ISO 24251-1:2025

International
Standard
ISO 24251-1
First edition
Prevention of hydrogen assisted
2025-08
brittle fracture of high-strength
steel components —
Part 1:
Fundamentals and measures
Prévention de la fragilisation par l'hydrogène des pièces en acier
à haute résistance —
Partie 1: Principes fondamentaux et mesures
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 4
5 Fundamentals . 5
5.1 General description of hydrogen embrittlement .5
5.2 Conditions for hydrogen embrittlement failure .5
5.2.1 General .5
5.2.2 Material susceptibility .6
5.2.3 Tensile stress .7
5.2.4 Sources of atomic hydrogen .7
5.3 Mechanism of hydrogen embrittlement of high strength steel .8
5.4 Fundamentals of metallic coatings regarding hydrogen uptake and diffusion .9
5.4.1 General aspects of metallic electroplated protection layers .9
5.4.2 Hydrogen generation during coating process .9
5.4.3 Corrosion protection mechanisms by metallic layers .9
6 Preventive measures with regard to hydrogen embrittlement .12
6.1 General . 12
6.2 Part design and manufacturing. 13
6.3 Material related measures and heat treatment . 13
6.4 Reduction of residual tensile stress (stress relief) .14
6.5 Measures related to coating processes .14
6.5.1 General .14
6.5.2 Pre-treatment — Cleaning processes . 15
6.5.3 Electroplating coating processes. 15
6.5.4 Stripping of coatings . 15
6.5.5 Corrective actions – Baking .16
6.6 Prevention of environmental hydrogen uptake .17
6.6.1 General .17
6.6.2 Environmentally appropriate design .17
Annex A (informative) Electroplating processes .18
Annex B (informative) Principles of electrochemical corrosion.20
Bibliography .23

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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The procedures used to develop this document and those intended for its further maintenance are described
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A list of all parts in the ISO 24251 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
High strength steel parts or components are broadly characterized by tensile strengths (Rm) above
1 000 MPa. They are often used in critical applications, such as in bridges, engines and aircraft, where a
failure can have catastrophic consequences. Preventing failures and managing the risk of hydrogen
embrittlement (HE) is a fundamental consideration that implicates the entire supply chain, including steel
mills, part manufacturers, coaters, application engineers, designers and end users.
HE has been studied for decades, yet the complex nature of HE phenomena and the many variables make it
hard to predict HE failures. Research is typically conducted under conditions that are either simplified or
idealized or both, and the findings cannot be effectively prescribed in industry standards and practices.
Circumstances are further complicated by specifications or standards that are sometimes either inadequate
or unnecessarily alarmist or both. Inconsistencies and even contradictions in industry standards have led to
much confusion and many preventable HE failures. The fact that HE is very often mistakenly determined to
be the root cause of failure as opposed to a mechanism of failure reflects the confusion.

v
International Standard ISO 24251-1:2025(en)
Prevention of hydrogen assisted brittle fracture of high-
strength steel components —
Part 1:
Fundamentals and measures
1 Scope
This document provides guidance on the prevention of hydrogen assisted brittle fracture, known as
hydrogen embrittlement (HE), that results from the manufacturing process or operating conditions.
This document is applicable to components or parts made of high strength steels.
This document provides guidance on the relationship between material selection, manufacturing (including
heat treatment) and coating.
NOTE 1 For hot-dip galvanizing components, see ISO 14713-2.
NOTE 2 This document does not consider applications under pressurised hydrogen.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 2080, Metallic and other inorganic coatings — Surface treatment, metallic and other inorganic coatings —
Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 2080 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
hardness
resistance of a metal to plastic deformation, usually by indentation or penetration by a solid object (at the
surface or in the core)
3.2
work hardening
increase of mechanical strength and hardness (3.1) when a metal is plastically deformed at ambient
temperature (by rolling, drawing, stretching, heading, extrusion, etc.) also resulting in a decrease of ductility

