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.摘要 采用盐酸中和催化剂是生物柴油生产流程中的关键工序，但该工序会因pH值下降以及介质中氯化物的存在而引发钢材腐蚀。此外，导致腐蚀乃至设备失效的另一诱因是生物柴油与酸性水的互溶性不佳，这会使管道及设备表面被生物柴油和酸性水不均匀覆盖，进而形成原电池并引发局部腐蚀。本文针对某生物柴油工厂内AISI 316L、904L奥氏体不锈钢（austenitic stainless steels，SS）以及SAF 2205、2507双相不锈钢（duplex SS）的腐蚀失效问题开展了识别与研究。低pH值、氯化物的存在、焊接热输入过高、温度梯度，以及最主要的靠近酸注入位点的位置，是引发本次研究中腐蚀失效的核心影响因素。腐蚀失效发生在输送生物柴油与酸性水的管道、酸性水注入管线段、搭载酸性水的静态生物柴油混合器、圆柱形沉降罐以及换热器中。