High Strength Bolting vs. Subsea Service Conditions Dr. Russell D. Kane – iCorrosion LLC Houston, Texas

Organization Introduction What are Subsea Service Conditions? Failure Scenarios High strength steels vs. hydrogen embrittlement Source: Internal Hydrogen from Processing Source: External Hydrogen from Cathodic Protection Variabilities in Conditions and Allowable Hardness Corrosion Resistant Alloys & Other Cracking Mechanisms Takeaway Messages

Introduction - 1 The concern in critical subsea bolting is the possibility and risk of failure. Minimizing this risk is essential for safe and efficient subsea operations. Bolting failures are categorized as: Ductile Overload / Fatigue Insufficient material Brittle Low toughness Environmental cracking Hydrogen Embrittlement Chloride or Sulfide Cracking

Introduction - 2 Environmental Cracking brings three important considerations: Environmental Conditions Stress Intensity/Mode Material Properties/Processing For hydrogen embrittlement this becomes: Can be multiple sources; variables in applications Can be affected by material properties, hardness and manufacturing methods Also dynamic loads & bolt torque

Why worry about hydrogen? Hydrogen is a very small atom. Diffuses easily into steel at ambient temperatures. Results in a form of Environmental Cracking: Hydrogen Embrittlement Fracture mode changes from ductile (high energy) to brittle (low energy) fracture. Can result in unexpected, sudden and catastrophic failure. Susceptibility to HE generally increases with strength/hardness & microstructure of steels & in other materials. Ductile air fracture Brittle HE fracture

Sources of Hydrogen - 1 Manufacturing: Many plating processes (e.g. Zn plating) involve cathodic current. Results in formation of atomic hydrogen on surface of the cathode (bolt). Acid cleaning also generates hydrogen. These are sources of “internal hydrogen” in the steel even before bolts go into service. Depending on strength/HRC baking is necessary. Issues w/internal hydrogen can also occur with Zn hot dip. H+ + e- = H0 From ASTM E850

Sources of Hydrogen - 2 Cathodic Protection: Two types of subsea CP: Impressed Current - drillship Sacrificial Anode – subsea structures & equipment. Also, applies to Zn plating/coating on steel PC is designed to provide current to a structure/equipment to impede the normal corrosion processes. While doing this, hydrogen atoms are liberated on the structure (cathode). CP w/ Impressed Current Commonly used on drill ships – current must be controlled CP w/Sacrificial Anodes Commonly used on subsea structure – current can vary with conditions H+ + e- = H0 H+ + e- = H0

CP Variables in Hydrogen Cracking - 1 Minimum cracking severity around -850 mV (Ag/AgCl): Potential of minimum hdrogen generation. Hydrogen is produced by CP at higher potentials If not controlled, impressed current CP systems can results in “over-protection” increasing hydrogen production and hydrogen cracking susceptibility. Hydrogen is produced by steel corrosion at lower potentials by active corrosion of steel in seawater. Hydrogen produced by active corrosion Hydrogen produced by CP Crack Opening Rate (from crack growth) Sandoz - 1972

CP Variables in Hydrogen Cracking - 2 Formation of calcareous films on steel reduces cathodic current In seawater, CP current demand usually decreases with time as calcareous films form. However, this is not always the case. Depth, temperature, pressure and flow can increase current demand on subsea equipment.

CP Variables in Hydrogen Cracking - 3 All subsea conditions are not the same. Other factors can increase CP current demand on subsea components: Low Dissolved Oxygen – reduces alkalinity on cathode Low Temperature – high solubility of calcareous deposits Seawater currents – lateral flow Salinity/resistivity Proximity to anodes or sacrificial coatings All of these factors increase CP current demand. Increased CP current demand increases potential for hydrogen embrittlement.

CP Variables in Hydrogen Cracking - 4 NACE SP0176-2007

CP Variables in Hydrogen Cracking - 4 Feasty - 2001 Deep Water brings Low T & high P Water currents Solubility of normally protective calcareous deposits Use of sacrificial (Zn and TSA) coatings. These are important considerationsfor both CP performance and hydrogen charging severity. Can to increased current and can lead to increased risk for hydrogen cracking. Hartt et.al. - 1986 Beavers et.al. - 1986

Consequences of Hydrogen in Steel Bolting Townsend - 1975: Questions: Where do we draw the line on hardness/strength for bolts? Zn coated steel should be similar to CP – most of the time. Does HRC 32-34 limit make sense for all types of subsea service based on: Material type & processing Criticality of service Type of service conditions

What about Corrosion Resistant Alloys? CRAs can be susceptible to hydrogen embrittlement under CP. A function of composition, microstructure and strength/hardness. But, also brings questions about other cracking mechanisms in the absence of CP… Stress Corrosion Cracking at elevated temperatures Cracking Susceptibility for Alloys under Cathodic Polarization Under CP - 8 days Alloy Alloy Type AYS (ksi) E ratio v air RA ratio v air TTF ratio v air X750 Ni-base 92 0.36 0.23 0.55 718 95 0.59 0.34 0.63 25Cr Duplex SS 110 0.27 0.29 0.58 410 Mart SS 79 0.64 0.41 0.72 F6NM 86 0.8 0.78 4140 Steel 145 0.67 0.31 0.84 4140 temp 0.85 0.68 0.9 4340 Q&T 116 0.83 A-286 PPT SS 131 1 0.82 Cathodic Polarization - 75 F with Aluminum Anode Wolfe et.al. - 1993

What About Other Environmental Cracking Mechanisms? caustic cracking stress corrosion cracking hydrogen blistering hydrogen embrittlement hydride embrittlement hydrogen-induced cracking (stepwise cracking) hydrogen stress cracking sulfide stress cracking liquid metal cracking Not likely in subsea environment Common in CRAs at high temp, but suppressed by cathodic protection Not common in high strength steels very common in high strength steels Typically in certain Titanium alloys Not common in high strength steels Common in high strength steels w/H2S Can occur in high strength steels w/LMP metals

Materials Hardness Limits Vary Spec hardness limits for internal/external hydrogen cracking in HS steel components vary. One reason is multiple service application requirements Path forward is to utilize API spec 20E (HRC 34) for steel rather than product specs not specifically related to subsea service. API spec 20F and 6A CRA for alloys. Specification Matl HRC Limit Comments NACE MR0175/ISO 15156 Steel HRC 22 Generic steel exposed to H2S " HRC 26 Controlled composition/heat treatment v H2S NORSOK HRC 32-39 Subsea applications (w CP) API 17A HRC 31 Resistance to hydrogen emrbittlement ASTM F1941 HRC <39 Coated fasteners no baking ASTM 2329 HRC 40+ Fasteners shall not be Zn coated   HRC >33 Risk of internal H.E. ASTM B633 HRC >31 Coated parts require baking ASTM E850 Baking requirements for HS Steel API Spec 20E HRC 34 HRC variance 3 HRC; no Zn plating; all plated parts baked API Spec 6A CRA CRAs HRC 34-43 Depends on alloy type/composition

Takeaway Messages High Hydrogen Charging High Strength/ Hardness High Mechanical Loads Subsea service conditions vary substantially from location to location. The industry has evolved from shallow to deep water drilling. Conditions of service, material requirements and risk levels have changed. These factors affect environmental, material and mechanical conditions increasing risk of hydrogen embrittlement. Equipment and use standards specific to subsea service are making progress but still evolving. Need is for better data on the affect of relevant conditions of CP to define material limits and acceptable processing conditions. No Cracking Goal: