Crevice corrosion alloy composition

Figure 37 Schematic illustrating the improvement in crevice corrosion behavior as the composition and microstructure of a series of a-Ti alloys are changed.

Has 21% Cr, 13% Mo, 3% W, 3% Fe, 60% Ni and suitable for use in oxidizing environments corrosion resistance better than C-276, C-4 in oxidizing media, better pitting resistance inferior to C-276, C-4 in reducing media and with respect to crevice corrosion Superior to C-22, C-276, super thermal stability attributed to the ternary system, Ni-Cr-Mo devoid of W, Cu, Ti and Ta Similar to C-276 in composition except for Cr level being 16-21% alloy is solution annealed at 1200°C and rapidly cooled to prevent precipitation of intermetallic phases thermal behavior not as good as Alloy 59 and its corrosion resistance was less than 59 1.6% Cu has been added to C59 lower corrosion resistance and thermal stability than Alloy 59... 

The alloy Haynes 6B is resistant to corrosion in organic acids, but subject to pitting and crevice corrosion and SCC in chloride media. The corrosion rate of 0.3 mm/yr or 12mpy has been observed in 30 wt % of NaOH it is likely that caustic cracking will occur at high concentrations of NaOH and temperatures in the case of all the cobalt alloys. The nominal composition of high-temperature cobalt alloys is given in Table 4.54. 

In the case of the nickel alloys, the stability of the passive layer is a problem. The alloys depend on the oxide films or the passive layers for corrosion resistance and are susceptible to crevice corrosion. The conventional mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects such as the acidification or the migration of the aggressive ions into the crevice solution [146]. These solution composition changes can cause the breakdown of the passive film and promote the acceleration and the autocatalysis of the crevice corrosion. In some cases, the classic theory does not explain the crevice corrosion where no acidification or chloride ion build up occurs [147]. 

Electrochemical potentials can arise from differences in electrolyte or electrode concentration as well as from differences in chemical composition. Thus, for example, there will be a difference in potential between two amalgam or alloy electrodes of the same basic type in which there is a difference in the activity of one of the alloy or amalgam constituents. The practical implication is that galvanic corrosion can occur between similar alloys of different composition. Crevice corrosion phenomena are often explainable in terms of differences in oxygen concentration. 

Effects of Alloy Composition on Pitting Corrosion Inhibition of Pitting Corrosion Crevice Corrosion... 

Some understanding of the corrosion of aluminium alloys used as cladding on research and test reactor fuel has been obtained from the CRP. Aluminium corrosion is extremely complex and the variables affecting localized corrosion (pitting and crevice corrosion) act both independently and synergistically. Additional information about the effects of deposited particle composition on the corrosion behaviour of aluminium alloys is needed. Surface finish affects the corrosion of aluminium alloys, and more information is required with respect to this parameter. Additional data on the effects of certain impurity ions in basin water on localized corrosion behaviour are necessary to better identify the ions that cause corrosion. A goal would be to develop an equation for pitting as a function of water chemistry parameters. 

In 1996 the IAEA initiated a CRP on the corrosion of aluminium clad spent research reactor fuels to help evaluate the state of the spent fuel assemblies and to inform pool/basin operators regarding maintenance and housekeeping procedures to extend the lives of the FAs. The main activities of this programme are related to exposing racks of aluminium alloy specimens (coupons) in different spent fuel basins around the world. Five racks were suspended in the 1EA-R1 reactor pool and were subsequently withdrawn after different time spans to evaluate the extent of corrosion of the coupons as a function of alloy composition, crevices, bimetallic effects and water chemistry. During this period the pool water was monitored for pH, conductivity, chloride ion content and radiometry (Table 6.3). The IAEA CRP racks are denoted as racks 1,2A, 2B, 3A and 3B. 

Although aluminium and its alloys have attractive nuclear properties, they have limited strength, poor compatibility with uranium at high temperatures and low corrosion resistance in water or steam at temperatures above 523 K. Hence their use is restricted to core components in research reactors, where temperatures do not exceed 423 K. However, various parameters, such as water quality, structural design (crevices, galvanic contact with other materials), alloy composition and irradiation, have significant influence on the corrosion resistance of aluminium in research reactors. 

High-temp Halide and Sulfate Solutions crevice corrosion non-Pd or Ru alloys, or alloys with <3.5 % Mo or <0.6 % Ni Dependent on alloy composition, brine pH and temp. Requires severe crevices. 

The critical pitting temperature (CPT) is defined at what temperature pitting occtus. A common range for stainless steels is 10-100 C and obviously depends on the alloy composition. [Pedrazzoli Speidel, 1991] has reported that the critical temperature for crevice corrosion (CCT) is appirox. 20 ° lower compared to pitting, see fig.l6 17 for details. 

Crevice corrosion is another type of localized corrosion relevant to non-alloyed and stainless steel. Narrow gaps- caused by the constructive design or the formation of deposits are a necessary requirement. Fig. 1 -9 c. Crevice corrosion as a result of the different compositions of the electrolyte inside and outside the crevice, i.e. depletion of oxygen in the crevice. In a crevice there is a large metal area/electrolyte volume ratio with a long diffusion path. Solution in the depth of a crevice is depleted of oxygen and therefore has a composition different from that of the bulk solution. As a result of the lack of oxygen anodic metal dissolution occurs at the depth of a crevice. Because diffusion in the crevice is almost impossible, the chemical composition becomes acidic... 

Type 317 stainless steel contains greater amounts of molybdenum, chromium, and nickel than type 316. The chemical composition is shown in Table 10.1. As a result of the increased alloying elements, these alloys offer higher resistance to pitting and crevice corrosion than type 316 in various process enviromnents encoimtered in the process industry. However, they may still be subject to chloride stress corrosion cracking. The alloy is... 

This alloy is similar to alloy 20Cb3 but with 4% molybdenum content instead of 2%, providing improved pitting and crevice corrosion resistance over alloy 20Cb3. The chemical composition is as follows ... 

The high nickel and molybdenmn contents provide improved resistance to chloride SCC. Copper has been kept to a residual level for improved performance in seawater. The high alloy composition resists crevice corrosion and pitting in oxidizing chloride solutions. 

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