Stainless steel corrosion potential

The subsequent oxide formation leads to a decrease in the overall oxidation rate, according to Equation 18.5. The value of n in this equation (which is the same as in the crack propagation rate. Equation 18.6) varies with the alloy chemistry (e.g., chromium content for a denuded grain boundary of Type 304 stainless steel), corrosion potential at the crack mouth, and the anionic activity in the bulk environment. 

P.E. Manning, D.J. Duquette, and W. Savage, The Effect of Test Method and Surface Condition on Pitting Potential of Single and Duplex Phase 304L Stainless Steel, Corrosion, Vol 35, 1979, p 151-157... 

Figure 7.3 presents stainless steel pitting potential with 13% and 18% chromium as a function of molybdenum content in the presence of 1 M NaCl at 20 °C. Pitting potential increases with an increase of Cr or Mo content, resulting in a decrease of pitting corrosion [8]. Pitting analyses is used to (i) develop new aUoys, (ii) rank aUoys, (iii) perform failure... 

N. Sridhar, G.A. Cragnohno, Applicability of repassivation potential for long-term prediction of localized corrosion of alloy 825 and type 316L stainless steel. Corrosion 49 (1993) 885-894. 

Sridhar, N. and Cragnolino, G. A., Applicability of Repassivation Potential for Long-Term Prediction of Loctilized Corrosion of Alloy 825 artd Type 316L Stainless Steel, Corrosion, Vol. 49, 1993, p. 885. 

Tsujikawa, S. and Okayama, S., Repassivation Method to Determine Critical Conditions in Terms of Electrode Potential, Temperature and NaCl Concentration to Predict Crevice Corrosion Resistance of Stainless Steels, Corrosion Science, Vol. 31, No. 1, 1990, pp. 441-446. 

The following mechanism of stainless steel corrosion in molten chlorides can thus be proposed. At the first stage a chemical exchange reaction between the alloy and the salt takes place. This interaction results in a gradual etching of samples. The rate of corrosion in the initial moment of time is highest due to a significant difference between the values of the red-ox potentials Me" /Me a" /A where Me is a component of steel... 

S. C. Dexter and G Y. Gao, Effect of seawater biofilms on corrosion potential and oxygen reduction of stainless steel. Corrosion 44 717-723 (1988). 

In the spht- or dual-pressure process, low to medium pressure gases (ca 0.3—0.6 MPa) containing nitrogen oxides are compressed to ca 1.1—1.5 MPa for efficient absorption in water to make nitric acid. Stainless steel is used for constmction in this corrosive environment and, because of the potential for... 

Duplex stainless steels (ca 4% nickel, 23% chrome) have been identified as having potential appHcation to nitric acid service (75). Because they have a lower nickel and higher chromium content than typical austenitic steels, they provide the ductabdity of austenitic SS and the stress—corrosion cracking resistance of ferritic SS. The higher strength and corrosion resistance of duplex steel offer potential cost advantages as a material of constmction for absorption columns (see CORROSION AND CORROSION CONTROL). 

Materials of Construction. GeneraHy, carbon steel is satisfactory as a material of construction when handling propylene, chlorine, HCl, and chlorinated hydrocarbons at low temperatures (below 100°C) in the absence of water. Nickel-based aHoys are chiefly used in the reaction area where resistance to chlorine and HCl at elevated temperatures is required (39). Elastomer-lined equipment, usuaHy PTFE or Kynar, is typicaHy used when water and HCl or chlorine are present together, such as adsorption of HCl in water, since corrosion of most metals is excessive. Stainless steels are to be avoided in locations exposed to inorganic chlorides, as stainless steels can be subject to chloride stress-corrosion cracking. Contact with aluminum should be avoided under aH circumstances because of potential undesirable reactivity problems. 

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

Note that low carbon or stabilized grades of stainless steel do not possess intrinsically greater corrosion resistance than their unadjusted counterparts. Their sole value in typical cooling water systems results from their resistance to sensitization and potential weld decay that can result when the metals are welded. It is therefore not economically justifiable to specify low carbon or stabilized grades of stainless steel for typical cooling water system components that are not to be welded. 

Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10,40-42]. 

Stainless steels in soil can only be attacked by pitting corrosion if the pitting potential is exceeded (see Fig. 2-16). Contact with nonalloyed steel affords considerable cathodic protection at f/jj < 0.2 V. Copper materials are also very resistant and only suffer corrosion in very acid or polluted soils. Details of the behavior of these materials can be found in Refs. 3 and 14. 

As in the case of corrosion at the insulating connection due to different potentials caused by cathodic protection of the pipeline, there is a danger if the insulating connection is fitted between two sections of a pipeline with different materials, e.g., mild and stainless steel. The difference between the external pipe/soil potential is changed by cell currents so that the difference between the internal pipe/ medium potential has the same value, i.e., both potential differences become equal. If the latter is lower than the former for the case of free corrosion, the part of the pipe with the material that has the more positive rest potential in the soil is polarized anodically on the inner surface. The danger increases with external cathodic protection in the part of the pipeline made of mild steel. 

When reviewing the materials of construction consider external corrosion concerns. Chloride stress cracking of stainless steel can be initiated by insulation capturing chlorides or insulation that contains chlorides (stainless steel should be primed). A weather barrier is needed. The principle here is to understand the potential hazards and their mechanisms. 

Figure 4-422. Corrosion characteristics of iron and 18% Cr stainless steel in diiute sulfuric acid and as a function of soiution oxidizing power (corrosion potential). (From Ref. [187].)...

Stainless steels each appear twice in the list. The more active potentials are those which the metal adopts when corroding as in a pit. The more cathodic potential is that adopted by the bare surface around the pit. The potential difference constitutes a significant driving force, analogous to the situation where the coupling of dissimilar metals such as copper and iron promotes the corrosion of the more anodic of the two (see below). 

These values are roughly constant across a range of electrolyte environments except where noted but the variations between alloys, heat treatment conditions, etc. creates a range for each metal. For some metals such as iron and steel the range is low ( 100 mV), but for lead, nickel, stainless steels a range is given. The corrosion potential is reported with respect to the saturated calomel reference electrode. 

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