Some disturbing trends in the levels of habitual physical activity in Hong Kong primary school children: preliminary findings

Hydrogen embrittlement has caused many service failures in linepipe steel. Service conditions present several avenues by which hydrogen can adversely affect the mechanical properties of such steel. Fesseler [1] reported that local mechanical damage can arise from handling and laying of pipe and that some cathodic protection schemes can in fact inject dangerous amounts of hydrogen into a steel. There can be synergism between these two aspects which can give extremely high hydrogen charging conditions resulting in failure [2]. Metallurgists in the oil and gas industries refer to this type of failure as hydrogen stress cracking (HSC). Future synthetic natural gas from coal gasification may contain as much as 50% H2 which will be very deleterious to linepipe properties. Sulfides present in natural gas, hydrogen sulfide or carbon disulfide, tend to inhibit the recombination of 2HADS → H2and therefore favor atomic hydrogen absorption into a steel [3]. Welding of linepipe is a potential source of nearly every known hydrogen problem depending on the circumstances of the technique [4]. Thus it is apparent that NDE techniques adaptable to field use will be important to the maintenance of thousands of miles of existing and future pipelines. The particular use of positron annihilation to be described could be one such technique