics/steel exerted a large influence on it. Joining of the ceramics to the platinum-plated steel with amorphous copper filler metal led to the great improvement of the detectability of hydrogen. This sensor will be useful for predicting hydrogen attack of steels in operating chemical plants.

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The corrosion resistance of titanium exposed in nitric acid vapor is generally inferior to that immersed in nitric acid solution because of an inhibitor effect of elements dissolved by corrosion of titanium in the solution.
A new type corrosion testing apparatus, which would not be influenced by such dissolved elements on the corrosion rate, was designed in order to easily estimate the corrosion rates in a nitric acid condensed zone, a vapor zone and a liquid zone simultaneously. This apparatus was composed of a condenser, an outer vessel, an inner vessel with a siphon pipe, and an Erlenmeyer flask. The solution condensed in the inner vessel was automatically removed to the flask through the siphon pipe and renewed at regular intervals. The nitric acid concentration and the temperature of the above each zone in this apparatus was similar to the data obtained from the previously reported vapor-liquid equilibrium line. As the concentration of dissolved elements in the solution of the condensed zone of this apparatus was sufficiently negligible, the corrosion rate in the condensed zone could be accurately estimated without the influence of dissolved elements from the corroded metal itself.
Using this testing apparatus, the corrosion rate of titanium in the condensed zone was 0.37 mm/year (1.17×10^–11 m/s), greater value than those in any other zone and general corrosion occurred. Moreover, the corrosion rate in the liquid zone was less than 0.01 mm/year (3.17×10^–13 m/s) and this rate was regarded as the perfect corrosion resistance. That is because of concentrated soluble titanium ions in the solution acted as a corrosion inhibitor.
In the case of 304L stainless steel, the corrosion rate was 4.5 mm/year (1.4×10^–10 m/s) in the liquid zone at the highest value, and the intergranular corrosion occurred. On the other hand, the rate in the condensed solution was small to be 0.095 mm/year (3.01×10^–12 m/s).

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The corrosion behavior, in particular passivation, of corrosion resistant alloys (CRAs) in sour environments was investigated intensively by using a duplex stainless steel and an austenitic stainless steel as representative CRAs. It was found that hydrogen sulfide significantly retarded the passivation of CRAs, whereas carbon dioxide did not. The changes in corrosion morphologies due to the changes in environmental aggressiveness were classified on the basis of the immersion test results. A new criterion for predicting the critical environments for active to passive transition was proposed. The critical environments for the transition of a given alloy can be determined by comparing the pH of environments with the depassivation pH (pH_d) of the material which is a function of environmental factors such as temperature and partial pressure of hydrogen sulfide. The locus of the points having pH=pH_d gave a good estimation for the critical environments. It was confirmed that the prediction was in good agreement with the long-term immersion test results for 6 months. Since this criterion is based on the corrosion mechanism, it has many advantages; the prediction is accurate; the results for CO₂-free environments can be extended to CO₂-containing environments; and the prediction can be performed very quickly.

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Sulfate-reducing bacteria can cause or influence corrosion in both carbon (or low-alloy) steels and stainless steels. The electrochemical reactions and rate-controlling processes are quite different in these two cases, the common feature being the catalysis of anodic dissolution by adsorption of hydrogen sulfide. In this paper the inorganic background to SRB corrosion is reviewed, and recent experimental results are described. For carbon steel, it is shown that corrosion occurs at some 100 μA/cm² in uniformly anaerobic conditions with hydrogen evolution on a large surface area of iron sulfide as the cathodic reaction. Electrochemical impedance spectroscopy and related techniques are not always useful in monitoring or understanding this corrosion, even when the attack is uniform, owing to effects introduced by the very large capacitance of the porous FeS film. Chloride ions and precorrosion help to create uniform rather than pitting-type attack. For stainless steel, a special chromium-depleted alloy (Fe-15Cr-10Ni) has been used to study and enhance the pitting induced by SRB in Cl^--containing media. Catalysis of pitting dissolution by H₂S requires a remote O₂ cathode to be effective; reduction of sulfur species such as polysulfide (HS_x⁻) is only barely able to sustain pitting under the most ideal conditions. Chloride must be the predominant anion, otherwise pitting is inhibited. A micro-cell arrangement is demonstrated that enables the stability of SRB-induced corrosion in an anaerobic micro-environment to be studied under conditions of bulk aeration.

