13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- data in [4] suggests that the depth of the pits in alloys such as 7075-T6 increases by the interconnection of the pits that have nucleated at constituent particles at various depths through the thickness of the exposed alloy. Rybalka’s study [5, 6] on pitting development on Stainless Steel 403 Steel and 20Kh13 showed that the size of pit is affected by the PH and temperature of the solution and the electrode rotation. Additionally, the depth of growing pits as a function of time can be described by the equation h = 2.25+3.39t1/2. While the depth h of growing pits on 20Kh13 steel in 0.01 M NaCl solution at ΔE= 30 mV increases with the time as h~t1/2, and an average pit diameter d obeys the relationship d= d0(1–e –0.07t). P. Ernst [7, 8] proposed that the pit width increases almost linearly with time, and the pit growth in depth follows a parabolic law with time (∝√t) and is independent of the potential, whereas lateral pit growth is linear with time and dependent upon potential. Harlow and Wei [9] assumed that the pit maintained hemispherical geometry and grew at a volumetric rate determined by Faraday’s law, and the aspect ratio is a continuous function of time. The relationship between the depth (a) and the diameter (2c) of pits was studied by Kondo [10] and showed that the pit growth occurred at the same aspect ratio a/c≈0.7. The corrosion pit growth law can be formulated as 2c∝t1/3. Cavanaugh [11] used optical profilometry and Weibull functions to characterize pit depth and diameter distributions and found pit growth kinetics varied by environment, but most followed approximately t1/3 kinetics. Sriraman [12] proposed the depth ap is considered proportional to the cube root of t through the relationship of ap=Bt 1/3. Buxton [13] described the pit growth law following a typical power law curve (x=Btβ) with a relatively large exponent value of 0.596. Turnbull [14] also assumed that the depth can be described by x=αtβ, and examples of the results [15] for three environmental exposure conditions in terms of the variation of aspect ratio with pit depth are illustrated. As illustrated above that various relationship was developed for pit depth and width. But the literature mostly suggests that the pit width follows a linear relationship with time shown in Fig. 1 (left) and the pit depth is linearly proportional to the square root of time in Fig. 1 (right). 0 100 200 300 400 500 600 700 800 900 0 200 400 600 800 1000 1200 1400 Average pit diameter in 0.01 M NaCl solution at ΔE=30 mV, d= 34.5(1-e-0.07t).[10] Radius of the area with changed surface state adjacent to the growing pit.[10] Pit wide from the edge of a 50 μm 304SS foil in 1 M NaCl at 10oC and 600 mV.[11] Pit wide in 1M NaCl at 20oC and 600mV.[12] Pit wide in 1M NaCl at 15oC and 600mV.[12] Pit wide in 1M NaCl at 10oC and 600mV.[12] Pit wide in 0.01MNaCl, at 28 oC and 600mV. [12] Pit wide in 0.005MNaCl, at 28 oC and 600mV. [12] Pit wide (1M NaCl+0.5MNa2SO4) at 15 oC and 600mV.[12] Pit wide 1M NaCl at 15oC and 600mV.[12] pit width, μm t, s [10] [10] [11] [12] [12] [12] [12] [12] [12] [12] 0 10 20 30 40 50 0 20 40 60 80 100 120 140 160 180 200 220 240 The square-root time dependence of the depth of growing pits. h=2.25+3.39t1/2[9] The dependences of the depth of the deepest pits on the time of exposure in 0.01M NaCl solution at ΔE= 30 mV.[10] Pit depth grown from the edge of a 50 μm 304 SS foil in 1M NaCl at 10oC and 600mV.[11] Pit growth kinetics from the edge of a 50μm 304 SS foil with 1D growth in an artificial pit in 1 M NaCl at 15oC and 600mV.[11] Pit depth in 1M NaCl at 20oC and 600mV.[12] Pit depth in 1M NaCl at 15oC and 600mV.[12] Pit depth in 1M NaCl at 10oC and 600mV.[12] Pit depth in 0.005M NaCl, at 28 oC and 600mV. [12] Pit depth in 0.01M NaCl, at 28oC and 600mV. [12] Pit depth (1M NaCl+0.5M Na2SO4) at 15 oC and 600mV.[12] Pit depth 1M NaCl at 15oC and 600mV.[12] pit depth, μm t0.5, s0.5 [9] [10] [11] [11] [12] [12] [12] [12] [12] [12] [12] Fig. 1. Corrosion pit width vs t and depth vs t0.5. 2.2 Pit shape development Pidaparti et al [16] noted that the pit/defect profile changing its shape (both depth and width) from slightly conical to more hemispherical shape with increasing corrosion time and stress distribution and levels vary non-linearly around a single pit/defect. Melchers [17] proposed that the sequence consists of the development of anodic areas, development of small pits and shallow broad pits, the apparent coalescence of small pits into larger localized corrosion and eventually the appearance of
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