Corrosion failures of austenitic and duplex stainless steels in a biodiesel plant

Abstract

The neutralization of catalysts with hydrochloric acid is an important step in a biodiesel process which generates steel corrosion due to the decrease in pH, and the addition of chlorides in the medium. Other factor responsible for corrosion and consequently failures is the poor miscibility between biodiesel and acid water generating a heterogeneous coverage of the surface of the pipelines and equipment by biodiesel and acid water, presence of galvanic cells and localized corrosion. In this paper, corrosion failures of AISI 316L and 904L austenitic stainless steels (SS), and of SAF 2205, 2507 duplex SS in a biodiesel plant were identified and studied. The low pH, presence of chlorides, high thermal input in welding, temperature gradient and mainly proximity to the acid injection site play the major roles in corrosion which generate the studied failures. Corrosion failures occurred in pipes transporting biodiesel and acid water, in a spool of acid water injection, in a static biodiesel mixer with acidic water, in a cylindrical decanter vessel, and in heat exchangers.

Biofuels-related materials deterioration in biorefineries, transportation and internal combustion engines: a technical review

“…Knowing the possible corrosion risks, researches have pin-pointed the metal category and mechanisms, as reported in Table 3. As expected, stainless (304-316) have shown the highest corrosion resistance by most chemically induced aversion phenomena [44].…”

“…The studies on corrosion behaviour of aluminium and its alloys are mentioned in The corrosion rate of mild carbon steel is known to be 12-13.5 times faster in biofuels than in most fossil diesels [56]. High quantity of oxygen and moistures in biofuels drastically increases the corrosion susceptibility of iron and steels; facilitates the formation of galvanic cells and localised corrosion [44]; forms corrosion products such as FeO(OH) (product of redox reaction between Fe, O 2 and H 2 O) [57], Fe 2 O 2 CO 3 (product of CO 2 solution in biofuels causing reaction between H 2 CO 3 and FeO(OH)) [58], FeCO 3 (product of RCOO − carboxylate radicals reaction with Fe radicals) [59], Fe(OH) 3 (final product of FeO(OH) reaction with dissolved H 2 O) [19] and Fe 2 O 3 (product of direct oxidation of iron with highly moist and oxygenated biofuels) [42]. The high concentration of α-FeOOH, β-FeOOH, δ-FeOOH and Fe 2 O 2 CO 3 are reported as corrosion byproducts for most iron-based materials.…”

“…Concise inspection of steel failures in biodiesel plants have shown that 45.5% has occurred in the biodiesel circuit (during decanting and washing process) and 54.5% in the glycerin circuits (during naturalising and distillation process). The corrosion failures mostly consist of pitting corrosion along contact area and crevice corrosion at junctions and welding zones [44]. The iron and steels have been the focus of corrosion study in biofuels, as the main constructive material in bio-production plants and ICEs (Table 5) - Copper and its alloys …”

Plant Maintenance Performance Measurements Shortfalls

“…Torres, Santos and Lins [75] investigated a biodiesel plant's pipeline in Brazil in order to establish why the plant had failed during its operation. After a thorough analysis it was found that production halts without proper acid phase drainage, an inefficient configuration of the neutralisation current insertion spool in the biodiesel stream, and a high concentration of the catalyst were the causes of the failures.…”

Laves phase precipitation and sigma phase transformation in a duplex stainless steel microalloyed with niobium.

“…Heating these steels to temperatures of 600º to 850°C leads to a reduction in ferrite content () and to the formation of the sigma phase (), a very common intermetallic compound that has a greater effect on the mechanical properties of the material and significantly reduces ductility and notched impact strength. It forms preferentially at the interface between ferrite and austenite, which is stable up to a maximum temperature of 1050°C [11][12][13][14].…”