Issue 20

A. Borruto et alii, Frattura ed Integrità Strutturale, 20 (2012) 22-31; DOI: 10.3221/IGF-ESIS.20.03 22 Analysis of the causes of failure in 5Cr-1Mo pipes mounted in a preheating furnace Adelina Borruto Dept. of Chemical and Mat. Engineering "La Sapienza", Via Eudossiana 18 - 00184, Rome, Italy adelina.borruto@uniroma1.it Giovanni Narducci Dept. of Chemical and Mat. Engineering "La Sapienza", Via Eudossiana 18 - 00184, Rome, Italy giovanni.narducci@uniroma1.it Pietro Pietrosanti Technip S.p.A Viale Castello della Magliana, 68 00148 Roma ppietrosanti@technip.com A BSTRACT . The oxidation resistance of iron-based alloys depends on the formation of thin protective films consisting of reaction products between the alloys and the ambient atmosphere. The high corrosion resistance of Fe-Cr-Mo alloys has been attributed to the rapid formation of a uniform, highly protective passive film. The pipes hardness and resistance to various forms of corrosion are determined by the composition of the oxide layer that plays an important role in determining the lifetime of the pipes themselves. The thermal stability of the oxide layer is influenced by its composition and thickness as well as by the bond to the underlying metal. Determining factors for the high temperature corrosion are generally: temperature, metallurgy, TAN (chloridric and naphthenic acid), local flow conditions, sulphur compounds. K EYWORDS . Low carbon steel; Corrosion products; Total Acid Number; Sulphur compounds. I NTRODUCTION he Cr-Mo steels are excellent structural materials that are used in power generation and petrochemical industry: typical products for these industries are boiler, heater, heat exchanger, reactors and hydrocrackers, usually build as heavy wall pressure vessels [1]. These steels are also used for turbine rotor of high-temperature and high-pressure power plant. The Cr-Mo steels combine, on one hand, creep resistance, strength and toughness that can be achieved through tempering, and, on the other hand, high corrosion resistance. In particular, Fe-5Cr-1Mo steel is used as corrosion resistant alloy in petrochemical plants. The oxidation resistance of Fe-Cr alloys depends on the formation of thin protective films consisting of the products of the reaction between iron-based alloys and the ambient atmosphere [2-5]. The characteristics of the oxide layer formed on the surface of a steel element play a significant role in determining the lifetime of the element itself. The thermal stability of the oxide layer is influenced by the composition and thickness of the oxide layer as well as the bond to the underlying metal [6]. However, the tempered Cr-Mo steels must be used judiciously in structures exposed to hydrogen gas [7]. T