Journal of the Society of Materials Science, Japan

Technical Topic

1993 Volume 42 Issue 479 Pages 905-911
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.905

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Influence of Suspended Solids on Pitting Corrosion of Carbon Steel in Cooling Water

Takahiko UCHIDA

1993 Volume 42 Issue 479 Pages 912-916
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.912

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Pitting corrosion of carbon steel is one of the most serious problems in cooling water systems, which shortens the life of equipments and sometimes induces unscheduled shutdowns. In order to solve these problems, pitting corrosion and its countermeasures were studied focusing on suspended solids in cooling water.
Pit formation at initial stage was investigated by a rotating disk method. Open pores and precipitation of streamlined rust around pits were observed. Increase of pits and enlargement of tubercles were observed with addition of suspended solids. Deposition of solids were typically observed behind the pores within the streamline. This is explained that dissolved iron ions from pits act as a coagulant, so that suspended solids coagulate by iron ions and deposit. After all, suspended solids accelerate pit formation and enlarge tubercles by corrosion products.
The influence of suspended solids on pit formation and its growth was confirmed by recirculating loop test. With addition of suspended solids, pit number and pitting depth increased with time, as expected, under the condition where less pits occurred without suspended solids. This indicates the additional countermeasures besides corrosion inhibitors are necessary to inhibit pitting corrosion in cooling water systems. Suspended solids in cooling water are the mixture of inorganic materials and microorganisms, and they are covered with sticky filamentous materials produced by microorganisms. Therefore sterilization by biocides and filtration were expected to inhibit deposition of suspended solids and pitting corrosion. These countermeasures were found effective to inhibit not only deposition of suspended solids on the surface of carbon steel but also under deposit corrosion.
Finally the effectiveness of practical methods were confirmed by pilot cooling tower test. The results showed that combination of continuous chlorination and filtration was more effective than conventional slug dose of chlorine or respective method itself. Moreover, the use of chlorine stabilizer was most effective to inhibit pitting corrosion.

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Study of Growth Mechanism of Ant Nest Corrosion in Copper Tube

Kazuhiro MIYA, Hisakatsu KAWARAI, Hiromasa MATSUOKA

1993 Volume 42 Issue 479 Pages 917-922
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.917

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It is a great problem for air conditioning units that leakage accidents of refrigerant happen in an extremely short running time after installation. Such a trouble may be caused by ant nest corrosion of copper tube used for a heat exchanger. This type of corrosion is characterized by complicatedly divided microscopic caverns, and can be observed in the cross section of the corroded copper tube by using an optical microscope. The surface of the corroded copper tube usually becomes discolored greyish-brown or blue-violet without showing any corrosion product. This corrosion is produced by organic acid vapor originating from the inside or outside of copper tube.
This study reports the investigation of the growth mechanism of ant nest corrosion. The ant nest corrosion could be reproduced in the heating cycle condition of (298K×12hr_??_313K×12hr) for 30 days in a humid oxygen atmosphere containing formic acid vapor. Porous accumulation was observed in the corrosion caverns. The chemical analysis of the corrosion caverns by an electron probe microanalyzer confirmed that the porous accumulation was copper oxide and no formic acid complex existed in the corrosion caverns. This result means that the formic acid complex dissolves in the solution of inner corrosion caverns.
Consequently, the growth mecanism can be explained as follows. A cathodic reaction occurs as O₂ +4H⁺+4e^-→2H₂O on the wet surface of copper tube, and an anodic reaction occurs as Cu→Cu⁺ +e^- in the inner corrosion caverns. Cu⁺ in formic acid solution is complexed by CO₂H^-: Cu⁺+CO₂H^-→Cu^(I)(HCO₂). Then Cu^(I)(HCO₂) changes to Cu₂O rapidly. Cu₂O takes the form of porous oxide. This oxide is dampened with formic acid solution. It is thought that the ant nest corrosion grows as the oxidation reaction of copper at the interface of the porous oxide and metallic copper. Therefore, the more the porous oxide becomes wet, the more corrosion caverns widen. The complicated divided pattern of corrosion caverns is caused by the cracks of porous oxide that are formed by the change in volume of the porous oxide.

