Development of Test Methodology to Evaluate High Strength Nickel Based Alloys Under Cathodic Protection

Abstract

The traditional material choice for subsea bolting applications with cathodic protection (CP) has been high strength low alloy steels. These steels provide acceptable performance up to yield strengths of 140 ksi (965 MPa) and a maximum hardness of 35 HRC. Development of deepwater fields has required larger size and higher strength fasteners than those that have been typically used for subsea applications. This has necessitated the use of alternative materials with higher yield strengths to prevent costly failures due to stress corrosion cracking (SCC) and hydrogen embrittlement (HE). Nickel-based alloys, such as UNS N07718, have been one alternative material choice for these applications. In-service failures of fasteners made from UNS N07718 subjected to cathodic protection have been reported. Prior to use in service, these alloys were tested using constant load and double-cantilever beam (DCB) specimens. The in-service failures indicate that the factors necessary for failure were not simulated in the constant load and DCB tests. Dynamic strain tests, such as fracture toughness tests, have produced cracking in hydrogen charged conditions. This paper discusses the Phase 1 results of an on-going joint industry project to evaluate the susceptibility of nickel based alloys to hydrogen assisted cracking under conditions of seawater with CP. Testing in Phase 1 focused on comparing dynamic strain test methods including, slow-strain rate testing, cyclic slow-strain rate testing, fracture toughness, and step-loaded fracture testing. The results produced by these test methods are compared to determine an accelerated test technique for evaluating susceptibility to hydrogen assisted cracking in seawater with CP conditions.

Introduction

High strength low alloy steels have had acceptable performance in cathodically protected systems. Their applications have been limited to maximum yield strength (YS) of 140 ksi (965 MPa) and a maximum hardness of 35 HRC (ISO⁽¹⁾ 13628-1, DNV ⁽²⁾ RP B401, ISO 21457), ^{2, 3}, although ISO 4042⁴ recommnds 32 HRC if low alloy fasteners are to be electroplated. Development of deepwater fields has required larger size and higher strength fasteners than those that have typically been used for subsea applications. Costly interventions have necessitated the need for increased reliability for subsea systems including fasteners. Steels with strength levels below 120 ksi (827 MPa) are resistant to stress corrosion cracking (SCC) and hydrogen embrittlement (HE) in sea water plus cathodic protection (CP) environments. However, above yield strengths of 120 ksi (827 MPa), KISCC decreases, with a typical threshold stress intensity value of 50 – 80 ksivin (56 – 88 MPavm)for 145 ksi (1000 MPa) yield strength material⁵, while at lower strengths K_1SCC can be as high as 100 ksivin (110 MPavm).