Chapter 11: HYDROGEN EFFECTS
page 11 - 1
Corrosion for Engineers
Dr. Derek H. Lister
HYDROGEN EFFECTS
Hydrogen can degrade metals by:
- hydrogen blistering;
- hydrogen embrittlement;
- decarburization;
- hydrogen attack.
Blistering
Hydrogen enters the Lattice of a metal diffuses to voids, creates high internal
stresses ~ blisters . ..
Blistering may occur during
exposure to:
- hydrocarbon;
- electroplating solutions;
- chemical process streams;
pickling solutions;
- H-containing contaminants
during welding;
- general corrosive environments.
Cross section of a carbon steel plate removed
from a petroleum process stream showing a
large hydrogen blister. Exposure time: 2 years.
Universify of New Brunswick, Canada
Chulalongkom Universify, Thailand
Chapter 11: HYDROGEN EFFECTS
page 11 - 2
Corrosion for Engineers
Dr. Derek H. Lister
Em brittlement
Similar to blistering . .. hydrogen enters metal lattice . ..
BUT . .. interaction with metal lattice different.
High-strength (and more brittle) steels are susceptible.
H-embrittlement different from see in nature of cracks . ..
•
stress-corrosion cracks usually propagate anodically;
hydrogen-embrittled cracks occur under cathodic conditions . ..
~
Oirection of Odvoncin; croekinQ into metal
t
v_
_v
Reqion of
hydrogen embfittlement
ReQ ion of anodic
stre,s corrosion crocking
."
+
" ,
Anodic current
M-M+++2s
+J
I~
o
Cathodic current
2s+2H+-2H
AnOdic stress
corrosion C(OCkinQ
Hydrogen
embrilliement
Schematic differentiation of anodic cracking and cathodically sensitive embrittlement.
University of New Brunswick, Canada
Chulalongkorn University, Thailand
Chapter 11: HYDROGEN EFFECTS
page11-3
Corrosion for Engineers
Dr. Derek H. Lister
Hydride-forming metal are susceptible to H- embrittlement ...
e.g., Zr-alloy pressur tubes (in CANOUs) and fuel sheathing (in all watercooled reactors) pick up hydrogen (or deuterium in heavy water) by
general corrosion. The hydrogen (0) migrates through the metal lattice
to cool regions and to regions of high tensile stress - can precipitate as
separate phase - zirconium hydride.
These hydrides are themselves brittle, and crack, and the crack can propagate
through the material, with more hydride progressively precipitating at the crack tip.
-
N.B. Enough
hydride can
precipitate to
form a "hydride"
blister ...
c.f. "hydrog,gn"
blister.
University of New Brunswick, Canada
Representation of tbe Growtb of a Crac~ !b~ougb
Failure of Zirconium Hydride tbat PreCIpitates
at Crack Tips
Chulalongkorn University, Thailand
Chapter 11: HYDROGEN EFFECTS
page 11-4
Corrosion for Engineers
Dr. Derek H. Lister
N.B. The mechanism of hydrogen uptake by metals must involve
Electrolyte
H+
H+
+
t
H .... H2.-H
_e
e
./
H
H
+
+
H
+
H
H
~
~
d~H26b
H
+
Schematic illustration showing
the mechanism of hydrogen
blistering.
Void
H-H 2-H
Air
BUT ... remember that molecular hydrogen may absorb and dissociate on
metal surfaces.
University of New Brunswick, Canada
Chulalongkom University, Thailand
Corrosion for Engineers
Dr. Derek H. Lister
Chapter 11: HYDROGEN EFFECTS
page 11 - 5
Decarburization and Hydrogen Attakk
High temperature process - C or carbide in steels can react with gaseous
hydrogen ...
C + 2H 2 -+ CH 4
Note that the reaction con occur with atomic H in the metal lattice ...
C + 4H -+ CH 4
May crack the steel from high internal pressure.
May cause loss of strength as C disappears.
University of New Brunswick. Canada
Chulalongkorn University, Thailand
Corrosion for Engineers
Dr. Derek H. Lister
Chapter 11: HYDROGEN EFFECTS
page 11- 6
PREVENTION
Blistering
•
Use steels with few or no voids;
•
Use coatings;
•
Use inhibitors;
•
Remove impurities that can promote hydrogen evoution ...
S-2 (particularly bad), As, CN", ect.
•
Use different materials (Ni-base alloys have low diffusion rates for
hydrogen).
EmbrittJement
•
Reduce corrosion rate (inhibitors, coatings, ect.);
•
Change electroplating process to minimize H effects (voltage, current
dens.ity, bath composition, ect.);
•
Bake material to remove H;
•
Minimize residual stresses;.
•
Use less susceptible material;
•
Maintain clean conditions during welding.
University of New Brunswick, Canada
Chufafongkorn University, Thailand