Volume 13A - Corrosion Fundamentals, Testing, and Protection Part 10 docx

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Evaluating Galvanic Corrosion

Harvey P Hack, Northrop Grumman Corporation

Component Testing

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Laboratory Testing

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Fig 2 Prediction of coupled potential and galvanic current from polarization diagrams i, current; io,

exchange current; Ecorr, corrosion potential

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Ω

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Fig 7 Specimen configuration for the ISO test for atmospheric galvanic corrosion 1, anodic plate, 1 piece; 2, cathodic plate, 2 pieces; 3, microsection, 2 pieces; 4, tensile test specimen; 5, bolt, 8 × 40 mm, 2 pieces; 6, washers, 1 mm thick, 16 mm diameter, 4 pieces; 7, insulating washers, 1 to 3 mm thick, 18 to 20 mm diameter, 4 pieces; 8, insulating sleeve, 2 pieces; 9, nut, 2 pieces

Fig 8 Specimen configuration for the wire-on-bolt test for atmospheric galvanic corrosion

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Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

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Tests for Stainless Steels and Nickel-Base Alloys

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≥

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Tests for Other Alloys

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References cited in this section

Acknowledgment

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Fig 1 Comparison of exfoliation of aluminum alloy 2124 (heat treated to be susceptible; EXCO ED rating) in various seacoast and industrial environments Specimens were 13 mm ( in.) plate Source: Ref 2

Evaluating Exfoliation Corrosion

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Fig 3 Examples of exfoliation rating EB (moderate) Specimens show notable layering and penetration into the metal Source: Ref 11

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Fig 5 Examples of exfoliation rating ED (very severe) Specimens appear similar to EC except for much greater penetration and loss of metal Source: Ref 11

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Acknowledgment

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Evaluating Stress-Corrosion Cracking

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Static Loading of Smooth Specimens

Table 1 Stressing methods applicable to various sources of sustained tension in service Residual stress Quenching after heat treatment Forming Welding Misalignment (fit-up stresses) Interference fasteners Interference bushings Rigid Flexible Flareless fittings Clamps Hydraulic pressure Deadweight

Faying surface corrosion

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Fig 5 Effect of loading method and extent of cracking or corrosion pattern on average net section stress in a uniaxially loaded tension specimen Behavior is generally representative, but curves will vary with specific alloys and tempers (a) Localized cracking (b) General cracking Source: Ref 8

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Fig 6 Comparison of the SCC response with bending versus direct tension stressing under constant load for Al-5.3Zn-3.7Mg-0.3Mn-0.1Cr T6 temper alloy sheet Tested to failure in 3% NaCl plus 0.1% H2O2 Source: Ref 9

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a b L S mm 3.2 6.4 9.5 13 19 25 38

Fig 7 Specimen and holder configurations for bent-beam stressing (a) Two-point loaded specimen (b) Three-point loaded specimen (c) Four-point loaded specimen (d) Welded double-beam specimen (e)

Bolt-loaded double-beam specimen Formula for stressing specimen (e): Δd = 2fa/3Et(3L - 4a), where Δd is deflection (in inches), f is nominal stress (in pounds per square inch), and E is modulus of elasticity (in

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Fig 8 Bent beam designed to produce pure bending Source: Ref 13

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Fig 10 Sampling procedure for testing various products with C-rings (a) Tube (b) Rod and bar (c) Plate Source: Ref 14

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Fig 13 Spring-loaded fixture used to stress 3.2 mm (0.125 in.) thick sheet tensile specimens in direct tension Source: Ref 12

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σ ε σ ε

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Fig 17 Effect of temper on SCC performance of aluminum alloy 7075 subjected to alternate immersion in 3.5% NaCl solution at a stress of 207 MPa (30 ksi) Mean flow depth was calculated from the average breaking strength of five specimens subjected to identical conditions Source: Ref 22

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B C D E F G α

Fig 19 Typical U-bend SCC specimens (a) Various methods of stressing U-bends (b) Typical U-bend specimen dimensions Source: Ref 27

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Fig 22 SCC test specimens containing residual stresses from plastic deformation Shown are 12.7 mm (0.5 in.) diam stainless steel tubular specimens after SCC testing (a) and (b) Annealed tubing that was cold formed before testing (c) Cold-worked tubing tested in the as-received condition Source: Ref 1

Fig 23 SCC test specimen containing residual stresses from welding (a) Sandwich specimen simulating rigid structure Note SCC in edges of center plate Source: Ref 12 (b) Cracked ring-welded specimen Source: Ref 1

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≈≈

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Fig 25(b) Proportional dimensions and tolerances for modified compact specimens Surfaces should be

perpendicular and parallel as applicable to within 0.002H TIR The bolt centerline should be

perpendicular to the specimen centerline within 1° Bolt of material similar to specimen where practical;

fine threaded, square or Allen head Thickness = B; net width (W) = 2.55B; total width (C) = 3.20B; half height (H) = 1.24B; hole diameter (D) = 0.718B + 0.003B; effective notch length (M) = 0.77B; notch width (N) = 0.06B; thread diameter (T) = 0.625B

Fig 25(c) Proportional dimensions and tolerances for double-beam specimens “A” surfaces should be

perpendicular and parallel as applicable to within 0.002H TIR At each side, the point “B” should be equidistant from the top and bottom surfaces to within 0.001H The bolt centerline (load line) should be

perpendicular to the specimen centerline to within 1° Bolt of material similar to specimen where

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Fig 27 Configuration and KI calibration of a double-beam plate specimen Normalized stress intensity KI

plotted against a/H ratio (W - a) indifferent, crackline-loaded, single-edge cracked specimen Source: Ref

33

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Fig 29 Comparison of determination of KISCC by crack initiation versus crack arrest (a) Constant-load

test (b) Constant crack-opening displacement test a0 = depth of precrack associated with the initial

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Fig 30(a) Wedge-opening load specimen loaded with instrumented bolt Source: Ref 58

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Fig 31 Ultrasonic crack measurement system for double-beam specimens Bolt-loaded specimen is mounted on translation stage at center Ultrasonic transducer is located above specimen, and the oscilloscope at left indicates (left to right) the top of the specimen, the crack plane, and the bottom face reflection Digital readouts of stage position and peak height for the crack front measurement used to make consistent positioning measurements are shown (right) This system has a crack growth resolution of approximately 0.127 mm (0.005 in.) Source: Ref 3

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Dynamic Loading: Slow-Strain-Rate Testing

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Table 2 Critical strain rate regimes promoting SCC in various metal/ environment systems

Aluminum alloys in chloride solutions

Copper alloys in ammoniacal and nitrite solutions

Steels in carbonate, hydroxide, or nitrate solutions and liquefied ammonia Magnesium alloys in chromate/chloride solutions

Stainless steels in chloride solutions

Stainless steels in high-temperature solutions Titanium alloys in chloride solutions

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Fig 35 Nominal stress versus elongation curves for carbon-manganese steel in slow-strain-rate test in

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Fig 37 Effects of beam deflection rate on stress-corrosion crack velocity in precracked cantilever bend specimens of a carbon-manganese steel Tested in a carbonate-bicarbonate solution at 75 °C (165 °F) and at a potential of -650 mV versus SCE Source: Ref 62

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Fig 40 Potentiodynamic polarization curves for carbon-manganese steel in 1 N sodium carbonate plus 1

N sodium bicarbonate at 90 °C (195 °F) showing the domains of behavior predicted from the curves

Source: Ref 78

Fig 41 Anodic polarization curves for aluminum alloy 7075-T651 in deaerated 3.5% sodium chloride solution showing the domains of behavior predicted from the curve Source: Ref 80

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Testing of Aluminum Alloys

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Table 3 Comparison of the SCC behavior of various aluminum alloys in the ASTM G 44 test and after 5 years in a seacoast atmosphere

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7075-T651 7075-T7651 7075-T7351

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Fig 43 Correlation of accelerated test media with service environment (industrial atmosphere) Combined data for five lots of rolled plate of aluminum alloy 7039-T64 (4.0Zn-2.8Mg-0.3Mn-0.2Cr) Tests in 3.5% sodium chloride were similar to ASTM G 44, except salt solution was made with commercial grade sodium chloride and New Kensington tap water Source: Ref 9

