Susumu Morioka

On the Corrosion of Chromium Steel in Nitric Acid Solution

As a supplement for the former experiment on the influence of added salt on the passivity of chromium steel in sulphuric and hydrochloric acid solutions, the writer carried out the similar experiment for nitric acid solution. It was ascertained that 10-14 percent chromium steels can be passivified in dilute (say, 5% HNO3) nitric acid solutions containing metallic cation of higher valency, for example, Cu++, Fe+++, but not passivified in solutions containing metallic cation of lower valency or that of non-variable. These relations are quite similar to that in the case of sulphuric and hydrochloric acid solutions. Next, 10-12 percent chromium steels are spontaneously passivified in dilute nitric acid solution containing no metallic cation of higher valency, or initial rate of attack of the acid gradually falls off, as the duration of the test increases. The passivity thus produced are explained as follows. (1) In the case of 12 percent chromium steel, the passivity is produced by the oxidizing power of the nitric acid solution, after being etched away the “shattered” metal which is difficult to render passive. (2) In the case of 10 percent chromium steel, the passivity is due to the oxidizing power of Fe+++ ions which is produced by attack and accumulated around the specimen (if specimen is rotated and Fe+++ ions are scattered, the passivity can not be obtained).

Surface-passivity and Inner-corrosion of 21% Cr Stainless Steel in Acidic Solutions containing Chlorine Ion and Cation of Higher Valency

An experiment of surface-passivity is made on 14% and 21% Cr stainless steels in a solution of 10% FeCl3, the former is severely attacked while the latter unattacked, retaining metallic lustre provided that the portion of the contact surface of the specimen and glass supporter is attacked to form a pit. When the specimen of 21% Cr stainless steel was immersed in the solution for a long time, the portion severely attacked formed a cavity, while the passive film on the surface of the inner-corroded portion slacken ed. The “inner-corrosion” of the 21% Cr steel occurs also in the case of the surface of the specimen in contact with non-cond uctive substances other than glass supporter, and a discussion of this phenomenon is also made, taking the non-metallic inclusions, pits and fissures on the surface of the specimen into account. These undesirable inclusions, pits and contact portion with non-conductive substances act as capillary crevices on the surface which becomes anodic while the other portions of the surface is subjected only to the action of oxidation and remain in a passive state. It is also ascertained that when the surface of the specimen is covered with glass cylinder having a capillary crevice and the specimen is held vertically, all the surface of the specimen was attacked and no passive film was formed. The interesting phenomenon of inner-corrosion occurs also in the solutions of (75g/L CuSO4+50g/L HCl) and 10% CuCl2, while in the solution of 10% FeCl2, the surface of the specimen was attacked, and in the solutions of (CuSO4+H2SO4) and (_??_M K2Cr2O7+10g/L HCl) all the surfaceof the specimen became passive state. The activity or passivity of the specimen is, of course, depends on the amount of chromium content in the specimen and the kind of acidic solution, however, the cause of the passivity of the surface and the inner-corrosion on a special portion of 21% Cr stainless steel is due to the following factors, - (1) the passivity depends on the strengths of the acidity and oxidation-power of the solution used on the surface of the specimen. In this experiment the existence of metallic cations of higher valency in the solutions of HCl and H2SO4, for example, Cu++, Fe+++, Cr+++ and Hg++ give an oxidizing power to the solution, and therefore the passivity of the 21% Cr steel in the solutions of FeCl3, CuCl2, (CuSO4+HCl) and (CuSO4+H2SO4) is produced. (2) The inner-corrosion of the specimen in which its surface is passive state is caused by the presence of capillary crevice on the surface which is in contact with the glass supporter, and it is also due to the existence of some amounts of chlorine ions in the solution. (3) The mechanism of the deep attack in the process of the inner-corrosion is elucidated by an experiment that if the portion on the surface, on which a capillary crevice is existing, is attacked by chlorine ions on account of poor oxidation of the solution, it will be dissolved by the solution and afterwards it cannot be so oxidized to form a passive state by the formation of reducing hydrogen, i.e. due to the impossibility of the formation of passive film on the portion of the specimen.

An Investigation of the Corrosion of Magnesium Alloys (The 8th Report)

An addition of chromium to magnesium alloys containing aluminium was tried by preparing a mother alloy with aluminium, and corrosion test was carried out in N/10 NaCl solution. It is shown that the addition of chromium for magnesium alloys containing aluminium, but not manganese, is markedly effective as that of manganese to reduce the rate of corrosion (Mg-Al-Zn-Cr alloys prepared with mother alloy Al-Zn-Cr containing 10-25% Cr, Table 2 and Fig. 2), but for magnesium alloys containing both aluminium and manganese it has no effect (Mg-Al-Mn-Cr alloys prepared with mother alloy Al-Mn-Cr containing 5-15% Cr, Table 1 and Fig. 1), or it is slightly effective (Mg-Al-Zn-Mn-Cr alloys containing more than 0.2% Mn which were prepared with mother alloy Al-Zn-Mn-Cr containing 5-25% Cr, Table 3 and Fig. 3). All alloys thus prepared, however, lost a large amount of chromium in melting, and found to contain only 0.01-0.03% Cr by analysis, irrespective of chromium content in mother alloys used and hence whether the beneficial effect is brought about by chromium addition or not is somewhat obscure. The effect of Sn, Cd, Pb, Ca, Sb, Bi, Si, Ce, Se, Ba, Mo, V or Zr added to Electron AZG alloy, containing small amount of chromium as above, has also been studied. The results obtained are shown in Fig. 4 and in Table 5 and 6.