Tribocorrosion-fatigue (multi-degradation) of stainless steel: A fundamental approach in practical conditions

Abstract

In marine and offshore industry, components and systems are exposed to corrosive media, wear and fatigue simultaneously (i.e. multi-degradation). These setups are vastly operated in critical and demanding conditions and their failure can result in irreversible environmental and health damages as well as introduce very high maintenance costs and operation shut downs. Numerous research works have been performed to study the effects of corrosion, wear and fatigue in offshore structures however, only limited studies have recently been dedicated to investigate multidegradation. In this PhD thesis, a fundamental approach has been taken to study the multi-degradation phenomenon in practical conditions of hydraulic tension risers used offshore. In house developed Lab-Scale Multi-Degradation (LSMD) test rig was used to simulate corrosion, wear and fatigue (cyclic and static loading) of super duplex (SDSS) and austenitic (ASS) stainless steels in offshore environment. Different mechanical and chemical characterization methods were applied to investigate the effect of parameters such as normal load, bending, electrochemical conditions, lubrication, etc. on multidegradation. The effect of 4-point bending on tribocorrosion of stainless steels in 3.4 wt% NaCl aqueous solution was investigated. It has been shown that formation of crack matrices as a mixture of smeared folds and subsurface cracks followed by enhanced premature detachment of these cracks matrices is the mechanism of debris formation. This mechanism is enhanced by cyclic and static bending where passive film properties are altered due to surface tension. Cyclic and static bending promoted the formation of a thicker passive film. A distinct correlation between the passive film thickness and the thickness of recrystallized zone beneath the wear track as well as wear was observed. This suggests that a thicker passive film enhances material detachment and wear by suppressing the annihilation of dislocations and promoting more brittle fracture. In marine and offshore industry, components and systems are exposed to corrosive media, wear and fatigue simultaneously (i.e. multi-degradation). These setups are vastly operated in critical and demanding conditions and their failure can result in irreversible environmental and health damages as well as introduce very high maintenance costs and operation shut downs. Numerous research works have been performed to study the effects of corrosion, wear and fatigue in offshore structures however, only limited studies have recently been dedicated to investigate multidegradation. In this PhD thesis, a fundamental approach has been taken to study the multi-degradation phenomenon in practical conditions of hydraulic tension risers used offshore. In house developed Lab-Scale Multi-Degradation (LSMD) test rig was used to simulate corrosion, wear and fatigue (cyclic and static loading) of super duplex (SDSS) and austenitic (ASS) stainless steels in offshore environment. Different mechanical and chemical characterization methods were applied to investigate the effect of parameters such as normal load, bending, electrochemical conditions, lubrication, etc. on multidegradation. The effect of 4-point bending on tribocorrosion of stainless steels in 3.4 wt% NaCl aqueous solution was investigated. It has been shown that formation of crack matrices as a mixture of smeared folds and subsurface cracks followed by enhanced premature detachment of these cracks matrices is the mechanism of debris formation. This mechanism is enhanced by cyclic and static bending where passive film properties are altered due to surface tension. Cyclic and static bending promoted the formation of a thicker passive film. A distinct correlation between the passive film thickness and the thickness of recrystallized zone beneath the wear track as well as wear was observed. This suggests that a thicker passive film enhances material detachment and wear by suppressing the annihilation of dislocations and promoting more brittle fracture.