In a study on the mechanism of pitting corrosion, a gas evolution in the pit has been observed. This gas has been gaschromatographically identified as hydrogen. The explanation of the evolution of hydrogen by the acidification of the pit electrolyte and the potential drop across the pit have been experimentally proved. By means of vacuum extraction experiments, the diffusion and dissolution of hydrogen in the metal has been shown. Experiments have been carried out to determine the effects of the dissolved hydrogen on the austenitic stainless steel. Replica techniques show that the hydrogen embrittles the metal around the pit. X-ray diffraction lines demonstrate that hydrogen causes phase transformations in the austenitic stainless steel. In the case of pitting corrosion, such transformations have not yet been confirmed. It is suggested that dissolved hydrogen can create new initiation places for pitting corrosion and in this way is responsible for the secondary pit initiation. The most important effect of hydrogen is connected with the internal stresses set up in the metal. There is no hydrogen development during the pitting corrosion of pure nickel. This leads to the conclusion that hydrogen is not of primary importance in the pitting corrosion process. A comparison is made between the pitting and the stress corrosion process. The electrochemical processes occurring in both localized corrosion forms are analogous. The hydrogen is responsible for the cracking in the stress corrosion process. Stress corrosion cracking is represented as a specific case of pitting corrosion.

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The influence of the duration of rapid mixing on a flocculation process is evaluated by investigating the characteristics of the hydroxide flocs formed under carefully controlled conditions. Two different methods are used for measuring floc dimensions and their distribution. From the experimental results other floc characteristics such as floc strength and size of the flocculi are deduced. Also the reaction constant, the destabilization factor and the growing constant of the coagulation and flocculation process are calculated. Using the influence of rapid mixing time on all floc characteristics and kinetic parameters minimum and maximum limits for the rapid mixing time are deduced.

Seys, van Haute and Brabers: Initiation process of pitting corrosion on austenitic stainless steel in chloride solutions 663 a) Auch bei Temperaturen < 0 ' C erfolgt Korrosion, ihre Geschwindigkeit ist aber vom S02-Zutritt und der Oberflachenwassermenge unabhangig. menge und die Temperatur die Korrosionsgeschwindigkeit besonders intensiv bei niedrigen SO2-Zutritten; die relativ hochste Geschwindigkeitssteigerung erfol t bei S02-Zutritten zwischen 10-30 mg . m-' . Tag-. In Anwesenheit niedriger Wassermengen (bis zu etwa 10 g . m-2) kontrollieren die Korrosionsgeschwindigkeit diejenigen Teilvorgange, welche zur Bildung von Sulfatnestern fuhren und die hydrolytische Rostentstehung bewirken. c) Bei gleichzeitiger Einwirkung hoher Oberflachenwassermengen und hoher Temperaturen begrenzt der gehemmte Sauerstofftransport die Korrosionsgeschwindigkeit. d) Die im Labor gefundenen ,,stationaren" Korrosionsgeschwindigkeiten stimmen mit Ergebnissen von Naturversuchen gut uberein. Eingegangen: 10. 4, 1974 b) Oberhalb von 0 O C beschleunigen Oberflachenwasser-P Schrifttum On the initiation process of pitting corrosion on austenitic stainless steel in chloride solutions