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Corrosion of Austenitic Stainless Steel in Phosphoric Acid Environments

Yoshio TANAKA

1993 Volume 42 Issue 479 Pages 923-929
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.923

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Although pure phosphoric acid is generally known to be mild to austenitic stainless steel, it is corrosive to austenitic stainless steel in some cases. This paper shows the corrosion behavior of stainless steel (mainly 316L) in a low concentration (about 1% H₃PO₄) phosphoric acid solution at high temperature (about 200°C), and a high concentration (about 33% P₂O₅) phosphoric acid solution at low temperature (about 70°C).
The polarization tests of stainless steels were carried out mainly in 220°C 1% H₃PO₄ solution with or without dissoluted oxygen. The corrosion rate decreased when Cr or Ni contents in the test specimens increased, but even Hastelloy C-22, Ni-based alloy, did not passivate by itself. Dissoluted oxygen also reduced the corrosion rate of 316L, but no self-passivation occurred.
The corrosion behavior of 316L stainless steel in the solution of a phosphoric acid plant was also tested. This solution contained phosphoric acid (33% P₂O₅), surfuric acid (1.5%), plaster (25%), fluorine ion (2.5%) and so on. It was found that corrosion in the plaster slurry settled from the solution was severer than that in the mixed acid solution without slurry, and the quantity of fluorine ions was higher in the slurry. Thus, it is suggested that CaF₂ in the plaster increases the amount of fluoric ions and accelerates corrosion. It is also suggested that ferric ions prevent 316L from corrosion in this environment.

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New Method for Analyzing Corrosion Potential Fluctuation in Stress Corrosion Cracking Tests

Hiroyuki INOUE, Koji YAMAKAWA

1993 Volume 42 Issue 479 Pages 930-933
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.930

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Corrosion potential fluctuation was analyzed to investigate the stress corrosion cracking of SUS304 stainless steel immersed in 42 mass% MgCl₂ solution. A potential difference between the specimen and the Ag/AgCl reference electrode was measured with a precise digital voltmeter at a sampling frequency of 2.56Hz. Elongation of the specimen was measured with a differential transformer at 10s interval, for the computation of elongation rate.
From an electrochemical point of view, the measured corrosion potential was divided into a component of the signal shifted to a more noble potential (Signal of Repassivation) and the one shifted to a less noble potential (Signal of Film Breakdown). These two types of signals were extracted from the sampling data by use of the differential coefficient at each data point. To avoid an errorr in the FFT calculation, the extracted signal was extended symmetrically. Then, a power spectra density (PSD) was calculated by FFT algorithm for each type of signal.
The difference in PSD level between two types of signal showed a good correlation with the elongation rate of the specimen. During the time when the elongation rate increased, the value of PSD for the signal of film breakdown became bigger than that of repassivation in the frequency range 10-100 mHz. Conversely, the PSD level for signal of repassivation exceeded that of film breakdown, when the elongation rate decreased or kept a constant value, in the same frequency range.

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Corrosion Resistance of Hot-Dip Galvanized Steel Under Marine Environments

Yoshihiko TAKANO, Tatsumi IZEKI, Tetsuya NAKADA, Koshi TAKADA

1993 Volume 42 Issue 479 Pages 934-940
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.934

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Hot-dip galvanized steel was exposed to sea air, sea spray, tidal flow, and continuous immersion in seawater in order to determine its corrosion resistance in various marine environments.
Corrosion resistance was evaluated by surface observation, microscopic observation of a cross section of the galvanized coating, and measurement of corrosion weight loss. The corrosion rate in a relatively dry marine environment (exposure to sea air) was approximately equal to the corrosion rate in rural areas. The corrosion rate in a wet marine environment (immersion in seawater) was high when compared with the corrosion rate when exposed to sea air, but the corrosion rate decreased with time. The corrosion rate under the alternating wet/dry conditions encountered in the sea spray and tidal flow exposure sites was high, and corrosion of the steel base was observed before zinc in the coating was depleted.
These exposure tests served to confirm the effectiveness of a hot-dip galvanized coating in various marine environments.