3.3
heat treatment
process cycle (controlled heating, soaking and cooling) of a solid metal or alloy product, to either obtain
a controlled and homogeneous transformation of the material structure or to achieve desired physical or
mechanical properties, or both
Note 1 to entry: Quenching and tempering, annealing, case hardening and stress relief are examples of heat treatment
for components or parts.
3.4
quenching and tempering
QT
heat treatment (3.3) process of quench hardening comprising austenitizing and fast cooling, under conditions
such that the austenite transforms more or less completely into martensite (and possibly into bainite),
followed by a reheat to a specific temperature for a controlled period, then cooling, in order to achieve the
required level of physical or mechanical properties
[SOURCE: ISO/TR 20491:2019, 3.4]
3.5
case hardening
thermochemical treatment process consisting of carburizing or carbonitriding followed by quenching which
induces an increase of hardness (3.1) in the surface of the steel
Note 1 to entry: This process is often used for gears, shafts, thread forming screws, self-drilling screws, etc.
3.6
stress relief
heat treatment (3.3) process by which parts or components are heated to a predetermined and controlled
temperature followed by a slow cooling, for the purpose of reducing residual stresses induced by
workhardening (3.2)
3.7
baking
heat treatment (3.3) for the purpose of hydrogen embrittlement relief after surface treatment
3.8
baking duration
period during which the coated material is held at specific baking temperature
3.9
crack
line of fracture (3.11) without complete separation
[SOURCE: ASTM F2078-15]
3.10
failure
loss of the ability of a part to perform a specified function, which in some cases can lead to complete
fracture (3.11)
3.11
fracture
break occurring when the plastic deformation in a part increases locally above its resistance limit, resulting
in the separation of the part into two or more pieces, during testing or in service
3.12
fracture morphology
structure and aspect of the fractured surface

3.13
ductile fracture
fracture with plastic deformation, where the resulting fracture surface is showing a fibrous ductile dimple
morphology that is typically dull or matte
3.14
brittle fracture
fracture with no plastic deformation, where the resulting fracture surface is showing a brittle morphology
that is typically shiny
Note 1 to entry: A brittle fracture along cleavage planes is known as a "trans granular fracture".
Note 2 to entry: A brittle fracture by separation at prior austenite grain boundaries is known as an "intergranular
fracture".
3.15
hydrogen embrittlement
HE
permanent loss of ductility in a metal or alloy caused by atomic hydrogen in combination with either load
induced or residual tensile stress or both that can lead to brittlefracture (3.14) after a certain time
Note 1 to entry: In the context of describing hydrogen embrittlement of high strength steel components, the term
“hydrogen” refers to atomic hydrogen and not molecular H -gas.
[SOURCE: ISO 1891-2:2014, 3.4.9, modified — Note 1 to entry has been replaced.]
3.16
internal hydrogen embrittlement
IHE
embrittlement caused by residual hydrogen from manufacturing processes, resulting in either delayed
brittle failure (3.10) of components or parts under load induced or residual tensile stress, or both
[SOURCE: ISO 1891-2:2014, 3.4.10, modified — The EXAMPLE has been deleted.]
3.17
environmental hydrogen embrittlement
EHE
embrittlement caused by hydrogen absorbed as atomic hydrogen from a service environment, resulting in
delayed brittle failure (3.10) of parts under tensile stress (i.e. either load induced or residual tensile stress,
or both)
[SOURCE: ISO 1891-2:2014, 3.4.13, modified — Note 1 to entry has been deleted.]
3.18
hydrogen embrittlement threshold stress
critical stress below which hydrogen embrittlement (3.15) does not occur, which represents the degree of
susceptibility of a steel for a given quantity of available hydrogen
3.19
stress corrosion cracking
SCC
category of environmental hydrogen embrittlement (3.17) where failure (3.10) occurs during service by
cracking under the combined action of corrosion generated hydrogen and load induced tensile stress
[SOURCE: ISO 1891-2:2014, 3.4.14, modified — The term "hydrogen induced" has been removed from the
preferred term. The admitted term "SCC" has been added.]

3.20
hydrogen diffusion
propagation of hydrogen and interaction with metallurgical features within the steel microstructure which
constitute areas of traps into the components or parts material
Note 1 to entry: Interactions with metallurgical features can include microcracks, dislocations, precipitates, inclusions
and grain boundaries.
Note 2 to entry: Traps can be non-reversible (characterized by high bonding energies and low probability of hydrogen
being released) and reversible (characterized by low bonding energies and hydrogen being released more readily).
3.21
hydrogen effusion
outward migration of hydrogen from the part material, occurring naturally at ambient temperature due to
concentration gradient or as the result of a thermal driving force and is accelerated by increased temperature
[e.g. baking (3.7)]
3.22
anodic protection
anodic corrosion protection
protection of selected metals, e.g. stainless steel by externally applied current
Note 1 to entry: The metal to be protected is polarized as anode thus causing passivity to the metal protecting it from
corrosion.
Note 2 to entry: Anodic protection requires external current supply as contact between different metals (galvanic
corrosion) cannot supply enough voltage.
Note 3 to entry: Compare also cathodic protection (3.23).
3.23
cathodic protection
cathodic corrosion protection
ability of a coating to act as a sacrificial layer thus protecting the basis metal of the work from corrosion in
case of coating damage
Note 1 to entry: In some countries, cathodic protection is sometimes misleadingly described as anodic protection
(3.22) as the basis metal is protected by an anodic coating, e.g. zinc or zinc alloy.
Note 2 to entry: Compare also anodic protection (3.22).
[SOURCE: ISO 1891-2:2014, 3.6.14, modified — Note 1 to entry and Note 2 to entry have been added.]
3.24
barrier protection
coating of a substrate by a nobler metal than the substrate or other material (e.g. lacquer) thus protecting
the basis metal of the work from corrosion by a closed barrier layer
Note 1 to entry: The protection is lost in case of coating damage.
Note 2 to entry: Compare also anodic protection (3.22) and cathodic protection (3.23).
EXAMPLE Nickel and chromium on steel substrate, copper and nickel and chromium on steel or zinc substrate.
4 Abbreviated terms
Table 1 explains the abbreviations used throughout this document.

Table 1 — Abbreviated terms
Abbreviated term Explanation
EHE environmental hydrogen embrittlement
IHE internal hydrogen embrittlement
HAC hydrogen assisted cracking
HE hydrogen embrittlement
HELP hydrogen enhanced local plasticity
HIC hydrogen induced cracking
SCC stress corrosion cracking
HaSCC hydrogen assisted stress corrosion cracking
HiSCC hydrogen induced stress corrosion cracking
HEDE hydrogen enhanced decohesion embrittlement
5 Fundamentals
5.1 General description of hydrogen embrittlement
Generally, HE is classified under two broad categories based on the source of hydrogen: IHE and EHE. IHE
is caused by either residual hydrogen from steelmaking or from processing steps such as pickling and
electroplating or both. EHE is caused by hydrogen introduced into the metal from external sources while it
is under stress, such as an in-service part.
SCC is used in relation to EHE that occurs when hydrogen is produced as a by-product of surface corrosion
and is absorbed by the steel part. Cathodic hydrogen absorption is a subset of SCC. Cathodic hydrogen
absorption occurs in the presence of metallic coatings such as zinc or zinc-nickel that are designed to
sacrificially corrode to protect a steel component or part from rusting. If the underlying steel becomes
exposed, a reduction process on the exposed steel surface simultaneously results in the formation of
hydrogen in quantities that are significantly greater than in the case of uncoated steel.
5.2 Conditions for hydrogen embrittlement failure
5.2.1 General
HE failure occurs when the following three elemental conditions are present at the same time (see Figure 1):
— material condition that is susceptible to hydrogen damage;
— tensile stress (typically from an externally applied load or residual stress);
— atomic hydrogen.
If all three of these elements are present in sufficient and overlapping quantities, and given time, hydrogen
damage results in crack initiation and growth until the occurrence of fracture. Time to failure can vary,
depending on the severity of the conditions and the source of hydrogen. Stress and hydrogen are considered
triggers, whereas material susceptibility is the fundamental requirement for HE to occur and is therefore
associated with the root cause.

