Seawater pitting potentials

Cathodically protect at a potential below the critical pitting potential. An impressed current can be used or in good conducting media (e.g., seawater), stainless steel can be coupled to an approximately equal or greater area of zinc, iron, or aluminum [44], Austenitic stainless steels used to weld mild-steel plates, or 18-8 propellers on steel ships, do not pit. 

Critical pitting potentials of 0.38 V (S.H.E.) in IV NaCl and 0.45 V in 0.1 V NaCl [6] indicate that the metal is vulnerable to pitting in seawater. It undergoes intergranular S.C.C. in anhydrous methyl or ethyl alcohol containing HCl, but not when a small amount of water is added [7]. This behavior, similar to that of commercial titanium, suggests that stress may not be necessary and that the failure is perhaps better described as intergranular corrosion. 

Figure 12.8 Pitting potentials of an austenitic (AISI 316) and a superaustentic (25Cr-7Ni-3MO-0.14N) stainless steel as a function of temperature in deaerated seawater [6].

The pitting potential of a passive alloy in aerated seawater at pH 8 and at 40 °C is equal to 0.80 V relative to the saturated calomel electrode. Is there a risk of pitting corrosion if this alloy is exposed to aerated seawater at this temperature ... 

Zhang, H. J. and Dexter, S. C., Effect of Biofilms on Critical Pitting Potentials for Stainless Steels S30400 and S31600 in Seawater, International Conference on Microbiologically Influenced Corrosion, P. Angell, Ed., NACE International, Houston, TX, 1995, pp. 70-71. 

Aylor and Moran [45], who conducted polarization experiments on Gr/Al MMCs in seawater, also h5rpothesized that diffiision of carbon into aluminum could lower the integrity of the passive film, rendering it more susceptible to breakdown. Wielage [46] reported that the pitting potentials of various squeeze-cast Gr/Al MMCs were approximately 20 mV more active than that of the pure aluminum matrix material in a chloride solution. Aluminum carbide was found at the Gr fiber-matrix interfaces. Shimizu et al. [37], however, found that pitting potentials of a squeeze-cast, short fiber Gr/6061 Al MMC had pitting potentials similar to that of monolithic 6061 Al in a chloride solution. 

Crevice corrosion also develops under deposits or fouling, e.g. under the basalt plates of barnacles. The microbiological slime often observed on the surface of stainless steels can shift the free corrosion potential in the positive direction so that the pitting potential of the steels is exceeded [108]. Pipes made of a chromium steel with 26% Cr and 1% Mo failed within a brief period with biological deposits, but remained resistant in deaerated and sterile seawater in desalination plants [109]. 

The materials Monit and Sea-Cure are characterised by good resistance to pitting, crevice and stress corrosion cracking in seawater. The critical pitting corrosion temperature in the FeCls test is 328 K (55 °C) and the critical crevice corrosion temperature is 318 K (45 °C). In Table 36, the pitting potentials of the two superferrites and the austenitic steels 1.4539 (SAE 904 L, XlNiCrMoCu25-20-5) and X3CrNiMol7-13-3 (SAE 316,1.4436) measured in 5% NaCl solution are presented. 

The current-potential curves in Figyre 37 show the breakthrough potentials for stable pitting corrosion (pitting potentials) of different steels (DIN-Mat. No. 1.3817, 1.4021,1.4016,1.4301,1.4401,1.4439) in seawater in comparison to redox potential [22]. Since a material cannot accept a potential higher than the redox potential of the relevant medium, only those steels with a breakthrough potential that is more positive than the redox potential are resistant to pitting corrosion. 

Figure 37 Pitting potential levels for different steels in seawater in comparison to redox potentiai [22]...

Based on a very good pitting potential of Uh = 1.1-1.2 V, the expected excellent pitting resistance equivalent of the steel X2CrNiMoN17-13-5 (317 LMN, 1.4439) was confirmed in every respect in practical application. The material proved resistant to pitting and crevice corrosion, even in subsurface exposure and in contaminated seawater and brackish water [22]. 

Figure 39 Dependence of pitting potential Upp on the pitting resistance equivalent in natural seawater at 323 K(50°C) [178]...

Cathodic metals in nonhalide salt solutions usually corrode aluminum to a lesser degree than in solutions of halide salts. This is because the aluminum is less likely to be polarized to its pitting potential. Galvanic corrosion is reduced in any solution when the cathodic reactant is removed. Therefore, the corrosion rate of aluminmn coupled to copper in seawater is reduced greatly when the seawater is deaerated. 

Pitting potentials of T1-3A1-2.5V and CP titanium are estimated to be 9.2 and 9.6 V (SCE), respectively, in seawater at 50 °C (120 °F). Commercially pure titanium exhibits improved performance over Ti-3A1-2.5V under these conditions, which could make a difference in an anode... 

Pitting corrosion always remains a worthy subject for study, particularly with reference to mechanism, and the problem conveniently divides into aspects of initiation and growth. For 6061 alloy in synthetic seawater, given sufficient time, pit initiation and growth will occur at potentials at or slightly above the repassivition potential . In an electrochemical study, it was found that chloride ions attack the passive layer as a chemical reaction partner so that the initiation process becomes one of cooperative chemical and electrochemical effects . 

Although less energetically efficient than the oxidation of hydrogen sulphide (Fig. 8.4), the oxidation of metal sulphides could potentially support chemo-synthesis at seafloor massive sulphide deposits long after hydrothermal activity had ceased, even in well-buffered seawater (Eberhard etal., 1995 Juniper Tebo, 1995). Newly formed sulphide deposits are rapidly subjected to oxidation upon contact with ambient seawater, and some show micro-scale weathering features (e.g. etch pits on mineral surfaces Verati etal., 1999),... 

Open circuit potential As defined by the McGraw-Hill Dictionary of Scientific and Technical Terms, open circuit potential (OCP) is the steady-state or equilibrium potential of an electrode in absence of external current flow to or from the electrode. OCP measures the corrosion potential of a corroding metal with regard to a reference electrode. For instance, increased susceptibility of stainless steels to pitting and crevice corrosion in seawater has been attributed to increase in OCP, which could partly be due to biofllm formation. Monitoring of OCP spectra can be used to rank the corrosion vulnerability of metals in comparison with each other. 

Nickel, containing 0.6 rf-electron vacancy per atom (as measured magnetically), when alloyed with copper, a nontransition metal containing no rf-electron vacancies, confers passivity on the alloy above approximately 30-40 at.% Ni. Initiation of passivity beginning at this composition is indicated by corrosion rates in sodium chloride solution (Figs. 6.12 and 6.13), by corrosion pitting behavior in seawater (Fig. 6.13), and, more quantitatively, by measured values of /critical and passive (Fig. 6.14), [41-43] or by decay (Flade) potentials (Fig. 6.15) [44] in IN H2SO4. 

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