ABSTRACT

Many alloys exposed in high-temperature process equipment corrode by sulfidation corrosion in the presence of steam or other oxidizing gases. This paper discusses the latest results of an extensive testing program for a diverse group of about 50 commercial alloys exposed to temperatures of 573 ? 1273 K with exposure times up to 6,000 hours for a total data compilation of nearly 4 million hr. The data compilation now allows engineering corrosion assessments for sulfidation and sulfidation in the presence of oxidizing gases and predictions for wide ranges of conditions. The effects of gas composition, temperature, exposure time and alloy type have all been analyzed and compiled to allow prediction of corrosion for wide ranges of conditions to allow engineering predictions of corrosionlimited lifetimes. Applications for this technology are found in industries such as oil refining, petrochemicals production, pulp/paper production, and power generation.

BACKGROUND

This paper discusses the potential of high-temperature gases to corrode metals by sulfidation in the presence of oxidizing gases and recommends how to predict sound metal losses for a wide range of conditions. Corrosion of metals and alloys used in equipment handling high-temperature, corrosive, sulfidizing gases is a potential concern in processes used in petroleum refining, gas processing, fired equipment, process heaters, thermocouples, instrumentation, hydrocracking, coking, vacuum flashing, hydrotreating, coal/coke/oil gasifying, petrochemical production, catalytic reforming and gasification of black liquor in pulp/paper production. Corrosion can often define the maximum allowable temperature or maximum allowable gas species concentrations for metals and alloys in equipment, although mechanical properties or other considerations may also define the maximum allowable temperature.

Determining the extent of sound metal loss by corrosion should allow assessment of the remaining useful load-bearing thickness of components as equipment corrodes. It is therefore important to review how the characteristics of the exposure conditions and the alloys can combine to influence the rate of corrosion. Most corrosion data for alloys exposed to high-temperature gases are reported in terms of weight change/area for relatively short exposures and inadequately defined exposure conditions. Unfortunately, the weight change/area information is not directly related to the thickness (penetration) of corroded metal, as illustrated in Figure 1 and is often needed in assessing the strength of process equipment components. Corrosion is best reported in penetration units, which indicate the sound metal loss, as discussed earlier1-2. Corrosion in high-temperature gases is affected by key parameters of the corrosive environments such as temperature, alloy composition, time, and gas composition. Summaries of metal penetrations for some typical conditions are shown in this presentation, which extends beyond the traditional corrosion weight change data by reporting total metal penetration for an extensive number of alloys over a wide range of conditions. Compositions of some of the alloys discussed in this paper are shown in Table 1.

SULFIDATION CORROSION MECHANISM

To better understand sulfidation in the presence of oxidizing gases, the corrosion mechanism of sulfidation in the absence of oxidizing gases will first be reviewed. Sulfidation is corrosion, which forms sulfide corrosion products, leads to metal loss (penetration), and occurs upon exposure of metals to gases containing H2S. The first step in determining the potential for equipment to sulfidize is to determine that sulfidation is the dominant corrosion mechanism. The key indicator

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