Figure 1 — Confluence of three necessary conditions for delayed hydrogen embrittlement (HE)
failure to occur
5.2.2 Material susceptibility
Susceptibility of a material to hydrogen damage (i.e. material susceptibility) is a function of the material
condition, which is comprehensively described by the metallurgical structure and mechanical properties
of a material such as steel. Examining material susceptibility is the fundamental basis for understanding
hydrogen embrittlement phenomena.
Material strength (i.e. either tensile strength or hardness or both) has a first order effect on HE susceptibility
of steel. As strength increases, steel becomes harder, less ductile, less tough and more susceptible to
hydrogen damage.
Second order effects of the microstructure (such as chemical composition, steel cleanliness, way of heat
treatment and grain size) can affect the material susceptibility. As only diffusible hydrogen can cause HE
failure, the trapping behaviour of high strength steels shall be observed.
Hydrogen “traps” refer to metallurgical features within the steel microstructure (such as voids, grain
boundaries, dislocations, precipitates and inclusions) to which hydrogen atoms can become bonded.
Hydrogen thus “trapped” is no longer free to diffuse (i.e. move) to areas of high stress where it can participate
in the mechanism of HAC. Traps are typically classified as reversible or non-reversible based on their
bonding energies. Reversible traps are characterized by low bonding energies: in other words, hydrogen
is more easily released from the trap. Non-reversible traps are characterized by high bonding energies: in
other words, hydrogen requires a great deal of energy (e.g. from heat or stress field) to be released from the
trap. Non-trapped hydrogen which is free to move in the metal lattice is called mobile hydrogen; it is also
known as interstitial or diffusible hydrogen. Reversible and irreversible hydrogen traps increase hydrogen
solubility and simultaneously decrease the hydrogen diffusion coefficient.
Reversible traps with low binding energy can provide a reservoir of hydrogen that can become diffusible
under certain conditions (e. g. temperature or mechanical stress) and diffuse to areas of lower hydrogen
chemical potential, such as notches or a crack tip with triaxial tensile stress during the application of a
mechanical load. Furthermore, the lattice structure of the material determines the diffusion velocity and
hydrogen solubility. Since the progress of HE is associated with the diffusion speed and the hydrogen
solubility in the material, the spatial arrangement of the metal lattice represents an additional influence
factor on diffusion. In face-centred cubic steels, the diffusion velocity is much lower, but hydrogen solubility
is higher in comparison to body-centred cubic steels.

Steels with the same tensile strength can have a totally different HE susceptibility. Given that hydrogen
embrittlement causes loss of ductility and, consequently, loss of strength, the foundation for studying and
quantifying the susceptibility of a material to hydrogen damage begins with mechanical testing aimed at
measuring the behaviour of the material under increasing stress, first without, and then with the addition of
absorbed hydrogen.
5.2.3 Tensile stress
Load induced stress is a normal service condition for many high strength steel parts. Sometimes, tensile
loaded parts are primarily subject to tensile stress and additionally subject to either torsional stress, shear
or bending loads, or all three. Given time, tensile stress in the components or parts can result in HE failure
provided it exceeds the HE threshold stress of the material. Hydrogen embrittlement threshold stress is
defined as the critical stress below which HE does not occur. HE threshold stress is a measure of the degree
of susceptibility of a material for a given quantity of available hydrogen. Time to failure is dependent on the
amount by which the threshold stress is exceeded. Time to failure decreases with increasing stress.
As HE is normally initiated at the points of greatest concentration of stress, any locations with notches,
intended or unintended, are potential HE failure locations in case of a susceptible material condition.
5.2.4 Sources of atomic hydrogen
5.2.4.1 Internal hydrogen
Steel inherently retains a small amount of residual hydrogen as it is produced. Even with advanced vacuum
degassing techniques, steel of standard quality contains hydrogen concentrations roughly in the order of
0,5 to1 ppm. This residual hydrogen is not normally cause for concern because it is typically in a trapped
state. Internal hydrogen can also be introduced into high strength steel parts during their manufacturing
processes. For example, during austenitizing or carburizing, hydrogen can be absorbed by the parts.
However, it is subsequently “baked out” during tempering. In a steel part that has been properly quenched
and tempered, any remaining residual hydrogen is typically trapped and innocuous.
The relevant manufacturing processes to consider with respect to internal hydrogen embrittlement are
primarily coating processes and related surface cleaning and preparation processes (see Figure 2). These
processes are critical because they are the final manufacturing steps, and coating materials (e.g. zinc) act as
a barrier to hydrogen effusion, i.e. the coating prevents or impedes hydrogen’s natural tendency to diffuse
out of the steel at room temperature.
Figure 2 — Internal hydrogen sources with regard to coating processes

