Formation Of Corrosion Pits Research Articles

MARINE DURABILITY OF 15-YEAR OLD UNCRACKED AND PRECRACKED CONCRETE MADE WITH DIFFERENT CEMENTS

普通セメント (OPC), 高炉スラグセメントA種 (SCA), B種 (SCB), C種 (SCC) およびフライアッシュセメントB種 (FACB) を用い, ひびわれの有無の異なるコンクリート試験体の海洋環境下における暴露15年試験を実施し, 圧縮強度, 可溶性・全塩化物含有量, ひびわれ治癒 (閉塞), および埋設鉄筋の電気化学的性状および物理的性状に関する試験を行った. 塩化物の浸入量および鉄筋の腐食量をセメント種で比較すると, 概ねOPC>FACB>SCA>SCB>SCC, の順になった. また, セメント種によらず, ひび割れ幅が概ね0.5mm以下の場合, ひびわれ治癒 (閉塞) が認められたが, 0.5mmよりも大きな場合, ひびわれ治癒は観察されなかった. コンクリートと鋼材の界面に空隙が存在する箇所において孔食が生じていた.

Metal sulfide semiconductor electrochemical mechanisms induced by bacterial activity

Various bacterial species (e.g. Thiobacilli, Leptospirillum) have adapted to utilize solid inorganic semiconducting sulfides (e.g. FeS 2, ZnS, CuFeS 2) as their energy source for carbon dioxide fixation. The interfacial electrochemical mechanisms which they apply have practical relevance for bioleaching of minerals, acid mine pollution, for the biocorrosion of steel, and for solar powered chemosynthesis based on the bacterial energy cycle. The bacterial attack on the sulfide surface is based on the use of recyclable chemical species (H +, Fe 2+, thiol-compound) which disrupt chemical bonds in the sulfide interface and thereby induce disintegration. Model experiments performed with 100-nm thin synthetic FeS 2 layers allowed a detailed study of the interaction between bacterial cells and the pyrite interface. Thiobacillus ferrooxidans uses an organic polysaccharide layer to extract sulfur in the form of colloids via a cysteine-based carrier molecule. The mechanism which leads to corrosion pit formation of bacterial size could be analyzed in great detail. Addition of a surface active agent was found to induce increasingly localized leaching activity of bacterial cells. Addition of cysteine stimulates bacterial activity and acts as a sulfide dissolving agent even in the absence of bacteria. Mechanisms to block bacterial attack were identified on the basis of thiol chemistry. Leptospirillum ferrooxidans was found to apply a different strategy. It can only exist on Fe 2+ oxidation and dissolves FeS 2 by pushing — via a very positive Fe 2+/3+ redox potential, generated within the organic capsula — the semiconductor towards the electrochemical dissolution potential. The FeS 2 interface thus disintegrates into small fragments from which free energy of electrons for Fe 3+ reduction is utilized. Two isostructural materials with analogous electronic structure, FeS 2 which serves as energy source for T. ferrooxidans and RuS 2 which cannot at all be oxidized, are compared in detail to understand the molecular aspects of bacteria-induced semiconductor electrochemistry.

In situ electrochemical scanning probe microscopy corrosion studies on duplex stainless steel in aqueous NaCl solutions

Electrochemical scanning tunnelling microscopy and atomic force microscopy have been used to observe, in situ and in real time, the early stages of corrosion on duplex stainless steel in aqueous NaCl solution. With the specimens polarised in the transpassive region, the in situ techniques monitored clearly a process involving first a progressive etching of the damage layer, second the appearance of austenite and ferrite phases, and finally the formation of corrosion pits. A summary of these results is presented and the advantages and disadvantages of such in situ approaches are illustrated by comparing these observations with those obtained by ex situ examinations of the corroded specimen surfaces.

In situ electrochemical scanning probe microscopy corrosion studies on duplex stainless steel in aqueous NaCl solutions

Electrochemical scanning tunnelling microscopy and atomic force microscopy have been used to observe, in situ and in real time, the early stages of corrosion on duplex stainless steel in aqueous NaCl solution. With the specimens polarised in the transpassive region, the in situ techniques monitored clearly a process involving first a progressive etching of the damage layer, second the appearance of austenite and ferrite phases, and finally the formation of corrosion pits. A summary of these results is presented and the advantages and disadvantages of such in situ approaches are illustrated by comparing these observations with those obtained by ex situ examinations of the corroded specimen surfaces.

Burner Rig Hot Corrosion of Silicon Carbide and Silicon Nitride

A number of commercially available SiC and Si3N4 materials were exposed to 1000°C for 40 h in a high‐velocity, pressurized burner rig as a simulation of an aircraft turbine environment. Na impurities (2 ppm) added to the burner flame resulted in molten Na2SO4 deposition, attack of the SiC and Si3N4, and formation of substantial Na2O. x(SiO2) corrosion product. Room‐temperature strength of the materials decreased as a result of the formation of corrosion pits in SiC and grain‐boundary dissolution and pitting in Si3N4.