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The effects of chlorination on the localized corrosion of stainless steel in saline environments have been studied by probing the detailed electrochemical environments with growing pits formed on UNS S30100, using micro-electrodes to determine the local pH, chloride ion concentration and potential.
In chlorinated systems, the reduction of free chlorine is the major cathodic electrode process. The strong reduction tendency of the free chlorine species increases the free corrosion potential of a steel. A model of chlorination-enhanced localized corrosion of stainless steels is proposed, wherein the growth of pits and crevices is influenced by the reduction of chlorine oxidant stimulated by the low local pH associated with sites of localized attack.

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Duplex stainless steel is widely used in various service conditions due to its high mechanical properties and high corrosion resistance. However, there have been few studies on the corrosion resistance of duplex stainless steel in highly oxidizing nitric acid, such as hot nitric acid containing Cr⁶⁺ ions. In this study, the effect of Si content (Si=0–4 %), austenitic phase (γ) ratio in volume and N addition (N=0.1 %) on the corrosion resistance of 23-25 Cr duplex stainless steel was investigated. Corrosion test was carried out in boiling 8N HNO₃+0.2 g//Cr⁶⁺ using TIG welded joint specimens. The relationship between microstructures and configuration of the corrosion attack was analyzed.
Results show that Si addition (Si>3 %) to duplex stainless steels is effective in improving corrosion resistance. Si-bearing duplex stainless steel without addition of N, however, was attacked at HAZ when γ ratio was less than 10 % and more than 40 %, because of α phase sensitization and σ phase formation, respectively. Nitrogen content affected γ ratio, nitride and carbide precipitation. A γ ratio more than 50 % was found to be most beneficial for the resistance to intergranular corrosion of the 25Cr-Ni-4Si-N duplex stainless steel.

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Since stress corrosion crack initiation/propagation process is essentially a stochastic one, a series of uniaxial constant load tests have been conducted on sensitized 304 stainless steel specimens in neutral chloride solutions to examine the effects of stress, temperature, crevice and chloride ion concentration on probability distribution of intergranular stress corrosion cracking lifetime. By examining the effects of various acceleration factors, the lifetime distribution can be satisfactorily presented by the exponential probability distribution model as that of sensitized stainless steels in high-temperature and high-purity water environment systems. The tendency of both the location parameter, i.e., lower limit of lifetime distribution, and the scale parameter, i.e., standard deviation of lifetime distribution, to increase with decreasing stress, temperature and chloride ion concentration, also the ratio of those two parameters is constant.

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Amorphous iron-base alloys typically contain large amounts of metalloids (B, P, C). The role of the metalloids is to facilitate the glass-formation but they very strongly influence the corrosion behaviour, too. Addition of phosphorus to iron-chromium alloys modifies the passivation mechanism and leads to self-passivation even in strongly acidic deaerated solutions. A highly stable chromium oxyhydroxide passive film with significant amounts of oxidized phosphorus incorporated is formed on the surface of the P-containing alloy. The amorphous alloy Fe-10Cr-13P-7C shows an extremely high resistance against localized attack in Cl^–-containing solutions. This is due to a low pit initiation rate and a high repassivation ability of the P-containing alloy.

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A new HIP and hot extrusion (HHE) process for manufacturing bimetallic pipes and tubes is proposed. The purpose is to produce bimetallic pipes and tubes the liner and substrate of which consist of those having a large difference in their hot deformation resistances. The most important point is to join the two metals metallurgically, by hot isostatic pressing, before hot working such as hot extrusion. After the feasibility of the process had been verified by the preliminary study at a laboratory scale, a trial production was conducted for a full scale corrosion resistant pipe for oil country tubular goods (OCTG) for sour service. The inner and outer materials of the pipe were a nickel-base superalloy and SSC resistant low alloy high strength steel, respectively. A bimetallic pipe with uniform thickness of both outer and inner metals was obtained by the new process. The corrosion resistance of the two layers of metals as well as the strength of the outer material exhibited satisfactory performance to comply with the requirements for OCTG use. The proposed process, HIP and hot extrusion process, is promising for producing bimetallic materials with excellent corrosion resistance and with metallurgical bonding between two materials that have much different hot workabilities.

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