Effects of Alloying Elements on the Pitting Corrosion of Stainless Steels

Abstract

Effects of alloying elements on the pitting corrosion resistance of 17Cr-16Ni steels with and without 4% Mo were evaluated by the corrosion rate in 10% FeCl3.6H2O and 4% NaCl added with hydrogen peroxide and by the polarization measurement in 0.1N NaCl+0.1-0.25N Na2SO4 at 40C.The elements, the content of which was varied, were carbon, nitrogen, silicon, phosphorus, sulfur, manganese and nickel. The supplementary alloying elements were aluminum, titanium, vanadium, cobalt, copper, zirconium, niobium, tin and tungsten. Tungsten, like silicon, was found beneficial in minimizing the pitting corrosion if added together with molybdenum.Carbon was favorable if no chromium carbides were formed. Copper added to the Mo-free steel was beneficial, but not effective in the Mo-bearing steel. Manganese was proved to be harmful in both Mo-free and Mo-bearing steels, but increasing its content more than that of ordinary stainless steels did not enhance its harmful effect. The very favorable effect of nitrogen was also recognized. The effects of the other alloying elements are summarized in a table. The more noble the electrode potential at a relatively high c.d., i.e. 10mA/cm2, in anodic polarization curves measured in sodium chloride solution, the lower was the corrosion rate in the ferric chloride test if a second phase was absent. The presence of a second phase increases especially the corrosion rate in ferric chloride solution. It was indicated that the alloying to increase the passivating ability of the steel would shift the pitting potential to the more noble direction by passivating the depassivated sites before they can grow as ordinary pits.

Influence of Oxygen Content on the Inclusion Formation and Pitting Corrosion Resistance of Hyper Duplex Stainless Steels

“…11,12) Suter and Böhni 13) confirmed that the pitting potential depends on the size of inclusions, and that this size has to be kept well below 1 µm in order to substantially improve the pitting resistance of stainless steels. This assumption is in good agreement with the observations reported many years ago by Osozawa et al, 14) in which stable pitting corrosion pit growth was not observed in stainless steels with inclusions of no more than 2.5 µm in size.…”

Effect of Nitrogen on Crevice Corrosion and Repassivation Behavior of Austenitic Stainless Steel

“…[1][2][3][4][5][6] Compared to other additives such as chromium or molybdenum, a minute nitrogen content is effective in improving resistance to localized corrosion. Moreover, nitrogen addition helps to refine the microstructure and increase the strength of the material, and it can be used instead of nickel as an austeniteforming element.…”

“…At relatively low potentials, solidified nitrogen dissolves with crevice corrosion to produce NH 4 þ in the solution and control the acidification inside the pit. 2) In the present research, increase of the nitrogen content in nitrogen-bearing austenitic steel results in (1) decreased of number of crevice corrosion spots and the corrosion weight loss, with a tendency to further decrease as the polarization potential reaches high values; (2) the amount of NO 3 À eluted into the solution showed a tendency to increase, and at the same time, the eluted NO 3 À was adsorbed onto the surface of the passivation film, which had a inhibitor effect suppressing the dissolution of the base metal; (3) the peak current density and the total quantity of electricity for the repassivation process decreased and indicated a high corrosion resistance which promotes repassivation.…”

Effect of Nitrogen Alloying on the Pitting of Type 310 Stainless Steel.

“…[1][2][3][4] Many authors have investigated corrosion property of nitrogen added stainless steels and recognized that nitrogen improves corrosion resistance especially for localized one. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Pitting, the most typical form of localized corrosion, is a crucial issue for passivating materials in practical use.…”