Zu der Arbeit ,,SpannungsriBkorrosion von Stahlen in flussigem Ammoniak" von W. Schniift und K. E. Heuslcr, diese Zeitschrift 35 (1984) Heft 7 . S. 329-337, erhielten wir folgende Zuschrift:The Clausthal Group have made several important contributions to the fundamental knowledge of electrochemistry in liquid ammonia and have been able to start their work on stress corrosion cracking on a good foundation. The published paper does, however, raise some questions, which I hope the authors will help to answer.The paper summarizes what is obviously a very great amount of work and concludes that the cracking phenomena must be explained by hydrogen induced SCC. This is in direct contradiction to the conclusions drawn by other researchers who have used very similar experimental approaches, and it is tempting to look for possible explanations.The first thing to be observed is that the tests were done in solutions which contained either ammonium chloride or ammonium perchlorate to increase the conductivity of the liquid ammonia. Ammonium chloride is acidic and must be assumed to promote active corrosion over a wide potential range, while ammonium perchlorate can probably be considered to be electrochemically inert. The authors discuss in the paper the electrochemical characteristics of other contaminants in ammonia, but not the effect of these two salts. The next observation concerns the strain rates used. It is mentioned in the text that the strain rate had no influence on the results, which is very astonishing. The only additional information can be found in the figure captions, which give examples of crosshead speeds. These can be translated to absolutc strain rates of 1.4 x and 3.4 x 10-5s-' for the cylindrical test pieces with a gauge length of 25 mm. Approximately, the same value is obtained for the notched flat sample if the 0.2 mm notch base is taken as the gaugc length.These strain rates are approximately 10 times higher than those used by Deegun and Wilde and by Poulson and myself (2 x W b s -' ) . Furrow, Hufchings and Sunderson used a strain rate of 9 x 10-'s-'. Tests at 2 X 10-'kl have consistently produced well developed cracks in passive samples, together with large reductions of ductility, while a strain ratc of 9 x W6s-' produced shorter cracks and a less pronounced effect on ductility. Still higher strain rates, as used by the authors, may give negligible anodic stress corrosion response, but may still reveal hydrogen damage in specimens exposed to corrosive ammonia with NH,C1 or cathodic polarisation.It would be appreciated, if the authors would supply additional information on the range of strain rates actually used, especially for tests in the "neutral" ammonia with perchlorate, in which anodic passivation can be induced.
H .Answer: Our paper produces positive evidence for the possibiiiity of hydrogen induced stress corrosion cracking (SCC) of steel in liquid ammonia. We did not say that cracking of steel in liquid ammonia always must be explained this way. In fact, we mentioned the possibility of a...
