Issue 20

A. Borruto et alii, Frattura ed Integrità Strutturale, 20 (2012) 22-31 ; DOI: 10.3221/IGF-ESIS.20.03 30 pipes. 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]. Determining factors for the high temperature corrosion are generally: metallurgy, TAN (chloridric and naphthenic acid), local flow conditions, sulphur compounds [12]. Sulphur compounds, originated from crude oils, include poly-sulphides, mercaptans, aliphatic sulphides, disulphides and thiophenes. The interaction of S with metal and protective passive film are of significance to the corrosion process since the presence of impurity elements, especially S, at the metal-oxide interface is associated with the instability of oxide over layers and the spallation of the oxide at high temperatures [6]. Moreover, naphthenic acid corrosion (NAC) and sulphidic corrosion (SC) usually occur in some similar environments and the interaction between them is an important topic for refineries [13]. The composition of protective passive film (scale), for an oxidation temperature below 400°C, consists mainly of Fe 2 O 3 , possibly Fe 3 O 4 and small amount of Cr 2 O 3 . Due to the penetration and the following protective scale’s fracture, the underlying metal layer is attacked [14]. In fact, the scale becomes extremely porous and no more protective. The environment’s acidity, due to the presence of chloridric [15-16] and naphthenic acids, is one of the most important causes of this penetration through the scale. The aforementioned corrosion is pitting like, localized between scale and base material, typically close to the grain boundaries or inclusions. A considerable dissolution of metal compounds takes place in such areas: this dissolution provokes the solution acidity to increase inside the pit. Moreover, the concentration of aggressive anions increases, due to migration from the bulk, hence solution becomes more corrosive and the pit moves ahead quickly. High temperature promotes this mechanism. According to the technical literature, the condition of pipes, when examined, after a long period in high temperatures and with the aggressive charge in the last two operating years, shows a clear pitting corrosion (see Fig. 4, 5, 6) with a strongly discontinuous and porous scale. Due to conspicuous infiltrations, this scale involves the formation of a pit under the deposit itself and even attacks grain boundaries (Fig. 8). The Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) analysis, executed on the craters of pits (see Fig. 10-11), clearly highlight the oxidized layer characterized by a porous and sponge-like structure. Moreover, the presence of chromium, oxygen and sulphur, shown by data, confirms oxides of iron and chromium as corrosion products. On the other hand, the Scanning Electron Microscope (SEM) analysis performed on the section of the pit (see Fig. 13-14) highlights the corrosion products present in the inner part of the cavity. The EDS analysis executed on areas where the scale is still compact (Fig. 15, Tab. 2 - Pt 1) highlights greater chromium’s concentration (maybe Cr 2 O 3 ) than that on areas less compact that is to say where the scale is strongly attacked and penetrated where oxidation is generated by Fe 2 O 3 and Fe 3 O 4 (Fig. 15, Tab. 2 - Pt 3-4). Mapping of a section of the crust has been performed (Fig. 18) in order to perform a more accurate investigation of the crust itself. It shows sulphur presence (sulphur compounds) in all non homogeneous scale, and a build-up immediately beyond the compacted one. The examination of the materials shows an ongoing highly developed corrosive process. In fact, looking at the first two operating years, Arabian Light crude and Arabian Medium crude constituted the only feed charges. A very low crude acidity level and a low Sulphur level (TAN = 0.1; S% = 2.12) feature both the abovementioned crudes. Feed charges made up of much more aggressive crudes were adopted in the last two operating years (TAN = 0.39; S% = 3.3%). These new environmental conditions provoked a heightened corrosive process, as shown by the analysis results. The severe fragmentation of the protective scale features the abovementioned heightened corrosive process, leaving behind sulphur compounds and severe pitting effects. R EFERENCES [1] J. Hilkes, V. Gross, In: IIW Conference, Singapore, (2009). [2] Metals Handbook, Properties and Selection: Irons, Steels, and High-Performance Alloys, 10 th ed., ASM International, Materials Park OH, 1 (1990) 140. [3] A.P. Greeff, C.W. Louw, J.J. Terblans, H.C. Swart, Corrosion Science, 42 (2000) 991.