SurTecTechnical Letter_____________________________________________________________

Hydrogen Embrittlement

Rolf Jansen and Patricia Preikschat May 20022. edition

In several process steps of electroplating zinc, hydrogen is formed at

the surface of iron parts. That is during pickling, cathodic electrolytical

cleaning and zinc plating itself. The hydrogen can diffuse into the bulk

material and, especially hardened parts can be affected badly by

hydrogen brittlement causing stress and evenbraking off the material.

How should a suited pre-treatment look like, particulary the pickling, to

minimize the adsorption of hydrogen?

Are there differences between the zinc processes and which effects do

they have; are all types of electrolytes suitable?

Which method is the best to remove adsorbed hydrogen out of the

bulk without distructing the zinc layer?

This Technical Letter answers these questions and shows concepts and

methods to avoid hydrogen embrittlement.

8

1 Formation of Hydrogen 31. Ways of the Hydrogen 3

2 Test for Hydrogen Embrittlement and Testing Parts 5

3. Methods to avoid the Penetration of Hydrogen 71. Mineral Acid Pickling 7

2. Electrolytic Cleaning 8

3. Zinc Bath 9

4. Reworking 9

4 Heat Treatment to Release the Hydrogen 101. Basics of the Heat Treatment 10

2. Usual Two Step Method (Old DIN) 11

3. One Step Method 12

4. New: Protected Heat Treatment 13

5. Diffusion Ability of Zinc Layers 14

1. Practical Research: Property of Effusion of

Zinc Layers out of Different Electrolytes 14

2. Evaluation 15

5 Conclusion 17

Important Normatives

DIN 50 961, 50 969

BOSCH-Norm N67F

ISO/DIS-9588

page 2

1 Formation of Hydrogen

During pickling in mineral acids, cathodic electrolytical cleaning and zinc plating

hydrogen is formed. In all cases due to a cathodic reduction. The anodic counter

reaction in case of pickling is the dissolution of metal and takes place at the same

location as the evolution of H2. In case of electrolytical cleaning or zinc plating, the

counter reaction is the formation of O2, taking place separately at the anodes:

in acid medium 2 H3O+ + 2 e– ® H2­ + 2 H2O

in alkaline medium 2 H2O + 2 e– ® H2­ + 2 OH–

These reactions take place in two steps. That means that the hydroxyle ions decharge

separately one by one. Also the water molekules are decomposed separately. In each

case, atomar hydrogen is formed first:

H3O+ + e– ® H + H2O

1.1 Ways of the Hydrogen

The atomar hydrogen is very reactive and quickly seeks a reaction partner. Until it meets

another atomar hydrogen, it forms a weak bond to iron atoms of the parts surface. This

is known as adsorbed atomar hydrogen Had. The adsorbed atomar hydrogen can react

to molekular hydrogen if combined with another atomar hydrogen. H2 is formed and

released from the electrolyte as a gas bubble. Alternatively, it diffuses into the bulk of

the part and damages the crystalline structure.

combination to the gas H2:

2 Had ® H2­

or: Had + H3O+ + e- ® H2­ + H2O

diffusion into the bulk:

Had ® Hmat

Metal surfaces being immersed in electrolytes always adsorbe a thin layer of

components of the electrolyte. These are water molekules, cations, anions or organic

compounds (inhibitors, surfactants), picture 1. The ratio of surface covered with one of

the components to the entire surface of the part depends on the electrochemical

potential of this component, on the temperature and on the concentration of all

components in the electrolyte.

page 3

adsorbedinhibitor

molecule

adsorbed,oriented

water dipoles

contact adsorbed anion,partially hydrated

hydratedcation

inner Helmholtz planeouter Helmholtz plane

hydrogen molecules

diffusinghydrogen atoms

adsorbedhydrogen atoms

picture. 1: metal surfaces in electrolytes are always covered with electrolyte components

page 4

According to the diffusion laws, the transport of atomar hydrogen gets faster the more

the concentration of Had on the surface differs from the hydrogen in the bulk. That

means, at a high concentration of hydrogen on the surface, the penetrating rate into

the bulk is high, too, and reverse.

Also important is the time of exposure: at short treatment times only few hydrogen is

able to penetrate into the material.

2 Test for Hydrogen Embrittlement and Testing Parts

Hardened parts normally are tested for hydrogen embrittlement at the end of the

plating process. Depending on the type of part, they are tested under tensile force,

stress or bending. However, these methods are destructive and only selected parts can

be tested. Nevertheless, the test should give security for the whole batch.

One possibility therefore is using some suited critical test pieces, which are plated

together with the parts. These test pieces can all be destroyed in successive tests. To

ensure that the result of this test is suitable for a quality control, the test pieces must

fulfill the following:

• the test pieces must be much more sensitive to hydrogen embrittlement than the

normal parts

• the test pieces must run through the same treament steps as the normal parts

• during the surface treatment, the test pieces must behave comparable to the normal

parts

• damages of the test pieces must be recognized reliable

• in addition to the test pieces, also a few normal parts should be tested if possible

As suited test parts, safety rings, DIN 471, 5 ´ 0,6, 650 HV, are recommended by the

central research department FV/PLO1 of BOSCH in Schwieberdingen (Germany).

page 5

The test is done as follows:

25 of these test rings are run through the

same surface treatment steps as the real

parts, including the heat treatment. After-

wards they are put up a glass rod of 5 mm

diameter using tongs. The tongs have an

arresting mechanism, so an overstreching of

the rings is not possible.

