Organic inhibitors for hydrogen permeation in iron
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MANYmetals when exposed to hydrogen suffer serious degradation in their mechanical properties? Degradation can occur due both to hydrogen present in the environment 2 and to hydrogen present internally in the metal? There is still considerable controversy as to the exact mechanism of hydrogen embrittlement. 4 However, it is generally conceded that hydrogen must be present as atomic hydrogen either in or on the metal to cause damage? Regardless of the exact mechanism of hydrogen damage, one potential method of preventing damage is to prevent or reduce the entry of hydrogen into the metal by treating the metal surface so that entry is made more difficult than for the clean metal surface. The present report describes measurements of the effect of surface treatments on the rate of hydrogen entry. The approach used was to measure the permeation of hydrogen through the metal and to attempt to decrease the permeation rate by surface treatments. Previous measurements have indicated that hydrogen entry can be retarded by using an oxide layer on a metal surface. 7 There are indications that gases added to hydrogen can reduce the extent of interaction of hydrogen with the metal, as evidenced by reduction in crack growth rates. Gases that are effective in reducing cracking include 502, 7 CS2, 7 and CartE .8 Gases that have been found to be ineffective include COil CO2, 7 CH48 and Nv 8 Wolfram and Morin 9 have recently performed calculations indicating that ethylenic type molecules should be strongly adsorbed on transition metal surfaces. The effectiveness of several such species was determined by performing hydrogen permeation experiments, with the potential inhibiting species added to the hydrogen gas on the inlet side of the permeation specimen. This paper describes the results of these experiments. EXPERIMENTAL PROCEDURE The materials used were high purity iron (99.9 + pct purity) received in the form of thin sheet. Disk specimens of about 5 cm diam were cut from the sheet and annealed for one hour at 900 ~ in a vacuum of 10 -5 WAYNE M. ROBERTSON is a Member of the Technical Staff, Physical Metallurgy, Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360. Manuscript submitted November 2 l, 1979.
torr. A specimen was then mounted in the permeation apparatus as described previously. 1~ The mounted specimen was heated to 450 ~ in hydrogen to help clean the surfaces before performing the permeation experiments. Steady state permeation experiments were performed by applying a hydrogen pressure, P l, to the inlet side of the specimen. The hydrogen which permeated the specimen was collected in a chamber of known volume on the outlet side of the specimen and the amount was determined by measuring the pressure, P2, in the chamber. High purity extra dry hydrogen, used directly from the tank as received, was passed continuously across the inlet side of the specimen at a flow rate of about one cubic centimeter per minute. Hydrogen permeability was measured over a range of temperatures on each specimen w
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