オーステナイトステンレス鋼の孔食および間隙腐食におよぼすMo, SiおよびNの影響

The resistance of X5CrNi18-10 stainless steel to pitting corrosion in a solution containing chlorides and sulphates was tested using the potentiodynamic polarization method. The obtained results show that the stainless steel is significantly resistant to pit formation, but it is susceptible to pit growth and crevice corrosion. Pits formed at the corrosion potential grow continuously. Statistical analysis of the results obtained during pitting corrosion testing was performed. It can be assumed with a probability of 95% that values of indicators of resistance to pit formation (the pitting potential Epit, the metastable pitting potential Empit and the difference Epit-Ekor) will be within the range of several percents. Values of indicators of resistance to pit growth (the amount of charge required for the pit growth q, the protective potential Eprot and the difference Epit-Eprot) will be within the broader range. In addition, the appearance of pits on the surface of the stainless steel, as well as the appearance of the pits bottom, were analyzed. It was shown that the structure of the stainless steel tested was not sensitized to pitting and intergranular corrosion, which means that the stainless steel was not previously thermally treated.

Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels

In this overview passivation mechanisms of high nitrogen steels (HNS) are decribed first and then the various corrosion properties related to general corrosion, pitting corrosion, crevice corrosion, intergranular corrosion (sensitization), stress corrosion cracking and hydrogen embrittlement are summarized based on the present knowledge. The effects of nitrogen on pitting corrosion of stainless steels have been studied most systematically and a number of different mechanisms have been proposed for the effect of nitrogen on pitting and crevice corrosion related to observed synergistic interactions with Mo in the alloy. There is a fairly consistent agreement that N additions improve the pitting corrosion resistance of austenitic and duplex stainless steels by increasing the pitting potential or temperature in aqueous chloride solutions or by decreasing the weight loss in standard FeCl 3 test. There is much less results on the effects of nitrogen alloying on active dissolution or general corrosion, and almost no systematic studies on the effects of N content on environment sensitive cracking of these steels.

Simulation of Crevice and Pitting Corrosion Using the Orthogonal Collocation Method

The effect of nitrogen alloying on the pitting corrosion resistance of duplex and austenitic stainless steels has been examined. In order to avoid alteration of the phase ratio as a result of nitrogen alloying of the duplex steels, a simultaneous decrease has been made in the nickel content. Austenitic alloys of compositions corresponding to the austenite phase of the duplex steels have been investigated and compared to the behaviour of austenitic steels in which the nitrogen content or the nickel content alone has been varied. Nitrogen has a beneficial effect on pitting and crevice corrosion resistance in all cases but the duplex stainless steel exhibit a lower resistance to pitting and a higher resistance to crevice corrosion than predicted from the austenite nitrogen content.

The decrease in pH caused by the hydrolysis of metal ions inside crevice is widely accepted to be the predominant factor affecting the initiation of crevice corrosion for stainless steels. The accumulation of chloride ions inside crevice is also thought to be of great importance for the initiation of crevice corrosion. 1, 2 In this study, a simultaneous measurement technique for pH and Cl− concentration was applied to the visualization of pH and Cl− distributions inside crevice in the initial stage of crevice corrosion of Type 430 stainless steel. A sensing layer with fluorescent dyes was made on one side of a SiO2 plate using the sol-gel SiO2 coating technique. Quinine sulfate (fluorescent dye) was used to measure Cl− concentration. The excitation wavelength was 350 nm. The emitted light was filtered through a 420-nm long-pass filter, since quinine sulfate has a peak wavelength of 451 nm (blue fluorescence). 1 Tb3+-DPA (dipicolinic acid) complex was used as the fluorescent dye for pH measurements. 2 The excitation wavelength of this dye was 270 nm, and the emitted light was filtered through a 510–550-nm band-pass filter. In this case, green fluorescence was observed. The crevice specimens were prepared from Type 430 stainless steel (Fe-16.5Cr). A crevice (3 mm × 3 mm) was made between the stainless steel and the sensing plate. 1,2 Crevice corrosion tests were carried out in a naturally aerated 0.01 M NaCl at pH 3.0, and the specimen was polarized at 0.0 V (vs. Ag/AgCl, 3.33 M KCl). Before the initiation of crevice corrosion, pH inside the crevice gradually decreased with time. And also, Cl− concentration increased with time. At pH 1.8 and ca. 0.1 M Cl−, crevice corrosion initiated, and a large pH drop and a large Cl− increase were observed.

Long duration measurements of the crevice corrosion rate of rolled, welded and cast materials of one high-alloyed austenitic stainless steel (UNS S31254) and one high-alloyed duplex stainless steel (UNS S32750) have been performed at 15, 25 and 40°C in seawater. The crevice specimens were artificially initiated before they were transferred to test loops. In most cases the crevice corrosion rate increased with the time until a sudden drop of the current density occurred due to repassivation. Propagation of crevice corrosion can thus occur at temperatures normally regarded as lower than the initiation temperature. The depth of the corrosion attacks was, however, restricted and active crevice corrosion may not necessarily lead to practical corrosion failures.

