The effect of additional elements in low-alloy steel on the protective rust layer has been discussed from the view point of atmospheric corrosion. The rust layer of corrosion resistant low-alloy steel is made up of two layers, the outer one which is α or γ FeOOH and the inner one which is amorphous or very fine spinel type oxide (Fe3O4). The inner layer is supposed to play an important role in the corrosion resistance of low-alloy steels in atmosphere. The method of detecting the above-mentioned dense amorphous oxide layer is presented, and the mechanism of its formation is also discussed.
The rust formed on the mild steel railing, which was used for 95 years in marine atmosphere, was investigated by means of infra-red absorption spectra, electron diffraction and of electron microscopy. The same measurements were carried out also about the corrosion products obtained from the test pieces of copper bearing low alloy steels, which were exposed in the seaside air for 6, 12, 42 or 64 months respectively. The effect of Cu2+ ion on the rust fomation on mild steels in aqueous solutions were studied and the electrochemical polarization measurement of the rusted steels was also performed. So far as the composition of the rust, the relative quantity of its constituents and the microscopic size of its particles were concerned, no difference was observed between mild steels and low alloy steels. On the other hand, with the rust on the low alloy steels the grain size was much smaller and the quantity of adsorbed water was larger than with that on the mild steels. While the Cu2+ ion in aqueous solution had no direct effect on the formation of FeOOH, it promoted the formation of Fe3O4. The best corrosion resisting film of rust was obtained in the solution containing about 10ppm of Cu2+ ion. The decrease, as well as the increase, in concentration of Cu2+ ion deteriorated the corrosion resistance of the film. The obtained results indicate that the copper ingredient in steel accellerates rust formation in the early stages of corrosion, and the layer of rust thus formed prevents the oxygen supply to the metalrust boundary, resulting in the formation of Fe3O4. It is considered that the Fe3O4 layer having large quantity of adsorbed water may play the most important role in resisting the atmospheric corrosion.
The corrosion-fatigue properties of weathering steel in sea water were investigated with reference to its utility and efficiency as shipbuilding material. Repeated tension (in low-cycle ranges) and reversed bending (in low-and high-cycle ranges) tests were applied on weathering steel plates. In low-cycle fatigue, the frequency, of stress is respectively 10cpm for repeated tension and 7 or 30cpm for reversed bending. In the high-cycle, only for reversed bending it is 1500 or 2200cpm. The results obtained are as follows:- (1) In the low-cycle range, but little effect of corrosion is seen on the fatigue strength in repeated tension, but a little effect is observed in reversed bending. (2) In the high-cycle range, there is remarkable influence of corrosion on fatigue of weathering steel, and there is no fatigue limit, such as seen in air. If the coefficient of corrosion-fatigue is defined as k=(NA-Ns)/NA×100(%), where NA and Ns are the number of reversals of stress to fracture in air and in sea water respectively, the smaller the applied stress is the larger the coefficient of corrosion-fatigue is. (3) The fatigue strength in aqueous solution saturated with H2S was also examined.
Experiments were conducted extending for three years in which low alloy high strength steels and stainless steels were exposed under static load to various types of atmospher i. e. in air in the industrial, in the marine and in the rural district. The results of the three years' exposure test showed no cracking either in the welded or in the non-welded specimens of low alloy high strength steels. Neither the non-welded specimens of stainless steels showed any cracking. Pitting and intergranular corrosion occurred, however, in the heat-affected zone of the welded stainless steels i. e. SUS 50 (13% Cr) and SUS 24 (18% Cr).
As a result of investigations of the mechanism through which the corrosion of stainless steels in boiling concentrated HNO3 could be accelerated the following facts were made clear. (1) The oxidizing power in terms of steady potential of a platinized platinum electrode attains its maximum at the azeotropic concentration. In case of addition of Cr6+, the power increases with the logarithmic amount added, indicating a positive slope of about 16mV which corresponds to the oxidation number 3, while in case of addition of Cr3+ a negative slope of about 33mV which corresponds to the number 1.5 is observed, the latter reason may be ascribed to partial oxidation of Cr3+ to Cr6+. Furthermore, increment or decrement of the power is dependent on the Langmuir adsorption type curve. (2) The corrosion of none-sensitized austenitic stainless steels can be accelerated either by addition of tri- or hexa-valent ions, the influence of which becomes maximum in the azeotropic acid, where the logarithmic corrosion rate is in direct proportion to the power. The cause by which the addition of Cr3+ can accelerate corrosion may be sought in the easer oxidation on the steel surfaces. (3) When Cr6+ is added, the higher the chromium content in the steels the higher becomes the corrosion rate, but when Cr6+ is not added the inverse result can be observed, i. e. the former corrosion may be in the transpassive region while the latter corrosion in the passive region. Nickel and molybdenum alloyed in the traditional amounts may, if anything, be taken as increasing the susceptibility to accelerating effect of Cr6+. (4) In the addition of Cr6+ corrosion of 18-9 and18-11-Mo type steels increases as the carbon content is decreased, whilst in no addition the state of affairs is inversed. As a rule, all above results may be influence some important factors as rate of escaping NO2 as well as the test duration, and so on.
