Page 184 - CW E-Magazine (1-4-2025)

Special Report

table 2: Preventive measures against HB
Measure description
Use of coatings Rubber, plastic coatings and brick linings; Nickel and steel cladding with austenitic steels.
Use of inhibitors Inhibitors reduce the rate of corrosion and rate of hydrogen ion reduction. Generally, used in closed
systems.
Use of clean steels Use material with minimum voids.
Removal of poisons Phosphorus compounds, sulphide ions and arsenic compounds hamper hydrogen ion reduction,
which in turn raises the concentration of atomic hydrogen. These poisons should be removed.
Corrosion, electroplating and cathodic protection are a few sources of hydrogen in metal.
Substitution of alloys Use nickel-containing steels or nickel alloys, as they have very low hydrogen diffusion rates.

Hydrogen Embrittlement (HE)
HE is the result of the absorption
of hydrogen by susceptible metals,
resulting in the loss of ductility and
reduction of load-bearing capability.
Stress above the yield stress of the
embrittled material can result in
cracking and catastrophic brittle
failure (Fig. 6).

There are three factors required for
failure due to HE: a susceptible mate-
rial; exposure to an environment that
contains hydrogen; and the presence
of tensile stress due to residual and/or Fig. 6: Failure due to hydrogen embrittlement. Fractured bolt (left) and scanning electron
microscope image (right) of fractured surface.
applied stress.
clude phosphating, acid pickling, elec- As for the stress to cause fracture,
High-strength carbon steel and low tro plating andarc welding. During even residual stress within a component
alloy steels are most vulnerable to HE. these processes, there is a possibility can be sufficient.
Steels with an ultimat etensile strength of absorption of hydrogen by the mate-
of less than 1000 MPa or hardness of rial. During use, hydrogen can be intro- Steps that can be taken to avoid
less than 30 HRC are not generally duced into metal as a result of corro- HE include reducing hydrogen expo-
considered susceptible to HE. sion, chemical reactions of metal with sure, as well as baking after plating
acids, or with other chemicals, notably and the other processing that lead to
Processes that can lead to HE in- H S. hydrogen absorption. Baking allows the
2
table 3: Preventing Scc and HE
Measure description
Reducing the rate Metal corrosion leads to evolution of hydrogen. Thus, reducing the corrosion rate decreases the rate
of corrosion of hydrogen evolution.
Baking A common way of removing hydrogen from steels is by baking at relatively low temperatures.
Altering plating If electroplating is performed under conditions of hydrogen evolution, hydrogen embrittlement and
conditions poor deposits are the result.
Proper welding Dry conditions should be maintained during welding since water and water vapours are a source of
hydrogen. Welding rods with low hydrogen content should be used if materials are susceptible to HE.
Substituting alloys Very high-strength steels are more prone to HE. Alloying with molybdenum and nickel reduces
susceptibility.

184 Chemical Weekly April 1, 2025

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