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

Evaluating Intergranular Corrosion Revised by Bopinder Phull, Consultant The Purpose of Testing... Evaluating Intergranular Corrosion Revised by Bopinder Phull, Consultant Tests for St

Evaluating Crevice Corrosion

Evaluating Galvanic Corrosion

Harvey P Hack, Northrop Grumman Corporation

Component Testing

Evaluating Galvanic Corrosion

Harvey P Hack, Northrop Grumman Corporation

Modeling

Evaluating Galvanic Corrosion

Harvey P Hack, Northrop Grumman Corporation

Laboratory Testing

Electrochemical Tests

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Fig 1 Galvanic series for seawater Dark boxes indicate active behavior of active-passive alloys

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

exchange current; Ecorr , corrosion potential

Specimen Exposures

Fig 3 Typical galvanic-corrosion immersion test setup using wire connections

Fig 4 Typical galvanic-corrosion test specimen using a threaded rod for mounting and electrical connection

Ω

Fig 5 Basic circuit for a zero-resistance ammeter

Fig 6 Conversion of a potentiostat into a zero-resistance ammeter WE, working electrode; CE, counter electrode; RE, reference electrode

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

Fig 9 Specimen configuration for the washer test for atmospheric galvanic corrosion

References cited in this section

Evaluating Galvanic Corrosion

Harvey P Hack, Northrop Grumman Corporation

References

Evaluating Galvanic Corrosion

Harvey P Hack, Northrop Grumman Corporation

Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

The Purpose of Testing

Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

Tests for Stainless Steels and Nickel-Base Alloys

Table 1 Appropriate evaluation tests and acceptance criteria for wrought alloys

S44800 S30400

S30403

S30908 S31600

S31603

S31700

S31703

S32100 S34700 N08020 N08904 N08825 N06007 N06985 N06625 N06690 N10276 N06455 N06110

Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

Tests for Aluminum Alloys

References cited in this section

Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

Tests for Other Alloys

Table 2 Media for testing susceptibility to intergranular corrosion

References cited in this section

Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

Acknowledgment

Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

References

Evaluating Intergranular Corrosion

Revised by Bopinder Phull, Consultant

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

References cited in this section

Evaluating Exfoliation Corrosion

Revised by Bopinder Phull, Consultant

Spray Tests

References cited in this section

Evaluating Exfoliation Corrosion

Revised by Bopinder Phull, Consultant

Immersion Tests

Fig 2 Examples of exfoliation rating EA (superficial) Specimens exhibit tiny blisters, thin slivers, flakes,

or powder, with only slight separation of metal Source: Ref 11

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

Fig 4 Examples of exfoliation rating EC (severe) There is penetration to a considerable depth into the metal Source: Ref 11

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

References cited in this section

Evaluating Exfoliation Corrosion

Revised by Bopinder Phull, Consultant

Acknowledgment

Evaluating Exfoliation Corrosion

Revised by Bopinder Phull, Consultant

Acknowledgment

Evaluating Exfoliation Corrosion

Revised by Bopinder Phull, Consultant

References

Evaluating Exfoliation Corrosion

Revised by Bopinder Phull, Consultant

References

Evaluating Stress-Corrosion Cracking

Introduction

Fig 1 Causes of premature fracture influenced by the corrosion of a structural component

Evaluating Stress-Corrosion Cracking

Revised by Bopinder Phull, Consultant

Static Loading of Smooth Specimens

Table 1 Stressing methods applicable to various sources of sustained tension in service

Faying surface corrosion

Constant-Strain versus Constant-Load Tests

Fig 4 Comparison of changing stress during initiation and growth of isolated SCC in constant-strain and constant-load tests of a uniaxially loaded tension specimen (a) Constant-strain test (b) Constant- load test σ M is the maximum stress at crack tip, σ N is the average stress in the net section, and σ G is the applied stress to the gross section Source: Ref 7

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

Bending versus Uniaxial Tension

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% H 2 O 2 Source: Ref 9

Elastic-Strain Specimens

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

pounds per square inch) Source: Ref 12

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

σ

Fig 9 Typical tuning-fork SCC test specimens (a) Source: Ref 24 (b) Source: Ref 1 (c) Source: Ref 25

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

Fig 11 Methods of stressing C-rings (a) Constant strain (b) Constant load (c) Constant strain (d) Notched C-ring; a similar notch could be used on the side of (a), (b), or (c) Source: Ref 14

Fig 12 O-ring SCC test specimen (a) and stressing plug (b) The O-ring is stressed by pressing it onto the plug, as shown in (c)