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Evaluation of Corrosion Resistance for Zn-5%Al Alloy Coating on Steel Structures

Yoshihiko TAKANO, Tatsumi IZEKI, Tetsuya NAKADA, Koshi TAKADA, Masamic ...

1993 Volume 42 Issue 479 Pages 941-947
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.941

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A new hot dip process for Zn-5%Al alloy coating on the steel structures was developed under suitable galvanizing conditions. The characteristics of the Zn-5%Al alloy coating were examined and its corrosion resistance was evaluated by a salt water spray test, an atmospheric exposure test and a coulostatic method.
It was clarified that from the results of chemical analysis, X-ray spectra, EPMA and hardness test, the coating prepared under the new type hot dip coating process exhibited superior qualities, especially, a coating layer consisting of Fe₂Al₅+Zn alloy formed in Zn-5%Al bath has the best corrosion resistance.
In addition, the results relating to the corrosion resistance clearly suggested that the coulostatic method may be useful for estimation of the rapid corrosion rate instead of the salt water spray test.

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Transmission Electron Microscope Observation of Grain Boundary Precipitates in an Al-1 mass% Mg₂Si Alloy

Susumu IKENO, Masanobu UWANORI, Masakazu ISURUGI, Kenji MATSUDA, Shizu ...

1993 Volume 42 Issue 479 Pages 949-954
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.949

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The crystal structure of grain boundary precipitates in an Al-1 mass% Mg₂Si alloy was investigated by transmission electron microscopy.
At a peak aged condition where the intergranular fracture occurs, the diffraction pattern of hexagonal system can be obtained from the grain boundary precipitates. These precipitates may be considered as an intermediate phase from comparing the diffraction patterns with matrix precipitates. The grain boundary precipitates have the preferential orientation relationship with the matrix as {001}_m||{0001}_p. The equilibrium precipitate (Mg₂Si) is scarcely observed at the grain boundaries.
The number of grain boundary precipitates decreases with increasing the misorientation of grain boundary, while the mean size of grain boundary precipitates increases with increasing misorientation.

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Statistical Distribution of Tensile Strength in Carbon Fiber

Tetsuya TAGAWA, Masanori TANIGUCHI, Takashi MIYATA

1993 Volume 42 Issue 479 Pages 955-961
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.955

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Tensile tests for monofilament of carbon fiber were performed and the statistical distribution of tensile strength and its dependence on gauge length were investigated. Four types of carbon fibers produced from mesophase-pitch and polyacrylonitrile were tested. For each type of carbon fibers, 40 tensile tests were repeated for one gauge length, and three levels of gauge length were adopted in tests. Based on the statistical analysis of experimental results, it was found that the dependence of the fracture strength on gauge length was almost similar irrespective of the type of carbon fiber, and the size effect on the fracture strength showed anisotropy between the axial and radial directions. Variation of filament diameter had a considerable influence on the tensile strength. Anisotropy in the statistical distribution of tensile strength was numerically analyzed by using the distribution model of defect density and the Reynolds-Sharp criterion.

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Effect of Specimen Size on the Behavior of Fast Crack Propagation and Arrest

Hiroshi NAKANISHI, Toshio FURUKAWA, Makoto SAKUWA, Megumu SUZUKI

1993 Volume 42 Issue 479 Pages 962-968
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.962

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The dynamic stress intensity factor of fast propagating cracks was calculated from the measured data of crack tip position versus time, through numerical simulation with an improved finite difference method. The data were obtained with a measuring system consisting of ladder gages and a digital wave memory unit. The cracks were propagated in the double-cantilever beam type specimens machined from the model material, polymethyl methacrylate(PMMA). The specimens were in various sizes where the ratio of initial crack length to specimen size was kept constant. The dynamic stress intensity factor decreased at the beginning of the crack propagation phase, remained almost unchanged in the next stage, and increased in the third stage. The above mentioned result appeared in all sizes of specimens, in common. But, after the third stage the behavior of the stress intensity factor varied with the specimen geometry. The characteristic values of the dynamic stress intensity factor in the process of the crack propagation and arrest were illustrated and investigated with respect to the following parameters: the specimen size, the initiation stress intensity factor, and the crack jump distance. The specimen size mainly affects the scatter of the results: the larger specimen gives the result with a smaller scatter.

