Corrosion testing alternating-immersion

Nickel-iron alloys are more resistant than iron to attack by solutions of various salts. In alternate immersion tests in 5% sodium chloride solution Fink and De Croly determined values of 2-8, 0-25 and 0-5 g m d for alloys containing 37, 80 and 100% nickel compared with 46 g m d for iron. Corrosion rates of about 0.4 g m d are reported by Hatfield for Fe-30Ni alloy exposed to solutions containing respectively 5 Vo magnesium sulphate, 10 Vo magnesium chloride and 10% sodium sulphate the same alloy corroded at a rate of about 1.2 g m d in 5% ammonium chloride. 

The ASTM have established a recommended procedure for alternating-immersion stress corrosion tests in 3-5% NaCl solution (ASTM G44 1988). 

Media considerations. SCC tests can be divided into those conducted in natural environments, such as atmospheric exposure tests and seawater immersion tests, and those which are conducted under laboratory conditions or other fabricating operations. The principal disadvantage of atmospheric exposure tests is the comparatively long time required for their completion however, they are reliable since they can reflect the projected use. There is a standard practice for evaluating stress-corrosion cracking resistance of metals and alloys by alternate immersion in a solution of NaCl 3.5%, pH 6.5. For spray testing, ASTM B-117, 2003 states the relevant conditions for conducting the test. (ASTM G44)4... 

Haidemenopoulos, G.N., Hassiotis, N., Papapolymerou, G., and Bontozoglou, V., Hydrogen absorption into aluminum alloy 2024-T3 during exfoliation and alternate immersion testing. Corrosion, 54, 73, 1998. 

FIGURE 20.41 Stress corrosion cracking behavior (a, b, c) of loop samples in 3% NaCl solntion (lifetime in alternate immersion test) and chemical activity (hydrogen evolution) (3% HCl), (d) the grain boundaries in hydrochloric acid for a 1-mm aluminum alloy sheet with 9.1 Mg. 

G 44 Recommended Practice for Alternate Immersion Stress Corrosion Testing in 3.5% Sodium Chloride Solution... 

FIG. 1—Corrosion rates of magnesium binary alloys determined in alternate immersion testing in 3 % sodium chloride [7]. 

Two frequent alternatives to ASTM B 117 testing used in the evaluation of general and galvanic corrosion performance have been (1) alternate immersion testing (30 s immersion in 3 % NaCl followed by 2 min in air [4,5,7], or (2) continuous immersion in 3 or 5 % NaCl for a period of three... 

Specific tests frequently used are (a) neutral 5 % Sodium Chloride salt spray (ASTM B 117, Test Method of Salt Spray (Fog) Testing), (b) 3.5 % Sodium Chloride by alternate immersion (ASTM G 44, Practice for Evaluating Stress Corrosion Cracking Resistance of Metals in 3.5 % Sodium Chloride Solution), and (c) exposure to various outdoor atmospheres. Guidelines for outdoor exposure are contained in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Generic types of atmospheres used are seacoast, industrial, urban, and rural. Sometimes specific geographical locations or local chemical conditions are important because they can produce unique results [2i],... 

To date there is no standardized filiform corrosion test, but it appears that chloride ions and high humidity are required for initiation and propagation. Laboratory tests that have been used are the 3.5 % NaCl alternate immersion test, ASTM G 44, or exposure to hydrochloric acid vapors... 

Corrosion of metals and alloys— Alternate immersion test in salt solution... 

Most inhibitors are required to function in aqueous environments in various states of aeration. Furthermore, there may be a requirement for them to be added to an environment to help prevent the further development of corrosion. The data referred to above were obtained in quiescent NaCl solutions open to air. The performance of R salt inhibitors under various conditions of aeration, i.e. alternate immersion, constant immersion in a quiescent test solution and constant immersion in an aerated test solution, has been studied by Hinton (1989). The weight-loss data obtained for tests with 7075 Al alloy in 0.1 M NaCl containing 1000ppm CeCls are shown in fig. 6. The R salts are effective inhibitors even under the most severe corrosion conditions, some of which are typical of service conditions. For example, the inhibition efficiency is greatest in a continuously aerated solution. This is consistent with a mechanism of inhibition based on the formation of the R oxide on the metal surface (see sects. 2.6 and 2.8). Continuous aeration will favour the reduction of oxygen with the formation of hydroxyl ions, and consequently promotes the precipitation of the R oxide and its coverage of the metal surface. 

The earliest work on REs and SCC and corrosion fatigue was by Hinton (Hinton, 1989b, 1995). This researcher studied the effects of several REs on the SCC of AA 7075 in NaCl solution. The specimens were loaded to two levels of stress, 0.5 and 0.75% of the yield stress level o= 450 MPa for the 7075, and subjected to alternative immersion in 0.1M NaCl containing various REs at 1000 ppm. The presence of RE chlorides increases the time to failirre at both stress levels. Furthermore, the addition of REs to the test environment delays the initiation of the cracking but also reduces the crack rate propagation. 

An application for these new rare earth organophosphate corrosion inhibitors is as additives to epoxy coatings, where their low solubility makes them ideal for slow, controlled inhibitor release to stifle corrosion of coated substrates. Forsyth et showed that Ce(dbp)3 and CeCdpplj, when dispersed in a simple epoxy formulation and applied to AA2024-T3, could suppress the initiation and propagation of filiform corrosion in both alternate immersion and high humidity tests. 

Standard test method for performing a stress corrosion cracking test of low copper containing Al-Zn-Mg alloys in boiling 6% sodium chloride solution Standard practice for alternate immersion stress-corrosion testing in 3-5% sodium chloride solutions. 

Stress corrosion test with alternate immersion-emersion... 

Tables are available in American Society for Metal, (1984, p. 283) that summarize filler alloy selection recommended for welding various combinations of base metal alloys to obtain maximum properties, including corrosion resistance. Care must be taken not to extrapolate the corrosion performance ratings indiscriminately. Corrosion behavior ratings generally pertain only to the particular environment tested, usually rated in continuous or alternate immersion in fresh or salt water.

Alternate Immersion in 3.5% NoCl. Exposure to 3.5% sodium chloride or to substitute ocean water (ASTM D 1141) by alternate immosion (ASTM G 44) (see Table 1) is a widely used procedure for testing smooth specimens of aluminum alloys. Aeration of the specimens, achieved by the alternate immersion, enhances the corrosion potential (Ref 26) and produces more rapid SCC of most aluminum alloys than continuous immersion. The ASTM G 44 standard practice consists of a 1 h cycle that includes a 10 min soak in the aqueous solution followed by a SO min period out of solution in air at 27 °C (80 °F) and 45% relative humidity, during which time the specimens are air dried, lliis 1 h cycle is repeated continuously for the total number of days recommended for the particular alloy being tested. IVpically, aluminum alloys are exposed from 10 to 90 days, depending on the resistance of the alloy to corrosion by salt water. This test ntethod is widely used for testing most types of aluminum alloys with all types of smooth specimens. 

Alternate immersion test using an aqueous solution containing 2.86% NaCl plus 0.52% magnesium chloride (total chloride equal to that in ocean water) proposed in Ref 39 as a less corrosive substitute for 3.5% NaCl solution for ASTM G44... 

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