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

Fig 14 Ring-stressed tension specimen for field testing Source: Ref 1

Fig 15 Equipment for constant-strain SCC testing (a) Constant-strain SCC testing frame Exploded view (left) showing the 3.2 mm (0.125 in.) diam tension specimen and various parts of the stressing frame Final stressed assembly (right) Source: Ref 21 (b) Synchronous loading device used to stress specimens The specimen is loaded to a prescribed strain value determined from a clip-on gage The applied stress is given by the product of the strain and the material elastic modulus A stressed assembly and one assembled finger-tight ready for stressing are shown

σ ε σ ε

Fig 16 Mean breaking stress versus exposure time for short-transverse 3.2 mm (0.125 in.) diam aluminum alloy 7075 tension specimens tested according to ASTM G 44 at various exposure stress levels Each point represents an average of five specimens Source: Ref 3

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

Fig 18 Influence of specimen configuration on SCC test performance (alternate immersion in 3.5% sodium chloride per ASTM G 44) Aluminum alloy 7075-T7X51 specimens stressed 310 MPa (45 ksi); each point represents 60 to 90 specimens Source: Ref 23

Fig 20 True-stress/true-strain relationships for stressed U-bends See text for discussion of (a) to (e) Source: Ref 27

Residual-Stress Specimens

Fig 21 SCC test specimens containing residual stresses from plastic deformation (a) Cracked cup specimen (Ericksen impression) Source: Ref 1 (b) Joggled extrusion containing SCC in the plastically deformed region Source: Ref 9

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

References cited in this section

Evaluating Stress-Corrosion Cracking

Revised by Bopinder Phull, Consultant

Static Loading of Precracked (Fracture Mechanics) Specimens

Test Specimen Selection

Fig 24 Classification of precracked specimens for SCC testing Asterisks denote commonly used configurations Source: Ref 33

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Preparation of Precracked Specimens

Testing Procedure

Fig 25(a) Proportional dimensions and tolerances for cantilever bend test specimens Width = W; thickness (B) = 0.5W; half loading span (L) = 2W; notch width (N) = 0.065W maximum if W >25 mm (>1.0 in.); N = 1.5 mm (0.06 in.) maximum if W = 25 mm (1.0 in.); effective notch length (M) = 0.25 to 0.45W; effective crack depth (a) = 0.45 to 0.55 W

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

practical; fine threaded, square or Allen head Half height = H; thickness (B) = 2H; net width (W) = 10H minimum; total width (C) = W + T; thread diameter (T) = 0.75 H minimum; notch width (N) = 0.14H maximum; effective notch length (M) = 2H

Fig 26 Alternative chevron notch (a) and face grooves (b) for single-edge cracked specimens

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

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

stress intensity KIo; Vpl = plateau velocity

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

Fig 30(b) Ring-loaded wedge-opening load specimen test setup Box to the left of loading rings contains analog signal conditioning for load and displacement signals The digital data-acquisition system consists

of a scanner connected to the analog load and displacement signals, a digital voltmeter, and a portable computer used to read and store data and to control the other instruments Source: Ref 3

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

Δ Δ

References cited in this section

Evaluating Stress-Corrosion Cracking

Revised by Bopinder Phull, Consultant

Dynamic Loading: Slow-Strain-Rate Testing

Fig 32 Effect of strain rate on SCC and hydrogen-induced cracking Source: Ref 66

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

Fig 33 Strain-rate regimes for studying SCC of various aluminum alloys Corrodent: 3% sodium chloride plus 0.3% hydrogen peroxide Source: Ref 64

Fig 34 Typical slow-strain-rate test apparatus Source: Ref 63

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

boiling 4 N sodium nitrate and in oil at the same temperature Source: Ref 62

Fig 36 Macrographs of two carbon steel specimens after slow-strain-rate tests conducted at a strain rate

of 2.5 × 10 -6 s -1 and 80 °C (180 °F) The ductility ratio in this example was 0.74 (original diameter: 2.54

mm, or 0.100 in.) Left: Ductile fracture in oil Right: SCC in carbonate solution

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

References cited in this section

Evaluating Stress-Corrosion Cracking

Revised by Bopinder Phull, Consultant

Selection of Test Environments

Fig 38 Cantilever beam specimens of PH13-8Mo stainless steel after testing Experiments demonstrate that electrochemical factors can override mechanical factors in determining initiation sites of SCC See text for details Source: Ref 75