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Crack Stability in Rate Sensitive Brittle Materials and Pseudo-Stable Crack Extension

Taketoshi NOJIMA, Osamu NAKAI

1993 Volume 42 Issue 479 Pages 969-975
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.969

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By assuming that the fracture toughness value (crack resistance force) R is expressed by a constitutive relation as R (a, a) ∝ R₁(a)·a^m^*, (a and a; crack length and crack velocity, m^*; rate sensitivity parameter), the stability condition of cracked specimens is analytically discussed for rate sensitive brittle materials such as glasses and ceramics. The stability criterion obtained is identical in its form to the one reported by Gurney and Hunt in which the rate effect is not taken into account. However, by introducing the rate effect into the analysis, it was found that an initial crack begins to extend before the load maximum appears. This fact that load gets larger, nevertheless crack extends, is named “pseudo stable phenomena (PSP)”.
Load-displacement and crack extension behaviour were simulated by numerical calculations including the rate effect. The PSP was confirmed to appear in rate sensitive brittle materials. Experiments on Pyrex glass also showed the appearance of PSP, which is prominent when EBΛ^* gets larger (E and B; Youngs modulus and thickness of the specimen, Λ^*; the compliance of the testing machine).

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Crack Propagation Behavior of Sintered Silicon Nitride under Cyclic Load at Elevated Temperature

Influences of Frequency and Stress Wave Form

Akira UENO, Hidehiro KISHIMOTO, Hiroshi KAWAMOTO, Tetsuya MURASAWA

1993 Volume 42 Issue 479 Pages 976-982
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.976

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In order to investigate the effect of elevated temperature on the crack propagation behavior of a sintered silicon nitride, crack propagation tests were carried out under square wave conditions where the test frequency f was varied from 1Hz to 0.01Hz and the test temperature ranged from 1073K to 1273K. At 1073K, the crack propagation rate increased as the frequency increased. This behavior is the same as that observed at room temperature. Above 1173K, the effect of frequency on the crack propagation rate almost disappeared, and the difference between the crack propagation rates under static load and cyclic load was not observed. The time-dependent crack propagation behavior was observed over the whole K-range. On the specimen tested above 1173K, a glassy oxide and dislocated grain boundary were observed. The thermal activation process seemed to change at about 1073K from the process related to stress corrosion cracking by H₂O to the process related to oxidation and viscous flow. From the above discussion, we concluded that the slow crack growth in the temperature range from room temperature to about 1073K was dominated by the stress corrosion cracking by H₂O, and oxidation and viscous flow accelerated the time-dependent fracture above 1173K.

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Effect of Tensile Orientation on Hydrogen Embrittlement in Single Crystals of High Purity Ferritic Stainless Steel

Keiji KOTERAZAWA, Hitoshi UCHIDA, Hisanori OHSHIBA

1993 Volume 42 Issue 479 Pages 983-989
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.983

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The tensile properties of single crystals of high purity ferritic stainless steel were investigated under cathodic charging. The susceptibility to hydrogen embrittlement (HE) and the fracture morphology were influenced prominently by the tensile axis orientation. In the [001]-oriented specimen, the susceptibility to HE had a maximum and the fracture surface was characterized by the cleavage growth along (001) plane which was perpendicular to the tensile axis. On the other hand, the susceptibility of the [101]-oriented specimen had a minimum and the fracture surface with some small cleavage facets was observed. From the results of stereographic analysis, these facets were composed of two equivalent {100} planes in schmid factor.

