Page 11 - Handbook of Materials Failure Analysis

1 Overview of Failure Mechanisms 3

seashore storage tank [9] and AISI 304 ethylene glycol condensate collector vessel
[10]. In case of oil refineries, SCC is very frequently encountered and has also been
correlated to the presence of sulfur-containing compounds. Specific examples
involve the effect of H 2 S on failure of SS347H tubes in a hydroprocessing reactor
heater [11] and of polythionic acid on the failure of AISI 321 hydrocracker heat
exchanger tubes [12]. SCC was also the dominant failure mechanism in a ring-type
component of hydrocracker unit [13]. The effect of sulfur compounds has been
reported to be of significant importance, especially in the case of naphtha feedstock
circulation, due to the presence of H 2 S in these fluids in relatively high concentra-
tion. In Ref. [14], transgranular cracking and intergranular SCC were noticed in case
of naphtha hydrotreater furnace tubes. The influence of corrosion environment on
fatigue-related processes is reviewed in Ref. [15]. The damaging role of corrosive
species, such as Cl and S, is underlined also in case of localized corrosion of steel
piping elbow in oil-gas separation system initiated from inorganic salts in crude
oil [16], as outlined by the chemical reactions 1.3–1.5.
CaCl 2 +H 2 O ! CaO + 2HCl (1.3)

Fe + 2HCl ! H 2 + FeCl 2 (1.4)

FeCl 2 +H 2 S ! FeS + 2HCl (1.5)
Hydrolysis of inorganic salts produces HCl (chemical reaction 1.3). The dissolution
of HCl in water creates aggressive corrosion conditions, leading to steel chemical
attack (chemical reaction 1.4). Under the presence of H 2 S, regeneration of HCl is
achieved, sustaining the corrosive conditions (chemical reaction 1.5). FeS can pro-
duce more stable FeS 2 reacting with elemental S, which could have been produced by
bacterial activity and the cycle is continued by the regeneration of fresh HCl. Metal
loss is mainly caused by HCl acid attack, produced by hydrolysis of inorganic chlo-
ride salts, such as CaCl 2 [16].
The detrimental effect of the presence of H 2 S aqueous solution was also
addressed in a case of steel absorber corrosion in a desulfurization plant, by the fol-
lowing chemical reactions [17]:
+
½
Fe + H 2 S+ H 2 O ! H 3 O + FeHS Š (1.6)
ad
+
Fe + HS ! FeHS½ Š + 2e (1.7)
ad
+
+
½ FeHS Š +H 3 O ! Fe 2+ +H 2 O+H 2 S (1.8)
ad
+

Fe 2+ +HS ! H + FeS (1.9)
where “ad” denotes adsorbed substance.