Although a wide variety of degradation modes can occur in aqueous environments for corrosion-resistant metallic materials, localized corrosion such as pitting corrosion, crevice corrosion, SCC, and hydrogen embrinlement (HE) is considered to be the primary mode. The evaluation of the susceptibility of candidate corrosion-resistant container materials to pitting and crevice corrosion is well underway using electrochemical polarization techniques described in the Activity Plan E-20-43144. The proposed activity (E-20-56) is aimed at evaluating the SCC behavior of these materials in susceptible environments using the linearelastic-fracture-mechanics (LEFM) concept. The mechanical driving force for crack growth, or the stress distribution at the crack tip is quantified by the stress intensity factor, K, for the specific crack and loading geometry. The critical stress intensity factor for SCC, KISCC for candidate materials will be evaluated in environments of interest, and their comparisons will be made to select the waste package inner container material having an optimum SCC resistance.

The resistance of high chromium austenitic and ferritic stainless steels to corrosion by saturated sodium chloride brines was investigated. The effects of temperature, solution pH, and the addition of one percent of sodium hypochlorite on pitting and crevice corrosion were studied on test coupons containing welds. Increasing temperature and decreasing pH were observed to increase corrosion. Addition of sodium hypochlorite markedly increased the corrosivity of the brine. Alloys with highest molybdenum content such as the new ferritic alloys, 29-4 and 29-4-2 and the new austenitic alloy, AL-6X, offered best resistance to the brines. These alloys were followed, in order of decreasing resistance, by E-BRITE® (ASTM XM-27), Type 316, Type 444 (18-2), and Type 304 stainless steels. Further limited tests showed that the alloys most resistant to pitting and crevice corrosion were also highly resistant to stress corrosion cracking in the brine solutions. Selective corrosion on welds was not observed on any of the alloys investigated.

A statistical evaluation of AISI 316 stainless steel resistance to crevice corrosion in 3.5% NaCl solution and in natural sea water after pre-treatment in HNO 3

The crevice corrosion behaviour of stainless steel in sodium chloride solution

Alloy 22 is a nickel base alloy highly resistant to all forms of corrosion. In very aggressive conditions (e.g. hot concentrated chloride containing brines) Alloy 22 could suffer localized attack, namely pitting and crevice corrosion. Chloride ion is known to be the most detrimental aggressive agent for Alloy 22 and is able to promote crevice corrosion when tight crevices exist in hot chloride containing solutions of different concentrations. Nitrate ion is an effective inhibitor of chloride induced crevice corrosion when present in a high enough [NO3-]/[Cl−] ratio. The occurrence of localized corrosion in a given environment is governed by the values of the critical potential (Ecrit) for crevice corrosion and the corrosion potential (Ecorr) that the alloy may establish in the studied environment. If Ecorr is equal or higher than Ecrit, localized corrosion may be expected. This paper discusses the evolution of Ecorr and corrosion rate (CR) of Alloy 22 specimens in 18 m CaCl2 + 9 m Ca(NO3)2 and 18 m CaCl2 + 0.9 m Ca(NO3)2 brines at 155°C. Two types of specimens were used, polished as-welded (ASW) creviced and non-creviced specimens and as-welded plus solution heat-treated (ASW+SHT) creviced specimens. The latter contained the black annealing oxide film on the surface. Results show that, in a few immersion days Ecorr reached a stable value higher than the open circuit potential of a platinum electrode in 18 m CaCl2 + 9 m Ca(NO3)2 for all specimens tested. Specimens tested in this solution did not suffer any type of localized attack. On the other hand, Ecorr showed oscillations of up to 600 mV in 18 m CaCl2 + 0.9 m Ca(NO3)2 during the entire immersion period. These oscillations were due to pitting corrosion development. Crevice corrosion was not observed in any testing case. Corrosion rates for specimens in the latter solution ([NO3-]/[Cl−] = 0.05) were one order of magnitude higher than for specimens in the second one ([NO3-]/[Cl−] = 0.5). Nitrate showed to be able to inhibit localized attack even in hot concentrated chloride brines when present in a ratio of [NO3-]/[Cl−] = 0.5. Localized corrosion occurred only in condition where Ecorr > Ecrit.

Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

Specialty steel producers have expended considerable effort to develop stainless steels that are resistant to pitting and crevice corrosion in seawater. Stainless steels with resistance to crevice corrosion and pitting combined with their excellent resistance to general corrosion and erosion, good formability, and good weldability would be ideal for use as plate and tubing fabricated into tanks, condensers and heat exchangers. Many applications for heat exchangers involve intermittent or continuous exposure to polluted seawater where the crevice corrosion and pit resistant stainless steels may be a viable replacement for copper based materials.

Dilation-balloon expandable coronary stents are made of implant grade stainless steels, UNS S31673, e.g., BioDur® 316LS. Boston Scientific/Interventional Technologies (BS/IVT) determined that addition of platinum to UNS S31673 could produce a stainless steel with enhanced radiopacity, which made such stents more visible radiographically. A goal of the program was to ensure the platinum additions would not adversely affect the corrosion resistance of the resulting 5–6 wt % PERSS® alloys. Corrosion resistance of PERSS and BioDur 316LS was determined using electrochemical tests for general, pitting, crevice and intergranular corrosion. Experimental methods included A262E, F746, F2129, and potentiodynamic polarization. The ∼ 6 wt % PERSS alloy (IVT 78) had a resistance to pitting, crevice and intergranular corrosion similar to base materials. IVT 78 was a single-phase austenitic PERSS alloy with no evidence of inclusions or precipitates; it was more resistant to pitting corrosion than the ∼ 5 wt % PERSS alloys. PERSS performance was not a function of oxygen content in the range 0.01 to 0.03 wt %.