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Tensile Properties and Stress Corrosion Cracking Resistance of Extruded High Strength Al-8Zn-1Mg System Alloys

Yoichi KISHI, Yukio HIROSE, Ichizoh TSUKUDA, Shigekazu NAGAI, Kenji HI ...

1993 Volume 42 Issue 479 Pages 990-996
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.990

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Tensile properties and stress corrosion cracking (SCC) resistance of Al-8wt.% Zn-1wt.% Mg system alloys with various Cu and La contents were investigated. The influences of additional elements on mechanical properties and SCC life were discussed on the basis of experimental results. The results obtained in the present work are as follows;
(1) Additions of elements of Cu and La were effective to decrease the grain size of those materials. In particular, the grain sizes in alloys containing La contents of more than 1.7wt.% were small at 20μm, comparing with that in the master alloy.
(2) For the present alloys containing La, the second phase particles, consisting of Zn and La, have the capacity of hydrogen storage. These particles were between 5 and 10μm in size, and were dispersed in the matrix homogeneously.
(3) Tensile properties were improved with increasing Cu or La content. For the alloys with Cu and La, the tensile strength was from 450 to 500MPa, and the elongation between 14 and 17%.
(4) The alloys containing La had high resistance to SCC. Their SCC lives were about 15 times as long as that of the master alloy. In conclusion, the alloys added Cu more than 0.6wt.% and La 4.0wt.% were much superior to the master alloy in both the tensile properties and the SCC resistance.

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DLC Films Deposited by RF Plasma CVD and Their Friction and Wear Properties

Kei DEMIZU, Takumi SONE, Kazuki NATSUKAWA, Yukio FUJISHIMA

1993 Volume 42 Issue 479 Pages 997-1003
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.997

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Hard diamond-like-carbon (DLC) films were deposited on silicon substrate by RF plasma CVD using methane as a reactant gas. Both CH₄ concentration and RF power were varied to find out a better deposition condition to make DLC film harder, since DLC film will be applied to tribological use. The deposition condition of CH₄: 100% and RF power: 300W was the best in this paper. The deposition rate of DLC film under the condition was 2.1μm/hr.
Friction and wear experiments were conducted with a reciprocating friction and wear test apparatus. EDX and XPS were used to analyze the sliding surfaces. The DLC film deposited under the above condition was slid against several kinds of metals to investigate fundamental friction and wear properties. And the results were compared with those of silicon substrate and steel (S15C). DLC film coating extremely reduced friction and wear of silicon substrate slid against all metals used in this paper. DLC film showed very low friction and wear especially against Ti alloy (Ti-6Al-4V), pure Ti, and stainless steel (SUS304). These 3 kinds of metals have characteristic of causing seizure easily during sliding or machining. The above mentioned results suggest that DLC film will have good tribological performance against metals that have high adhesive characteistic.

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Breaking of Transparent Glass Plate by Irradiation of YAG Laser

Toshiji KUROBE, Noriyasu KAWAMUKAI, Toshiyuki TAKAO

1993 Volume 42 Issue 479 Pages 1004-1010
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.1004

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Cutting of brittle materials such as glass is usually made with a diamond scriber or a rotary roller chisel. It is said that the mechanical cutting of glass using the scriber or the chisel leads to lower cutting quality. A non-contacting cutting method using a CO₂ laser is recently being studied enthusiastically. Precision breaking of glass and other brittle materials, however, has not been made.
Breaking of transparent glass plates with a YAG laser was studied after the surface of the specimen was painted in marker ink (magic ink). Ink was used as an absorber of laser beam. And also, the influence of surface roughness on breaking was investigated. It was found from the experiments that painting of magic ink had a considerable effect on breaking of glass, especially in blue color. Chamfering of the corner of the specimen by lapping yielded easy breaking and it reflected directly to high quality breaking of glass.

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Fundamentals of Reliability Engineering

3. Statistical Properties of Load

Hiroshi ISHIKAWA, Masaru ZAKO, Mamoru MIZUTANI, Masakuni KUBO

1993 Volume 42 Issue 479 Pages 1011-1016
Published: August 15, 1993
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.42.1011

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