Authors studied surface hardening of shot-peened or cold rolled Type 316 stainless steels during aging at 340℃ in air. We first introduce our shot peening method using PSZ （yttrium partially stabilized zirconia） shots which has been utilized as the countermeasure against the chloride stress corrosion cracking （SCC） of chemical process and nuclear energy equipment, and reveal the structure and physical properties of the peened Type 316 stainless steel. Next, we study change of both structure and properties of the peened or cold rolled Type 316 stainless steel during aging at 340℃ in air. We discuss mechanism why the cold worked and then aged Type 316 stainless steel in air tends to suffer intergranular cracks by few percent surface strains. Feasibility of the grain boundary hardening and cracking by internal diffusion of oxygen via thin layer of strain-induced martensite is proposed.

This research was focused on the growth of rusted areas on the surface of carbon steel in tap waters, which were collected at 66 locations in Japan.

Rusted area ratio （θ） was defined as θ＝A_rust/A, where A was total surface area and A_rust was the area covered with corrosion products. The changes of θ and corrosion potential （E_corr） with time were investigated.

The followings （1）-（6） were resulted.

（1）As immersion time elapsed, θ increased and E_corr became less noble.

（2）The abrupt changes in θ and E_corr with time appeared simultaneously. Stagnations of both θ and E_corr followed the above. The time of the abrupt change depended on quality of the tap water.

（3）The relationship between log{θ/（1－θ）} and E_corr was similar regardless of water quality.

This meant that the rusted area could be estimated only by measuring E_corr.

（4）The abrupt change was in θ seen from 0.2 to 0.5 and E_corr from －300 to －500 mV vs.SSE, the time at which E_corr reached －400 mV was defined as t_initial. For larger t_initial, the corrosion at small θ rusted longer.

（5）Both average and maximum localized corrosion rates were closely related to t_initial. Insight into those relations surmised the following mechanisms;

（a）When θ＜0.2, the anodes were fixed, inducing localized corrosion.

（b）When θ＞0.5, anodic and cathodic reactions were distributed even on the whole surface, resulting in then uniform corrosion.

（6）The authors found that the maximum localized corrosion rate, v_max（mm y^－1） was a function of t_initial as v_max＝0.32×t_initial^1/3. The unit of t_initial in this equation was hour.

It has been reported that crevice corrosion of stainless steel is accompanied by the generation of micro pits with a diameter of several μm in the crevice. Such micro pits are called metastable pits because they are immediately repassivated on free surfaces without a crevice structure. Metastable pitting potential （V’_{C, MS}） of stainless steel in aqueous solution of pH＝1.0, [Cl^－]＝3.0 mol・dm^－3, which was obtained from anodic polarization, was consistent with the critical potential for the initiation of crevice corrosion （V_{C, CREV}）. Therefore, it was found that the measurement of V’_{C, MS} in the aqueous solution simulating the environment in the crevice when the micro pits are generated is useful for the simple estimation of V_{C, CREV}.