Fatigue crack growth behavior of inconel 600 at cathodic potentials
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cathodic relative to the carbon steel support plate. At room temperature, Inconel 600 is cathodic relative to carbon steel and the reduction of protons to form atomic hydrogen on the tube surface is the likely cathodic reaction, particularly in deaerated water. Adsorbed atomic hydrogen can either recombine to form molecular hydrogen, or become absorbed into the metal. It is this uncombined or atomic hydrogen that is the possible source of embrittlement and the object of this study. EXPERIMENTAL APPARATUS AND PROCEDURES Corrosion Cell Fatigue crack growth rate tests under controlled potential were conducted in a specially designed 200 ml Plexiglas e' cell, Fig. 1. The front face of the cell is fitted with a glass window for visual crack length measurements. A platinum gauze counter-electrode is mounted to the inside of the back face. The Luggin probe penetrates the side of the cell through two O-rings, and is attached to a micrometer marked in 0.01 mm gradations. Since this is the same gradation spacing as on the traveling microscope dial, the moving crack tip can be followed by the probe with greater accuracy. The Luggin probe is attached by a rubber hose to a beaker containing the same solution as that in the cell ( I N H2S04). A distilled water bridge separates this beaker from an intermediate solution (1N K2SO4) in which the reference electrode is immersed. This arrangement keeps the chloride ions in the saturated calomel reference electrode far from the test cell. The same approach is used in potentiodynamic polarization tests and has been found to be very successful. Specimen Preparation The Inconel 600 stock is supplied in the form of 0.32 cm plates from which 7.6 cm • 15.2 cm blanks are cut. The blanks are wrapped in stainless steel foil and clamped between two 1.3 cm thick blocks of stainless steel to maintain flatness during the annealing and
ISSN 0360-2133/81/0811-1409500.75/0 9 1981 AMER1CAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF A1ME
VOLUME 12A, AUGUST 1981--1409
quenching operations. The appropriate thermal treatment is given to each specimen. The specimens are then machined down to 7.4 cm • 12.7 cm and the holes are drilled to finish the single edge notch (SEN) specimen design, Fig. 2. A notch 1.0 mm wide and 5.1 mm long is cut with a band saw and a smaller notch 0.25 mm wide and 2.5 mm long extending from the tip of the band saw cut is made with a string saw. A two inch wide horizontal strip on both sides of the specimen is ground using 240, 360, 400 and 600 grit silicon carbide paper, 3/0 emery paper, and then polished with 6 and 3/~m diamond paste to a high luster. The surface and notch are cleaned ultrasonically in acetone. RTV ~ silicone rubber is applied as a thin film over the polished horizontal strip to limit the total current
flow between the electrodes. An area ahead of the notch 0.48 m m wide by 4.1 cm long is left exposed by protecting it with tape during the application of RTV ~. This leaves an exposed area of 3.87 cm 2 on both sides and allows for easy tracking of the
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