溶融硝酸塩中における鋼材の腐食疲労挙動

Corrosion fatigue behaviour of plant materials such as 21/4;Cr-1Mo, SUH309 and S20C was investigated in a molten salt environment of (NaNO240%, NaNO37%, KNO353%) at 723K. The rotating bending fatigue strength at 4×107 cycles of 21/4;Cr-1Mo steel in the molten salt environment was 8 percent lower than the fatigue limit in air. The fatigue strength at 4×107 cycles of SUH309 in the molten salt environment was deteriorated by 27 percent. Corrosion fatigue cracks were initiated from corrosion pits and propagated accompanying striation predominantly in the molten salt environment. The morphology of the corrosion products on the specimen surface was examined by X ray-diffractions. The major corrosion product of these steels was Fe3O4. It may be concluded that the formation of corrosion pits at the initiation of fracture surface is the major cause of fatigue strength decrease of these steels in a molten salt environment containing Cl-.The corrosion fatigue strength of S20C steel was higher than the fatigue limit in air. This may be caused by the retardation of crack initiation from the hardened surface layer due to nitriding in a molten salt environment. Neither the initiation nor the propagation of stress corrosion cracking of these steels was observed in the molten salt environment during the initiation test up to 2880hrs and the propagation test up to 2160hrs in the stress intensity factor range KIO of 248 and 372MPa√m, respectively.

Corrosion fatigue strength of high-tension steels in seawater. (The effect of cathodic protection on corrosion-fatigue strength of notched and welded specimens).

The effect of cathodic protection on corrosion-fatigue strength in synthetic seawater was investigated for notched specimens of high-tension steels (HT50 and HT80). For slight notches (stress concentration factor, ak≤2), fatigue strengths with cathodic protection were almost the same as those in laboratory air, because the cathodic protection retarded the formation of corrosion pits. When ak was further increased, corrosion-fatigue strengths reached a value which is probably the fatigue strength of the circular cracked specimen. The value is higher than that in laboratory air due to the initiation of nonpropagating cracks. The fatigue strengths of welded specimens were compared with those obtained for the notched specimen.

Effect of rare earth metals and the stability of sulfide inclusions on Localized corrosion of low alloy steels

AbstractEffect of alloying with rare earth elements RE (La, Pd + Nd, or so called “Mischmetall” containing Ce, Pr, Nd, La and traces of the other RE) on the ability to passivate and susceptibility to chloride breakdown of low alloys steels was investigated. Electron microprobe analysis revealed that the effect of RE depends upon the element and is connected with a modification of the composition and morphology of MnS inclusions present within the steels without RE addition. La seems to be most detrimental. The nonstability of La sulfide inclusions in air results in their spontaneous oxidation and growth into fantastic “flower‐like” forms which facilitates the formation of corrosion pits.

３％食塩水中のスリーブ付軸材の疲労強度

Pump shafts are used being fitted with impellers and sleeves. Since they are often made from different metals and form crevices, galvanic corrosion or crevice corrosion occurs in a corrosive solution. Therefore, corrosion fatigue strength (CF) of these components may differ from that of bare metal specimen. In this paper, effects of galvanic couples and crevices on CF were investigated. Round bar materials tested are a 0.35%C steel (S35C) and a 13Cr stainless steel (SUS 403). Sleeves used are Cu, Al and S35C for S35C specimen and brass and SUS 403 for SUS 403 specimen. All tests were carried out at 1450 or 2770rpm in 3% NaCl solution. Test results are summarized as follows.(1) The corrosion fatigue strength σwc of a S35C at N=107 was 10kgf/mm2 (33% of that in air) and considerably decreased at higher cycles. For SUS 403 σwc was 19.6kgf/mm2 (59% of that in air) and decreased little at higher cycles.(2) The effect of sleeve materials on CF of S35C bar specimen was explained by considering the difference in corrosion potentials between the specimen and the sleeve. Namely, the highest σwc was obtained for Al sleeve (12.5kgf/mm2 at N=2×107), followed by S35C sleeve (10kgf/mm2: the same as that in bare specimen) and Cu sleeve (6.7kgf/mm2).(3) In the case of SUS 403 bar specimen, the same σwc was obtained for two different sleeve materials, while both SUS 403 and brass sleeve considerably decreased σwc because of the formation of corrosion pits at the ends of the sleeves. (10kgf/mm2 at N=5×107, 30% of that in air)(4) To clarify the effect of pit on CF, corrosion fatigue test of etch pitted round bar specimens was carried out, in which etch pits were induced by the potential method and the size of the pits was controlled to be as large as those found on fracture surface of the sleeve fitted specimens. It was shown that σwc of etch pitted specimens was roughly equal to that of the sleeve fitted specimen in the high cycle regime.

Effects of pH of corrosive liquids on corrosion resistance of boehmite films

Studies were made on influence of chemical conversion coatings (see Table 3) on corrosion resistance of aluminum and aluminum alloys by immesing coated metals in corrosion liquids with pH of 311. The followings are the results.(1) Boehmite films showed the highest resistance to acid solutions containing H2SO4 (pH = 3 or 5). Other films were inferior to boehmite films.(2) Films made with chemical treatments shown in Table 3 all showed satisfactory resistance to alkaline solutions containing NaOH (pH = 9 or 11). Difference of the resistance among various films was small.(3) After giving chemical conversion coatings shown in Table 3, specimens in Table 1 were immersed in acid corrosion liquids (pH = 3 or 5) at 45°C for 12 weeks. Among these specimens, only A1050P with boehmite films showed no corrosion pit. Other films were found to be ineffective in preventing aluminum alloys from corrosion pit formation.