Pitting resistance equivalence number PREN

Some physical properties such as coefficient of thermal expansion and modulus of elasticity of the alloys are given in Table 7.12. Other factors such as resistance to general and pitting corrosion, pitting resistance equivalent number (PREN) and the relative cost ratios are also given in Table 7.12. 

Pitting resistance equivalent number (PREN) is derived from a formula involving chromium, molybdenum and nitrogen contents of the alloys. These numbers are used as a qualitative indication of the pitting resistance of the alloys. The higher the PREN value the greater the resistance of the alloys to corrosion due to chloride. 

Small amounts of tungsten and nitrogen in the alloy decrease susceptibihty of stainless steel to pitting. Sedriks [69] introduced pitting resistance equivalent number (PREN) as an index for stainless steel ... 

The molybdenum (Mo) content of types 316 and 317 stainless resists the onset and development of pitting. The presence of nitrogen (N), a strong austenitic former, remarkably increases resistance to pitting and crevice corrosion. The relevant effect of Cr, Mo, and N on crevice corrosion is denoted by the widely used Pitting Resistance Equivalent Number (PREN). 

Performance in the area of pitting and crevice corrosion is often measured using critical pitting temperature (CPT), critical crevice temperature (CCT), and pitting resistance equivalent number (PREN). As a general rule, the higher the PREN, the better the resistance. The PREN is determined by the chromium, molybdenum, and nitrogen contents PREN=%Cr-l-3.3 (%Mo)-l-30(%N). Table 1.4 lists the PRENs for various austenitic stainless steels. 

Relative resistance can also be described by the chloride concentration below which there is little likelihood of crevice attack occurring. Pitting, particularly at or near welds and in crevices, has often resulted in perforation within a few months. It is necessary, therefore, to chose an alloy with high resistance to localized attack, which is often defined as an alloy with a high pitting-resistance equivalent number (PREn). PREn is derived from an empirical relationship and can take several forms. The most widely used form to predict the pitting resistance of austenitic and duplex stainless steels is expressed as ... 

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