Details of the performance of chloride stress-corrosion cracking (SCC) resistant stainless steels (SS) in Japan operating plants have been collected and reported.
In cases where originally selected 300 series SS experienced chloride SCC, Japanese refineries and petrochemical plants tended to select duplex SS or 316 SS as countermeasures. Duplex SS and 316 SS materials were usually selected for more-severe conditions than the other 300 series SS.
Although there are few applications, high nickel alloy in acid environments with a high amount of chlorides experienced few SCC incidents. Type 444 SS had similar superior results.
These are the conclusions based on a survey conducted in 1995-1997 by The Society of Chemical Engineers, Japan (SCEJ), and The Japan Petroleum Institute (JPI).
These results were presented by Jun`ichi Sakai, general manager of Kokan Keisoku KK, and Keiichi Matsumoto, deputy manager of Toyo Engineering Corp., at the National Association of Corrosion Engineers Corrosion/99 conference in San Antonio at the end of April.
SCEJ and JPI received 107 responses from member companies of the two organizations.
The survey asked respondents to comment on:
- Damage which occurred by chloride SCC
- Countermeasures taken before and after the damage
- Environmental conditions (that is, chloride concentration, temperature, pH oxidizing agents, sulfides, process fluid) of the equipment in consideration
- Performance of adopted countermeasures.
In 72 of the 107 responses, the owner had not considered SCC-resistant materials during its initial application. In the remaining 35, the owner indicated that material had been chosen specifically to avoid SCC.
Most of the materials employed for the 72 cases were 300 series SS (61 out of 72). The remaining five cases were duplex SS.
SCC occurred within 2 years in 13 cases, and after 2 years in 51 cases. Thus, Sakai pointed out, most material selections for these 72 cases were reasonably chosen-with the severity of the service having been taken into account.
Some SCC occurred after more than 10 years of operation.
For the 35 cases in which the owner selected SCC-resistant material in advance, 15 were duplex SS, 13 were high purity ferritic SS (such as SUS444, which is 19Cr-2Mo), and 7 cases were other materials.
Of the 15 cases with duplex SS, 6 cases experienced cracking. One SUS444, one SUS405 (13Cr-Al), and one SUS316L also cracked.
No cracking was observed for the high alloy applications. For temperatures above 200° C., high nickel 600 alloys were generally employed, and for temperatures around 100° C., 800 alloys were used.
Duplex SS is often used as a countermeasure for 304 SS or 316 SS.
In general, duplex SS is a reliable, SCC-resistant material. The fact that it is usually selected for very severe services must be taken into consideration when evaluating it in cases in which cracking has occurred.
Like duplex SS, Type 444 SS showed superior results. Type 444 SS, however, is not good in situations with localized corrosion and is therefore found in more mild services than duplex SS.
Other 400 grade SSs were generally used in the overheads of main columns in crude distillation units. Most of them performed well. One owner of SUS405 reported cracking at a weld, and another reported general corrosion in SUS410.
The most popular countermeasure material, chosen after an SCC occurrence in 34 cases, was duplex SS. SUS444 and 316 SS were adopted in 19 cases. Regular-grade ferritic SS and high alloys (iron content less than 50%) were adopted in 10 cases.
Notably, said the authors, 316 SS was adopted in 19 cases as countermeasures although the occurrence of SCC was high in 316 SS.
Table 1 shows the countermeasure selection trend. The number of each cell shows the number of cases of material adoption. Numbers in parentheses indicate unsuccessful results.
In 44 cases, respondents reported SCC damage to exchangers, particularly to exchanger tubes. Towers and vessels accounted for 23 cases.
For exchangers, originally, mostly 300 series SS was selected. As a countermeasure for exchanger SCC damage, duplex SS was the most popular material.
For towers and vessels, 316 SS originally was the most popular material. As countermeasures, duplex SS and ferritic SS were selected.
Other equipment included piping, reactors, and pumps.
SCC incidents were widely scattered among various temperatures and chloride concentrations for both 304 and 316 SS. For long service periods, no significant differences between the two materials were observed from this survey.
Reports of cracking seemed more prevalent with 316 SS than 304 SS. The authors believe, however, that the 316 SS materials were generally placed in more-severe services than that in which the 304 SS was placed. Therefore, 316 SS is an upgrade for applications in which 304 SS has cracked after 3-4 years.
Figs. 1 and 2 illustrate respondents` countermeasures based on service temperatures and chloride concentrations.
Most materials were in a temperature service below 200° C., which represented many of the heat exchanger cases with operating temperatures of about 100° C.
Four cases were reported with temperatures above 200° C.: Two reported using duplex SS and two reported using high alloy metals.
The performance of the high alloy applications was reportedly good.
One duplex SS case, which services a weak alkaline with low chloride concentration, had not experienced SCC after 1 year of operation.
The other cracked at a weld within 1 year of operation. This SCC-damaged case was operating in a weak acid with high chloride-concentration environment.