Steel environmental conditions affecting

Environmental Conditions Affecting Vessel Steel in Hydrogen... 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Factors that influence SCC response will generally affect one of four basic variables in the process the material or alloy system, the chemical service environment, the electrochemical state of the system relative to surroundings, and the state of mechanical stress. Nearly all structural alloy systems can be found susceptible to SCC under certain alloy chemistry, metallurgical condition, and service environmental conditions. SCC behavior relative to alloy system was detailed in an American Society for Metals (ASM) publication [6], for a broad range of structural alloys, and has also been covered extensively for aluminum, titanium, and high-strength steels [8]. 

The danger that cracks may adversely affect the durability of the material is proportional to their width, strongly related to their connectivity, and depends on the environmental conditions. Also, the location of the cracks and function of the element are of importance. Table 9.1 lists approximate values of the crack width, which are considered as maximum permissible according to various national and regional standards or other regulations. These values are indicated as examples only and should be considered together with such aspects as the quality of execution and control, dimensions and importance of examined elements, their role as structural or non-structural ones, the minimum cover depth required for steel bars, etc. 

Severe loss of ductility of a metal (or alloy) loss of load carrying capacity of a metal or alloy the severe loss of ductility or toughness or both, of a material, usually a metal or alloy. Many forms of embrittlement can lead to brittle fracture and many can occur during thermal treatment or elevated-temperature service (thermally induced embrittlement). Some of these forms of embrittlement, which affect steels, include blue brittleness, 885 °F (475 °C) embrittlement, quench-age embrittlement, sigma-phase embrittlement, strain-age embrittlement, temper embrittlement, tempered martensite embrittlement, and thermal embrittlement. In addition, steels and other metals and alloys can be embrittled by environmental conditions (environmentally assisted embrittlement). Forms of environmental embrittlement include acid embrittlement, caustic embrittlement, corrosion embrittlement, creep-rupture embrittlement, hydrogen embrittlement, bquid metal embrittlement, neutron embrittlement, solder embrittlement, sobd metal embrittlement, and stress-corrosion cracking. 

Different types of wood, equipment design, and processes can affect the performance of stainless steels. It is typical of the pulp and paper industry that corrosion conditions vary considerably from one plant to another. In general the corrosivity of the pulp and paper process has increased since 1970 as a result of environmental regulations. Some advice on design materials for the Kraft process is given below. 

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