Corrosion Behavior of Welded Joints for Cargo Oil Tanks of Crude Oil Carrier

E32 grade corrosion resistant steel was welded with welding wires with three different S contents. The mi crostructure, mechanical properties, inclusions, and corrosion behavior of welded joint were investigated. The joint coupon corrosion test and potentiodynamic polarization test were carried out under the simulated corrosion environ- ment of the inner bottom plates of cargo oil tanks. The pitting initiation and propagation mechanism of the weld metal were studied by scanning electron microscopy and infinite focus. The results indicated that the microstructures of three kinds of weld metals are all composed of acicular ferrite, ferrite side-plate and proeutectoid ferrite. The micro- structure of heat affected zone is composed predominantly of bainite. Joint welded with low S filler wire has good me- chanical properties. S can decrease free corrosion potential and increase the corrosion tendency. The pitting initiation is oxide inclusion or sulfide oxide inclusion complex. S can induce the formation of occluded area and promote the corrosion propagation. The chemical compositions of weld metal is similar to base metal, which can limit the galvanic corrosion between weld metal and base metal, and avoid formation of corrosion step.

Influence of Chromium on Corrosion Behavior of Low-alloy Steel in Cargo Oil Tank O_2-CO_2-SO_2-H_2S Wet Gas Environment

As international maritime organization （IMO） draft 289 was adopted to develop a low-alloy anti-corrosion steel for the deck of cargo oil tank and to understand corrosion mechanism, corrosion behavior of a low-alloy steel with chromium contents was studied in O2-CO2-SO2-H2 S wet gas environment. Corrosion rate was measured, and the microstructure and morphology of corrosion product film were characterized by scanning electron microscopy （SEM）. The phase and chemical composition of the corrosion product film were investigated by X-ray diffraction （XRD） and energy dispersive spectroscopy （EDS）, respectively. The effect of misorientation distribution on corro- sion resistance of steel was evaluated by electron backscattered diffraction （EBSD）. The results showed that corro- sion rate decreased with increasing chromium content in the low-alloy steel, and the corrosion type was general corrosion. The phenomenon of chromium enrichment was found in corrosion product film consisting of a-FeOOH, γ- FeOOH, sulphur, FeS2 and Fel-xS. The increase of chromium content decreases the amount of high-angle grain boundaries, thus resulting in the improvement of corrosion resistance.

Influence of Inclusion on Corrosion Behavior of E36 Grade Low-alloy Steel in Cargo Oil Tank Bottom Plate Environment

Corrosion behavior of low-alloy steel was investigated in simulated cargo oil tank （COT） bottom plate service environment （10% NaCl solution, pH = 0.85）. The corrosion behavior of inclusion was studied by in-situ scanning electron microscopy （SEM） and energy dispersive spectroscopy （EDS）. It was found that pitting corrosion was inclined to occur around the place where inclusions exist. After initial corrosion, an area of 10-20μm in diameter was formed as a cireinate cathode around the edge of inclusion. MnS inclusion dissolved in the simulated COT corrosion solution before low-alloy steel matrix, and pitting was formed at the place where MnS dissolved. TiO2 inclusion dissolved in the simulated COT corrosion solution after low alloy steel matrix, and pitting was formed at the place where steel matrix dissolved. The corrosion tended to occur at the area where the curvature radius of inclusion is smaller. The size of round TiO2 inclusions had little influence on corrosion behavior in this certain environment.

Corrosion behavior of low alloy steel for cargo oil tank under upper deck conditions

A simulated corrosion test apparatus was used to investigate the corrosion behavior of a low alloy steel under simulated upper deck conditions in a cargo oil tank. The estimated corrosion loss of conventional E36 class ship plate steel is 4.27 mm, which is clearly inadequate compared with the standard adopted by International Maritime Organization. Outer rust layer of specimens starts to peel off after 77 days and becomes fragmented after 98 days. X-ray diffraction, scanning electron micros- copy, and Raman spectroscopy revealed that the resulting rust is composed of σ-FeOOH （the main protective phase）, Fe2O3, FeS, elemental S, and FeSO4. FeSO4 in the interface of the base and rust layer leads to localized corrosion. Elemental sulfur forms on the surface of σ-FeOOH, and the quantity and size thereof increase with increasing corrosion time. Furthermore, layered elemental sulfur promotes fracture and peels off the rust layer.