In general, typical sources of internal hydrogen due to the manufacturing process are:
— welding and humidity during welding;
— heat treatment, e.g. austenitizing, carburizing or nitro-carburizing;
— metal stripping;
— pre-treatment, e.g. cathodic degreasing or pickling;
— electrochemical deposition including post-treatment, e.g. passivation;
— chemical deposition.
5.2.4.2 Environmental hydrogen
Environmental hydrogen is introduced in high strength steel parts as a result of corrosion during service.
Contact with water and corrosive substances can generate hydrogen that can be absorbed by the parts.
More critically, galvanic corrosion of a sacrificial cathodically protecting coating (e.g. Zn, ZnNi) generates
hydrogen, which can then be absorbed by exposed steel surface areas of a part (i.e. cathode). This condition
occurs when the coating is damaged, cracked, porous or partially consumed by corrosion. The quantity of
hydrogen absorbed in this manner is orders of magnitude higher than under normal corrosion conditions
(i.e. steel part without coating).
These conditions can lead to SCC, also called HaSCC or HiSCC.
From a failure analysis perspective, any amount of corrosion prior to failure of an in-service part can lead
to EHE as the dominant failure mechanism, independently of the presence of internal hydrogen. With the
passage of time, the localized contribution of corrosion generated hydrogen is cumulative, and the relative
contribution of internal hydrogen becomes negligible.
NOTE Typically, EHE failures take much longer to occur than IHE failures. EHE failures can occur anywhere from
weeks to years after installation of a part, as hydrogen is absorbed during corrosion processes.
5.3 Mechanism of hydrogen embrittlement of high strength steel
High strength steel is broadly defined as having a tensile strength (Rm) above 1 000 MPa. When high
strength steel is tensile stressed, the stress causes atomic hydrogen within the steel to diffuse (i.e. move) to
the location of greatest stress (areas of high stress concentration, e.g. notched areas). As increasingly higher
concentrations of hydrogen collect at such locations, steel that is normally ductile gradually becomes brittle.
Eventually, the concentration of stress and hydrogen in one location causes a hydrogen assisted (brittle)
microcrack. The brittle microcrack continues to grow as hydrogen moves to follow the tip of the propagating
crack, until the part is overloaded and finally fractures. This phenomenon is often called HAC (or HIC). The
hydrogen damage mechanism as described causes the part to fail at stresses that are significantly lower
than the basic strength of components or parts as determined by a standard tensile test.
The figure of 1 000 MPa as the limit for incipient HE susceptibility (e.g. in ISO 19598) is a guideline.
Depending on material and microstructure, steels with tensile strengths below 1 000 MPa can under certain
circumstances be susceptible to hydrogen-assisted failure. On the other hand, advanced high-strength
steels with much higher tensile strength can have significantly lower susceptibility to HE and can withstand
considerably higher tensile loads and hydrogen levels. However, for high strength steel parts with tensile
strengths above 1 000 MPa which are subject to tensile load, and which can absorb hydrogen either during
manufacturing or during service or both, appropriate measures shall be taken regarding HE.
In the case of high strength steel, two theoretical models are proposed that describe hydrogen damage
mechanisms under idealized conditions. These models are based primarily on two complementary theories
of HEDE and HELP.
With quenched and tempered high strength steel parts, the fracture surface resulting from HAC is typically
characterized by brittle intergranular morphology which is caused by a crack growth path that follows the
grain boundaries. The morphology of a fracture surface varies based on the susceptibility of the material and

