tailieunhanh - Mechanical Engineers Reference Book 12E Episode 8

Tham khảo tài liệu 'mechanical engineers reference book 12e episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Corrosion 7 163 Most material combinations are prone to fretting corrosion but the presence of oxide films aggravates it especially when oxygen is present. Failure occurs by accumulation of oxide debris seizing galling or loss of tolerance. Fatigue may initiate at fretted parts see Figure caused by small displacements but large displacements tend to rub out the initiating fatigue cracks. Fretting corrosion is eliminated by stopping relative motion and alleviated by lubrication using harder materials fitting a gasket or by increasing relative motion. Solid-stale diffusion. Almost any two metals that will form an alloy can in theory corrode each other. A typical example was the corrosion penetration of the prototype aluminium cans of the first uranium metal reactors. On heating at temperatures simulating reactor conditions the uranium alloyed locally with the aluminium can to form UAh pyramids which penetrated the can allowing ingress of oxygen. This destroyed the fuel element by oxtde jacking. Inserting an uranium oxidc graphite layer between uranium and can prevented failure by this mechanism in the reactor. Liquid metal corrosion General liquid metal corrosion Any liquid metal that comes into contact with a solid metal with which it will form an alloy for example molten aluminium in a steel melting pot is liable to corrode it away. The effect is enhanced if the liquid metal forms a circuit operating between a range of temperature. For example molten bismuth corrodes steel slowly if at all but if it is used in a heat-transfer circuit the steel is dissolved at the higher temperature and deposited at the lower. Liquid metal penetrations Far more potentially dangerous is liquid metal penetration a form of stress corrosion in which the aggressive agent is a liquid metal which can penetrate rapidly along the grain boundaries of a metal stressed in tension and cause cracking. Many metals including lead bismuth tin and zinc cracking from which caused the .