Stress corrosion cracking oxygen levels

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

Low-carbon and chromium-nickel steels, certain copper, nickel and aluminium alloys (which are all widely used in marine and offshore engineering) are liable to exhibit stress-corrosion cracking whilst in service in specific environments, where combinations of perhaps relatively modest stress levels in material exposed to environments which are wet, damp or humid, and in the presence of certain gases or ions such as oxygen, chlorides, nitrates, hydroxides, chromates, nitrates, sulphides, sulphates, etc. 

As an industrial problem stress corrosion cracking is of considerable importance. There is a long history of major and minor failures, particularly in the chemical industry and in the transport industry, particularly of components in ships and planes. It is a major potential source of failure in the nuclear power industry in which, for excunple, austenitic stainless steels may fail in high purity water containing oxygen and chloride ions at the level of ppb. 

In a similar manner carbon steel is sensitive to pH. In the pH 4.5-9 range the corrosion rate is governed by dissolved oxygen. (See the following discussion on velocity effects.) Below pH 4.5 the corrosion rate is controlled by hydrogen evolution and above about pH 9, the rate is suppressed by an insoluble film of ferric hydroxide (Fig. 5) [IS]. At very high pH levels, especially at elevated temperatures, steel is susceptible to stress corrosion cracking [2i]. 

A new disclosure of stress corrosion cracking in a fully-refrigerated ammonia storage tank was reported a year later [89]. Two identical tanks were inspected of which one showed a significantly higher level of attack by SCC than the other which could not be correlated with differences in the oxygen or water content of the ammonia. 

Metallic chloride salts, such as ferric chloride and cupric chloride, can be very corrosive to CF8M. Above 160°F (71°C), chloride can also cause SCC. The combination of chlorides, water, oxygen, and surface tensile stress can result in cracking at stresses far below the tensile strength of all austenitic SSTs. Although a threshold chloride level may exist, one is difficult to set because chlorides concentrate in pits, crevices, and under deposits until the minimum concentration is reached. One must be concerned about SCC any time a few hundred ppm chlorides is present and the temperature exceeds about 160°F (71 °C). SCC may develop at lower temperatures if the pH is low. 

---