the degree of HE. Clearly either defined grain facets (i.e. sharp and angular features) or a high proportion
of brittle versus ductile features or both are indicative of a high degree of embrittlement. Less susceptible
material conditions can present fracture surfaces that contain a mix of intergranular and cleavage (i.e.
trans-granular) morphologies.
5.4 Fundamentals of metallic coatings regarding hydrogen uptake and diffusion
5.4.1 General aspects of metallic electroplated protection layers
Metallic electroplated layers in general are not uniform. The thickness of the metal layer on a given part can
vary due to characteristics of the electrolyte and the basis metal composition as well as the shape and size
of the part. The layers show typical structures, such as columnar, lamellar, unoriented, coarse grained, fine
grained, rough and porous. These structures can lead to either interactions with hydrogen recombination
or uptake or both. Catalytic effects can cause additional effects on the hydrogen interaction with the basis
material. Generation of hydrogen during electroplating does not generally lead to HE and shall be considered
in combination with the material condition, i.e. the susceptibility of the part to be electroplated.
5.4.2 Hydrogen generation during coating process
Electroplating including pre-treatment processes as acid pickling generates hydrogen, but there is always
an interaction between hydrogen formation, hydrogen diffusion, hydrogen recombination and hydrogen
effusion due to the material microstructure and the surface conditions.
The process of zinc-flake coating itself does not generate hydrogen. As cleaning pre-treatment normally is
applied mechanically (e.g. shot blasting), there is no risk of IHE.
Possible absorption of hydrogen by the steel during coating processes shall be controlled by appropriate
measures (see 5.1).
5.4.3 Corrosion protection mechanisms by metallic layers
5.4.3.1 General
There are two mechanisms for protection by electroplating layers:
a) cathodic corrosion protection;
b) barrier protection.
For electroplating processes and description of corrosion effects, see Annex A.
5.4.3.2 Cathodic corrosion protection
Cathodic protection is given when the layer is less noble than the basis material, e.g. zinc or zinc alloy on
steel. In case of corrosive attack to the coated part, the coating layer becomes the anode of the system and
corrodes while forming metal cations. The related electrons stay with the substrate building the cathode of
the system. The substrate is therefore protected as cathode.
A side-effect of cathodic corrosion protection is the generation of hydrogen due to the corrosion of the
coating material in cases of local damage of the coating, as demonstrated in Figure 3.

Key
1 corrosive environment
e.g. aqueous solution
2 steel
3 tensile stress
4 crack propagation
5 stress concentration
a
6 corrosion
7 hydrogen diffusion
b
8 coating
a +
Corrosion leads to: low molecular hydrogen ions (e.g. H ), which are adsorbed (H ) on the surface of the steel; diffusion
ad
into the steel due to absorption (H ); and crack propagation primarily in zones with high stress concentration.
ab
b
The coating can be an anodic or cathodic protecting layer. An anodic protecting layer does not promote hydrogen
uptake but enforces corrosion rate of the substrate. A cathodic protecting layer promotes hydrogen uptake by
cathodic polarization of the substrate, indeed the crack tip within the notch. This can speed up crack propagation.
Figure 3 — Local damage of the coating
When a cathodic protecting coating on steel (e.g. zinc) is locally damaged, which preferably happens in areas
of high stress concentrations such as notches, the protection mechanism is initiated, i.e. zinc as the less
++ -
noble part acts as the anode and dissolves forming Zn cations and electrons e which migrate to the steel.
+
As a by-product of zinc corrosion, H atoms are generated. They can be adsorbed at the active steel surface
-
which, as the nobler partner, acts as the cathode. By receiving the electron e from the cathode, hydrogen
can be absorbed by the steel (see Figure 4) and diffuse to areas of highest stress concentration.

Key
1 hydrogen recombination
2 hydrogen diffusion into the metal
3 Volmer-reaction
4 Tafel-reaction
5 Heyrovský-reaction
Figure 4 — Mechanisms during electrochemical discharge
5.4.3.3 Barrier protection
To achieve different properties, high strength steel parts are frequently coated by more noble metals. This
includes layers of copper, nickel, chromium or tin. It can also include combinations of layers, e.g. copper
and nickel and chromium. Corrosion protection is given as the layer is nobler than the basis material and
therefore protects the steel from corrosive attack. However, the protection is only given if the layer is closed
and without defects. If there are pores, cracks or uncoated areas, the basis material forms the anode of the
system and corrodes under the nobler metallic layer.
Typically, a strong local corrosive attack causes a mechanical failure of the part due to loss of load bearing
capability. This is much more likely than a failure by HE.
NOTE 1 Some metals (e.g. stainless steel) become passive by anodic polarization which significantly reduces the
corrosion speed. This effect is defined as anodic corrosion protection. However, an external current source is required
to supply the protection current. The voltage difference between an electroplated layer and the substrate is not
sufficient to generate anodic protection.
NOTE 2 Additionally, an organic layer can serve as a protecting barrier, e.g. hydrogen formation by corrosion.
5.4.3.4 General
Under practical conditions, hydrogen can form on ferrous materials in particular:
— for acidic corrosion media;
— for crevice corrosion (acidification and oxygen depletion in the crevice);
— for oxygen-free/low-oxygen corrosion media;

