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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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.

10. 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.

11. CP Variables in Hydrogen Cracking - 4
NACE SP

12. CP Variables in Hydrogen Cracking - 4
Feasty
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
Beavers et.al

13. Consequences of Hydrogen in Steel Bolting
Townsend :
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 limit make sense for all types of subsea service based on:
Material type & processing
Criticality of service
Type of service conditions

14. 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

15. 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

16. 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

17. 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: