Difference Between NACE MR 0175 and NACE MR 0103

There are several major differences between MR0103 and MR0175/ISO 15156:

_ The refinery standard guidelines for determining whether an environment is "sour" are

quite different from the sour environment definitions provided in previous and current

versions of MR0175

.

_ The refinery standard does not include environmental restrictions on materials.

_ Materials and/or material conditions are included in MR0103 that are not listed in

MR0175/ISO 15156.

_ Materials and/or material conditions are included in previous and/or current versions of

MR0175/ISO 15156 that are not listed in MR0103.

.

_ Because welding is prevalent in refinery piping and equipment, MR0103 places extra

emphasis on welding controls in several material groups, most notably the carbon steels.

Applicability of MR0103. Both MR0175 and MR0103 include sections that describe applicability

of each of the standards. Both describe material factors and environmental factors, and provide

guidelines to the user on how the standard should be applied. Note that both standards require

the user to specify whether the environmental conditions are such that the material requirements

of the standard should be applied.

One of the key differences between the MR0175 and MR0103 standards lies in the the environmental conditions under which SSC is likely to occur. MR0103 covers a

broader range of sour environment conditions experienced in downstream process units. These

are based on:

_ User's plant experience and practices

_ Existing NACE and industry recommended practices and reports (i.e., NACE RP0296,3

8X194,4 8X294,5 API Publication 5816)

_ A fundamental understanding of atomic hydrogen generation in the sour service corrosion

reaction and the subsequent rate of hydrogen flux into the process-contacted steel, i.e.,

combined effects of pH, H2S and HCN.

A significant difference between upstream and downstream sour environments is that, in many

refinery sour water environments, dissolved ammonia is present. This increases the pH, thereby

increasing the solubility of H2S, which in turn increases the bisulfide ion concentration and

corrosivity. Ammonium bisulfide corrosion in these high-pH environments generates a relatively

high rate of hydrogen flux.

From: D. R. BUSH, Emerson Process Management / Fisher Controls Intl. LLC, Marshalltown, Iowa, J.

C. BROWN, Motiva Enterprises, Convent, Louisiana, and K. R. LEWIS, Shell Global Solutions

Intl., BV, Amsterdam, The Netherlands.