The anodic behavior of Fe-Si alloys containing 2∼22% Si in deaerated H2SO4 solution (20°C) was studied by the potentiostatic method. The effects of the Si content on the corrosion potential Ecorr., passivation potential Ep, passivation current density Ip and passive current density ik were determined. The effects of chloride ion on the passivity of some high Si alloys were also studied. The results are summarized as follows. (1) The effect of Si addition on the corrosion potential of iron is not so remarkable: it shifts slightly in negative direction when Si content is 2∼6%, but the shift is in positive direction when Si content is 12∼16%. (2) Low Si alloys (2∼8% Si) exhibit polarization behaviors similar to that exhibited by iron except that their passive current densities are considerably higher than that of iron. (3) High Si alloys especially containing over 15% Si have an excellent passivating ability, i.e. more negative value of Ep and very low values of Ip and ik. Also these alloys are characterized by the large difference between the potential of passivation and that of the complete passivation. From this behavior of passivation, it is suggested that silicon contributes greatly to passivation in case of high Si-Fe alloys containing over 12% Si and the mechanism of passivation of these alloys differs from those of iron and low Si alloys. (4) During the active dissolution of high Si alloys such as 15% Si-Fe, the preferential dissolution of iron occurs, and the corrosion current decreases remarkably with time until it drops as low as ik value, but the potential of specimen shifts only slightly in the positive direction, showing that the passivity is not attained. (5) The flade potential of 15% Si alloy shows the value of +0.29V (S.C.E.) as a clear arest on the potential decay curve. It is more negative than Eps (+0.38V), but far more positive than Ep (About-0.2V). (6) The effect of chloride ion in case of high Si alloys, appears as the continuous increase of passive current density with increasing chloride ion concentration without being accompanied by pitting corrosion within the scope of our experiments.
The evaluation by electrochemical study was made of the corrosion resistance of copper-nickel-chromium electroplates. Polarization curves were obtained in acid sulfate solution with or without chloride ion by the potentiostatic method. The metal specimens examined in the present study were as follows: (a) mild steel (b) copper plated from cyanide bath (c) sulfur containing bright nickel electroplate (d) sulfur-free semi-bright nickel electroplate (e) chromium plated from conventional bath (f) contact electrode in which cross section of the electroplated articles was used as a working electrode (g) coupling electrode in which two metal electroplates were coupled with each other externally without any contact between them in the electrolyte (h) electroplated nickel-chromium plate. In general, bright nickels obtained from Watts type bath with sulfur containing addition agents such as 1, 5-naphthalene disulfonate, saccharin, p-toluene sulfonamide and thiourea were found to be more active than sulfur-free semi-bright nickels. With increased sulfur content in the nickel deposits, the polarization curves were shifted to less noble potentials and the rate of corrosion of nickels increased remarkably inhibiting the passivation of nickel. It is found that chromium electrodeposits tend to become passive at less noble potentials than nickel in acid sulfate solution and the dissolution of chromium is very small compared with that of nickel at the potentials of -0.34∼+0.10V vs. SCE. Corrosion of nickel-chromium electroplates at the potential region of the active state of nickel might correspond to the anodic dissolution of nickel through the pores or cracks in the chromium coating. By using the contact electrode and also coupling electrode systems in the measurement of polarization curves, it can be predicted that sulfur containing nickel plating on a sulfur-free semi-bright nickel will dissolve in preference to the semi-bright nickel exposed at coating defects and may result in the effective catholic protection of the electroplated articles owing to the retardation of the rate of penetration into the double-layer nickel coatings. However, rapid corrosion penetration of the underlying nickel layer can be expected if the two layers do not have sufficiently different sulfur contents.
The measurement of polarization in metals in electrolytic solutions is an effective method of determining the corrosion rate and its mechanism because the local action of the cell on the metal surface, which is one of the typical electrochemical phenomena, is considered to be the causative of aqueous corrosion of metals in general. To this general statement, however, there are limitations as follows: (1) That even with perfect polarization of the metal its active sites on the surface remain constant. (2) That the mechanism of the anodic and the cathodic reactions remain as it is for a wide range of polarization. Through experiments of examining iron pieces of different species, for their contamination at 40°C, in aqueous solution of sulfuric acid, as it was or in the same solution with ferrous ion, the parameters of hydrogen electrode reactions on the metal cathode, their Tafel coefficient, and their exchange current density were obtained as well as their static potential and corrosion current density. The hysteresis on the potential vs. current density curve with increasing and decreasing current was observed in all the sample species while the potential vs. log current density curve made a straight line when the surface of the electrode was sufficiently stabilized by electrolysis at high current density in pure solution of the acid. The report of the experiments is concluded with demonstration to prove that the measurement of polarization is still an effective method of corrosion test.