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Fig 45 Examples of SCC crack growth in various aluminum alloys with relatively high resistance to SCC S-L (see Fig 28) double-beam specimens bolt-loaded to pop-in and wetted with 3.5% sodium chloride three times daily; relative humidity 45% The numbers 1 to 7 indicate different test materials listed in order of decreasing resistance to SCC (see Table 4); dashed lines indicate plateau velocities The alloy 2 specimen failed by mechanical fracture rather than intergranular SCC Source: Ref 47, 52

Table 4 Correlation of SCC plateau crack velocities with smooth specimen SCC ratings

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Fig 46 Effect of corrosive environment on SCC velocity and threshold stress intensity for 7079-T651 plate (64 mm, or 2.5 in., thick) stressed in the short-transverse direction (S-L; see Fig 28) Double-beam specimens bolt-loaded to pop-in No SCC occurred during 3 years of exposure to dry air in a desiccator; however, the plateau velocity (horizontal part of each curve) and the apparent threshold stress intensity

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Fig 47 The variable effects of corrosion-product wedging on SCC growth curves in a K-decreasing test

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Fig 48 SCC propagation rates for various aluminum alloy 7050 products Double-beam specimens (S-L; see Fig 28) bolt-loaded to pop-in and wetted three times daily with 3.5% NaCl Plateau velocity averaged over 15 days The right-hand end of the band for each product indicates the pop-in starting stress

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Fig 49 Initial stress intensity versus time to fracture for S-L (see Fig 28) compact specimens of various

aluminum alloys exposed to an aqueous solution containing 0.06 M sodium chloride, 0.02 M sodium dichromate, 0.07 M sodium acetate, and acetic acid to pH 4 Asterisk indicates metallographic

examination showed that SCC had started Source: Ref 44

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Fig 50 Effect of various alloying elements on the SCC behavior of a low-alloy ferritic steel in two different corrosive environments Behavior indicated by time-to-failure ratios in a slow-strain-rate test

(a) Immersed in 1 N sodium carbonate plus 1 N sodium bicarbonate at 75 °C (165 °F) (b) Immersed in

boiling 35% sodium hydroxide Source: Ref 116

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Fig 51 Method of plotting results of sulfide stress cracking tests Open symbols indicate failure; closed symbols indicate runouts Source: Ref 120

Table 6 Influence of cutoff time on apparent KISCC using the SCC initiation method

MPa ksi 100

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Table 7 Comparison of KISCC values determined by initiation and arrest methods

Initiation Arrest 10Ni, normal purity

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Fig 52 Comparison of SCC behavior of several high-strength steels based on threshold stress-intensity

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Fig 53 Use of electrochemical polarization to distinguish between SCC and hydrogen embrittlement mechanisms in a high-strength steel immersed in sodium chloride solution See text for explanation of curves A through H Source: Ref 124

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Testing of Magnesium Alloys

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Testing of Magnesium Alloys

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Table 9 Environments and temperatures conducive to SCC of titanium alloys

Hot dry chloride salts

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Fig 56 Variation of crack initiation load with potential in 0.6 M halide solutions for Ti-8Al-1Mo-1V

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Fig 57 Effect of sustained-load cracking compared to SCC in Ti-8Al-1Mo-1V mill-annealed sheet

Hydrogen concentration, 48 ppm; yield strength, 850 MPa (123 ksi); cantilever bend specimen (T-S); B =

6.35 mm (0.25 in.) See Fig 28 for an explanation of specimen orientation and fracture plane identification Source: Ref 148

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Special Considerations for Testing of Weldments

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μ μ

Reference cited in this section

Interpretation of Test Results

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Fig 60 Relationship of applied stress and flaw depth to crack propagation in hydrogen gas Dashed lines

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Table 10 Stress-corrosion crack growth rates in aluminum alloy 7075 obtained by different test methods 7075-T651 7075-T7X1 7075-T7X2 m/s in × 10-5/h m/s in × 10-5/h m/s in × 10-5/h Breaking load test using smooth tensile bar

stressed 207 MPa (30 ksi); 4 or 6 days

Bolt-loaded double-beam, pop-in stress

Plateau velocity obtained from V-KI curves Average growth 0–15 days

Average growth 0–42 days

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