— for cathodic polarization of the iron substrate, e.g. cathodic protection by means of coatings;
— contact corrosion (due to direct contact of materials with different electrochemical potential).
After hydrogen formation, two reactions can occur:
— Atomic hydrogen can be absorbed by the steel;
— Recombination of atomic hydrogen to molecular hydrogen (H gas): H gas leaves the surface, see
2 2
Figure 4.
NOTE Corrosion can produce hydrogen, but there is always an interaction between hydrogen formation, hydrogen
diffusion, hydrogen recombination and hydrogen effusion due to the microstructure material and the surface
conditions.
5.4.3.5 Sour corrosion media
In sour corrosion media, the potential of hydrogen increases in accordance with the Nernst Equation so that
the hydrogen reduction (formation of atomic hydrogen) can dominate [e.g. by the influence of CO or exhaust
gas condensates (SO , NOx)].
5.4.3.6 Crevice corrosion — Acidification and oxygen depletion in the crevice
Crevice corrosion can occur in the case of construction-related crevices. An increased risk of hydrogen
formation can be derived from the main mechanisms for crevice corrosion (oxygen depletion in the crevice
and as a result of local acid formation in the crevice).
The oxygen content depletes in the crevice by diffusion limitation in cases of corrosion. This leads to spatial
separation of the anode (in the crevice) and the cathode (outside the crevice) in this ventilation element.
By exceeding the solubility product, the metal hydroxide precipitates. This leads to an acidification in the
crevice by the remaining protons of the water dissociation, see Formula (1).
2++
Me +→22HO Me()OH + H (1)
5.4.3.7 Oxygen depletion
In the event of oxygen depletion, a corrosion potential can establish at which hydrogen is reduced.
The real mixed potential of the reduction reaction establishes (at pH = 7) between the standard potential
equilibriums of the hydrogen and oxygen reactions at −0,4 V to +0,8 V. The larger the percentage of oxygen
reduction, the more positive the mixed potential will establish and as a result will be further away from the
standard potential equilibrium of the hydrogen formation. As a result, hydrogen formation is reduced.
5.4.3.8 Cathodic polarisation by metallic coating
For cathodic corrosion protection of ferrous materials (e.g. zinc), the corrosion potential of the substrate is
moved to the negative direction and thus cathodic reaction is enabled.
6 Preventive measures with regard to hydrogen embrittlement
6.1 General
During the early stages of designing high strength components and selecting materials, prudent measures
shall be taken to minimize the risk of possible HE failures.
Regarding minimizing the risk of IHE, information about either tensile strength or hardness of high strength
parts or both shall be provided to decide about avoiding or adapting processes which generate hydrogen.

Manufacturing, joining technologies and surface treatment processes shall be carried out in such a way that
a risk of HE failure is minimized. The necessary measures and tests in accordance with the state of the art
shall be specified in a suitable manner. These measures can include:
a) the selection of a material with the appropriate chemical composition;
b) the minimum tensile strength necessary for a specific application;
c) the minimization of tensile stress;
d) the choice, composition and control of chemicals to be used;
e) physical and chemical process limits;
f) type of tests;
g) testing frequency;
h) the number of specimens to be tested.
The result of tests carried out shall be documented.
6.2 Part design and manufacturing
Measures during part design to reduce the risk of HE failures shall include the avoidance of sharp stress
concentrators (notches, punched edges, abrupt cross-section variations, holes in radii, sharp grooves from
machining, unintended burrs, laps, etc.).
6.3 Material related measures and heat treatment
As tensile strength and hardness have a first order effect on susceptibility to HE, they should be kept the
minimum necessary for a given application. Appropriate material selection shall be considered. Higher
tensile strength/hardness steels with an addition of more alloying elements like Cr and Mo are advisable
to increase ductility and to provide hydrogen traps. In addition, detrimental tramp elements as sulfur and
phosphorus shall be kept to a minimum. Steel cleanliness shall be specified and controlled. Finally, especially
for steels with high tensile strength/hardness, enough ductility shall be ensured to keep the sensitivity to
stress embrittlement in notched areas as low as possible.
For each steel, the relationship between the cooling process, cooling rate, microstructure and material
hardness can be derived from continuous and isothermal state diagrams. Continuous time-temperature
transformation diagrams are the basis for evaluating the transformation behaviour of steels
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