Hereunder is given a report of the investigation made of the corrosion of various kinds of steels of commercial type owing to the action of molten salts used as heat exchange medium in chemical industries, not the salts used for heat treatment in metal industries. Study was made also of the effects of carbon and silicon ingredients on the corrosion resistance. The following molten salts were used: (a) NaNO3-KNO3-NaNO2, (b) NaNO3-KNO3, (c) LiCl-ZnCl2-BaCl2-CaCl2-NaCl, (d) NaCl-BaCl2-CaCl2 The test temperatures were 400°C, 500°C and 600°C. The results obtaind are as follows: (1) In the salts (a) and (b), increase in chromium content of steels, largely improves corrosion resistance of the steels. Rimmed steel is better than killed steel. Influences of carbon and silicon contents on the corrosion resistance appear at above 500°C. That means that the corrosion rate increases with carbon content up to about 0.16%, then decreases. Silicon also increases the corrosion rate up to about 0.2-0.4%, and then decreases it. Aluminized steel shows the resistance as well as stainless steels. (2) Chlorides such as salts (c) and (d) are more aggressive than nitrate salts (a) and (b) below 500°C. The use of chlorides below 500°C is thought to be undesirable. In molten chlorides, chromium alloying to steels is not so effective as in nitrate salts.
Hereunder is given a report of studies made of the corrosion behavior and resistivity of aluminum brass pre-treated with an aqueous solution of K2Cr2O7 at high temperature. The samples were prepared with 0.5W/O solution at various temperatures: 180, 235 and 265°C, and the surface of the metal was covered with thin film. The corrosion rate and static potential of the rotating cylinder-type sample in synthetic sea water with and without small amount of Na2S and NH4Cl as contamination were measured at 30°C. The weight loss of the inside surface of the pipe line with stream of contaminated saline water from the Bay of Tokyo for 150 days was determined. The corrosion rate of the sample treated with dichromate solution at 265°C was low, while the sample prepared at temperature lower than 180°C was attacked considerably. The static potential of the sample treated at high temperature was stable in comparison with bare metal whose potential varied by 115mV or more. The chemical analysis shows that the protective film consists of chromic, Cr (III), without chromate, Cr (VI).
Surface preparation is believed to be fundamentally the most important factor that affects the efficiency of paint. In recent years the sand blast cleaning method has been increasingly adopted in the special marine coating system. The blast condition consists of three factors as follows. PB: The pressure at gage in kilogram per sq. cm. (Discharging the abrasive in a stream of high pressure air.) VB: The velocity of movement of blast cleaning nozzle in cm. per sec. NB: The number of repeated blast cleaning when PB and VB are constant. Experiments have been performed in order to investigate the relation between the efficiency of zinc silicate paint for its anticorrosive function and the blast condition, and the following results have been obtained as pointing to the way to raise the efficiency of the paint for its anticorrosive purposes. (1) The relation between NR per sq. cm. and the blast condition can be expressed by NR/cm2(TP-1)=116.3747+0.3466VB+PB(3.6555PB-38.0895)-4.5806NB NR/cm2(TP-2)=23.0572+0.0927VB+PB(0.8327PB-8.2335)-0.9123NB where NR/cm2: the number of pinpoint rusting per sq. cm. TP-1: the steel surface with millscale before the sand blast cleaning. TP-2: the steel surface treated by shot blast and having zinc rich epoxy type shop primer applied to it before the sand blast cleaning. (2) The effect of PB on the efficiency of the paint is estimated at 60∼70per cent. PB is the condition of prime importance to promote the efficiency of zinc silicate paint for anticorrosive purposes. Pressure of 6 kilogram per sq. cm. is required in this case. (3) VB has but little to do with the efficiency of paint by itself. It is important, however, relative to the interaction of PB and VB. The following rate of velocity of transition of blast nozzle is required under PB of 6 kilogram per sq. cm. VB=10cm/sec for TP-1 VB<40cm/sec for TP-2 (4) NB has litte to do with the efficiency of the paint when PB and VB are constant. (5) When PB, VB and NB are constant, high efficiency of zinc silicate paint is obtained by the use of TP-2 before the sand blast cleaning.