Hydrogen Embrittlement
Is a process resulting in a decrease of the toughness or ductility
of a metal due to the presence of atomic hydrogen.
Hydrogen embrittlement has been recognized classically as being of two
types. The first known as internal hydrogen embrittlement, occurs when
the hydrogen enters molten metal which becomes supersaturated with hydrogen
immediately after solidification. The second type, environmental hydrogen
embrittlement, results from hydrogen being absorbed by solid metals.
Hydrogen embrittlement is a major cause of fastener
failure. Prevailing thought is that steels with Rockwell
hardness above C30 are vulnerable. The phenomenon
is well-known although the precise mechanism
has eluded extensive research. A number of proposed
mechanisms have been proposed, and most have
at least some merit. Current thinking is that the
susceptibility to hydrogen embrittlement is related directly
to the trap population. Generally, hydrogen
embrittlement can be described as absorption and adsorption of hydrogen
promoting enhanced decohesion of the steel, primarily as an
intergranular phenomenon.
Electroplating is a major cause of
hydrogen embrittlement. Some hydrogen is
generated during the cleaning and pickling
cycles, but by far the most significant source is
cathodic inefficiency, which is followed
by sealing the hydrogen in the parts. Baking must
be performed on high strength parts to reduce this risk, and
the ASTM, in 1994, issued a specification for baking
cycles, as shown below. For the plater, having to adhere to the post plating
baking cycles is mandatory to certify meeting the mil specs.
Deterioration which can be linked to corrosion and corrosion-control
processes, involves the ingress of hydrogen into a component, an event
that can seriously reduce the ductility and load-bearing capacity, cause
cracking and catastrophic brittle failures at stresses below the yield
stress of susceptible materials. Hydrogen embrittlement occurs in a number
of forms but the common features are an applied tensile stress and hydrogen
dissolved in the metal. Examples of hydrogen embrittlement are cracking
of weldments or hardened steels when exposed to conditions which inject
hydrogen into the component. Presently this phenomenon is not completely
understood and hydrogen embrittlement detection, in particular, seems to
be one of the most difficult aspects of the problem. Hydrogen embrittlement
does not affect all metallic materials equally. The most vulnerable are
high-strength steels, titanium alloys and aluminum alloys.
Sources of Hydrogen
Sources of hydrogen causing embrittlement have been encountered in
the making of steel, in processing parts, in welding, in storage or containment
of hydrogen gas, and related to hydrogen as a contaminant in the environment
that is often a by-product of general corrosion. It is the latter that
concerns the nuclear industry. Hydrogen may be produced by corrosion reactions
such as rusting, cathodic protection, and electroplating. Hydrogen may
also be added to reactor coolant to remove oxygen from reactor coolant
systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can
be facilitated in a number of ways summarized below: (Defence Standard
03-30, October 2000)
a.by some manufacturing operations such as welding, electroplating,
phosphating and pickling; if a material subject to such operations is susceptible
to hydrogen embrittlement then a final, baking heat treatment to expel
any hydrogen is employed
b.as a by-product of a corrosion reaction such as in circumstances
when the hydrogen production
reaction (Equation 2) acts as the cathodic reaction since some of the
hydrogen produced may enter
the metal in atomic form rather than be all evolved as a gas into the
surrounding environment. In
this situation, cracking failures can often be thought of as a type
of stress corrosion cracking. If the
presence of hydrogen sulfide causes entry of hydrogen into the component,
the cracking
phenomenon is often termed “sulphide stress cracking (SSC)”
c.the use of cathodic protection for corrosion protection if the process
is not properly controlled.
Hydrogen Embrittlement of Stainless Steel
Hydrogen diffuses along the grain boundaries and combines with the
carbon, which is alloyed with the iron, to form methane gas. The methane
gas is not mobile and collects in small voids along the grain boundaries
where it builds up enormous pressures that initiate cracks. Hydrogen embrittlement
is a primary reason that the reactor coolant is maintained at a neutral
or basic pH in plants without aluminum components.
If the metal is under a high tensile stress, brittle failure can occur.
At normal room temperatures, the
hydrogen atoms are absorbed into the metal lattice and diffused through
the grains, tending to gather at inclusions or other lattice defects. If
stress induces cracking under these conditions, the path is
transgranular. At high temperatures, the absorbed hydrogen tends to
gather in the grain boundaries and stress-induced cracking is then intergranular.
The cracking of martensitic and precipitation hardened steel alloys is
believed to be a form of hydrogen stress corrosion cracking that results
from the entry into the metal of a portion of the atomic hydrogen that
is produced in the following corrosion reaction.
Hydrogen embrittlement is not a permanent condition. If cracking does
not occur and the environmental conditions are changed so that no hydrogen
is generated on the surface of the metal, the hydrogen can rediffuse from
the steel, so that ductility is restored.
To address the problem of hydrogen embrittlement, emphasis is placed
on controlling the amount of
residual hydrogen in steel, controlling the amount of hydrogen pickup
in processing, developing alloys with improved resistance to hydrogen embrittlement,
developing low or no embrittlement plating or coating processes, and restricting
the amount of in-situ (in position) hydrogen introduced during the service
life of a part. As well as relieving the embrittlement via post-operation
baking at 357F for a minimum of 3-hours.
How much Baking Do Electroplated Parts need?
(ASTM B 850-94)
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