13Cr Tubular Service Limits and Guidelines for Sweet and Sour Environments
Authors: Chen Wen Ching 
Published: October 2008

This paper discusses the use of 13Cr tubulars in sweet and sour service. Field service factors that may affect 13Cr performance, such as acidizing, galvanic/crevice corrosion, galling, thread connection, and descaling etc, are discussed, and the precautions to resolve these concerns in field operations are provided.

INTRODUCTION

Tubulars made with 13% Chromium (13Cr) are the least expensive corrosion-resistant alloy (CRA) tubulars that can be used to combat C02 corrision and H2S environmental cracking without inhibitor treatment. These stainless steel tubulars cost only half as much as duplex stainless steel and one-fourth to one-tenth as much as high-nickel CRA materials. Using 13Cr tubulars might also be less expensive and more cost-effective than using carbon/low-alloy steel with inhibition. The 13Cr tubulars are especially attractive for tubing applications in offshore production operations where working space is limited and workover costs are high. Tubulars made with 13% Cr have been widely used in North America, the North Sea, Gulf of Mexico and North Slope of Alaska.

13Cr is susceptible to corrosion at high temperature, to sulfidestress cracking (SSC) in moderate to high H2S environments and to crevice corrosion. Therefore, application of 13Cr stainless steel tubulars does require precautions in tubular design, procurement, handling, storage, and field operations.

Many 13Cr OCTG manufacturers have suggested service limits of temperature and H2S partial pressure for using 13Cr in sour environments [1-3]; however, there is no common agreement on these limits. Manufacturers' data typically have placed the operating limits at 0.15 to 10 psi (1.0 to 69 kPa) H2S and 200 to 400°F (93 to 204°C). The test results of recent investigations [4,5] indicate that many existing limits of H2S partial pressure can be relaxed for Grade L80.

This paper provides the basic background for 13Cr metallurgy, mechanical characteristics and corrosion/cracking performance. The objective of discussing guidelines for use of 13Cr tubulars is to ensure operational safety, to prevent environmental casualties, and to ensure cost-effective use of 13Cr tubulars for sweet and sour service.

METALLURGY

The 13Cr stainless steel and its equivalents are martensitic stainless steels that contain 11.5-14.0 percent chromium. Table I lists the chemical compositions of Type 410, Type 420 and API 5CT L80 13Cr. Type 410 and Type 420 differ primarily in carbon content; 0.15 percent maximum for Type 410 versus 0.15 percent minimum for Type 420. Type 410 has been widely used for wellhead and Christmas trees in the oil and gas industry because it has been listed in NACE MR0175 [6] for sour service.

It is easier to achieve a uniform martensitic and delta-ferritefreemicrostructure with Type 420. Delta-ferrite is detrimental to impact and environmental cracking resistance. Type 420 is superior to Type 410 in sulfide-stress-cracking (SSC) resistance [7]. Weldability is not a concern for oil country tubular goods (OCTG). Therefore, Type 420 has become the basis for making 13Cr OCTG. API 5CT [8] has designated a standard grade, L80 13Cr, by reducing phosphorus and sulfur content and adding traces of nickel and copper.