As reported in the previous paper, the inhibiting effect of hexamethylenetetramine (HMT) was most excellent as a vapor phase rust-inhibitor among various compounds investigated. The vapor phase rust-inhibiting paper, powder, tablet and oil were made of HMT together with various sub-components. The optimum conditions in preparation were investigated and at the same time the vapor transfer mechanism was discussed. The followings are results of this research: 1) The water-soluble binder of the vapor phase rust-inhibiting papers was found to be most excellent in inhibiting effect at the addition of 9-10% for milk casein, while the oil-soluble binder, shellac, showed a good effect at 4-6% addition. 2) As extenders of the vapor phase rust-inhibiting powders, the inhibiting effect was excellent in the following order, zinc oxide>titanium white>lithopon>white lead carbonate>calcium carbonate>barium sulfate and white lead sulfate. The most of them showed enough effect when more than 40% of HMT was put into the extender. Particularly, zinc oxide and titanium white showed inhibiting effect when only 10% of HMT was put into the extender. In cases of the addition of dolomite, casein, talc, and kaoline, the inhibiting effect became poor. 3) In the case of vapor phase rust-inhibiting tablets, the extender had not a contact inhibiting effect. But the white pigments that showed good effect in the case of powder were also found to be excellent in an inhibiting effect, and the effect was good when more than 30% of HMT was added into the extender. 4) The vapor phase rust-inhibiting oils showed a good inhibiting effect when more than 0.7% of HMT was added. 5) It seems probable from the discussion based on the vapor transfer mechanism that hygroscopic HMT decomposes as a result of absorption of moisture and the decomposition products neutralize the sealed atmosphere.

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Measurement of an exact pitting potential of 18Cr-8Ni stainless steel open surface was attempted by preventing the crevice corrosion under insulating coating. The method was as follows: the specimen was passivated previously, covered with an insulating coating except the testing area, then the exposed area was abraded or pickled except the metal/insulating coating interface. The pitting potential of the abraded open surface was estimated at ca. +0.72 volt (Eh) in 0.1M NaCl solution at 25°C. A conventional specimen gave an observed pitting potential of +0.3 volt in the same solution which is about 0.4 volt less noble than the above value, because the crevice corrosion occurred under the insulating coating. The specimen hung with a platinum wire did not give an exact pitting potential because a slight crevice corrosion occurred at the contact area with the platinum wire. The protection potential obtained in this method was also about 0.1 volt more noble than that obtained with the conventional specimen. The preparing method of specimens can be also applied to a chemical pitting test, which causes no crevice corrosion, in a neutral sodium chloride solution.

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The effect of alloying elements (Cr, Ni, Mo, N, Mn, Cu etc.) on susceptibility of stainless steels to pitting corrosion has been investigated to develop a new stainless steel which possesses good weldability. It was confirmed again that Cr, Ni, Mo and N have a beneficial effect to pitting corrosion. Especially the effect of nitrogen was remarkable. Nitrogen reduces not only dissolution rate of pitting corrosion but also initiation of pitting corrosion. Furthermore nitrogen reduces dissolution rate of stainless steel in sulfric acid and hydrochloric acid. Nitrogen is a strong austenite former and improves hot workability of low nickel stainless steels. Solubility of nitrogen in austenitic structure increases with chromium content. A 25% Cr steel can dissolve 0.38% N by usual solution heat treatment. As results of these beneficial effects of nitrogen, we have developed a pitting corrosion resistant and economical austenitic stainless steel. Characteristics of this steel are described.

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