The test rings have to be put up the rod

without breaking. The test is successful, if

after 24 h no rings are broken. picture. 2: testkit for hydrogen embrittlement

The rings also are suited for examining separate treatment steps, for instance to test

different pickling additives or the effectivity of the heat treatment.

In case of examining a pickling additive, first the rings

are put up the glass rod and then immersed into the

pickling, picture 3. It is advisable to use a high glass

cylinder, since the rings may break vehemently and

parts of them may fling out of the solution. Protecting

glasses are necessary!

In intervals of 1 to 5 minutes, the broken rings are

counted. The number of broken rings versus the time

are maked into a diagram.

Comparing pickling additives leads to curves with

different ascents, each depending on how effective the

additive inhibited the attack of acid (see picture 4 in

the next chapter).

page 6

picture. 3: test rings while a pickling trial

3. Methods to avoid the Penetration of Hydrogen

Generally, the surface covered with atomar hydrogen should be as small as possible and

the exposition to hydrogen should be as short as possible.

3.1 Mineral Acid Pickling

During the pickling, a lot of hydrogen is developed and the treatment time sometimes is

very long. Therefore, critical parts should not be pickled at all. But in most cases they are

contaminated or passive and without pickling they can not be treated further. For these

parts, the pickling time must be very short. Important normatives [1] prescribe the

pickling of critical parts to last less than 5 minutes. It is better to use a short pickling at a

higher concentration than a long pickling at a low acid concentration.

The oxide and scaling layers as well as rust are dissolved in the pickling without

hydrogen generation:

Fe2O3 + 6 HCl ® 2 FeCl3 + 3 H2O

Hydrogen only is formed if acid attacks the base material. The protons are the oxidising

agents: iron is oxidised and the protons are reduced to elementar hydrogen.

Fe0 + 2 H+ ® Fe2+ + H2

Suited inhibitor molekules are adsorbed by the pure iron surface. With oxides they will

not interact. Behaving like this, they prevent the dissolution of iron and, thus, the

hydrogen evolution. The task of the pickling, dissolving oxides, is not impaired.

Furthermore, by the inhibitor covering it, the hydrogen can not reach the pure iron

surface.

But not all inhibitors for picklings are recommendable. Some impede the formation of

molekular hydrogen, the atomar hydrogen actually is forced to diffuse into the bulk.

These unsuited inhibitors are called promotors.

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0 5 10 15 20 25 30

100

80

60

40

20

0

SurTec 425

SurTec 424

% defects Additive 1(Promotor)

HCl 1:1(without additives)

Additive 2

Additive 3

Additive 4

Additive 5

Additive 6

pickling time in min

picture. 4: Some inhibitors act better, some act worse impeding the hydrogen diffusion (Pickling solutions in all trials: HCl 1:1 at room temperature)

By the aid of specially chosen pickling inhibitors, it is possible to work even with

concentrated hydrochloric acid. These inhibitors impede nearly all hydrogen evolution

and no aerosols and HCl fogs are formed.

An excellent inhibiting is reached with SurTec 424. It was developed especially to avoid

hydrogen embrittlement of critical steel parts.

page 8

3.2 Electrolytical Cleaning

Hardened parts are not allowed to be cleaned cathodically. They have to be cleaned

anodically in the electrolytic cleaning step. The amount of free hydrogen is too high and

the treatment time too long, therefore it is forbidden in the relevant normatives to clean

cathodically.

3.3 Zinc Bath

Zinc has the property to impede the diffusion of atomar hydrogen. Thus, a common

opinion is, that the formation of hydrogen during zinc plating only is critical at the

beginning when the zinc does not cover the surface completely. However, in reality also

parts which were pretreated carefully without hydrogen adsorption, can adsorb the

hydrogen during zinc plating, especially in alkaline electrolytes.

Weak acid zinc electrolytes have a better current efficiency compared to the alkaline

baths and less hydrogen is developed. On the other hand, the small amount of

hydrogen that is formed in a weak acid electrolyte is sufficient to cover the surface of

the part completely. And this results in a maximum diffusion speed.

The effect of electrolyte type and bath composition on the diffusion of hydrogen has to

be discussed in context with current efficiency and metal distribution. All steps for rising

the current efficiency are good for a low H2 penetration. And with a better metal

distribution, also the areas of low current density are covered quickly with zinc and the

immigration of hydrogen is reduced.

3.4. Reworking

Occasionally, parts are plated with faults and they have to be dezinced and plated

again. For hardened parts, this has to be done very carefully. To remove the old zinc

layer, normally a pickling in hydrochloric acid is used and a lot of hydrogen is developed.

Especially at areas which are free of zinc first, a very strong hydrogen evolution can be

observed. That is due to the lower hydrogen overtension of iron compared to zinc.

However, at this point even the best hydrogen inhibitor is uneffective. The parts adsorb

great amounts of hydrogen and are not allowed to be plated directly again. Hardened

and dezinced parts have to be heat treated in any case before a new zinc plating. The

processing has to be discussed with the customer.

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4 Heat treatment to Release the Hydrogen

4.1 Basics of the Heat Treatment

Several metals, for instance iron, are able to adsorb atomar hydrogen, forming the

respective metal hydride. As a consequence the structure of the bulk changes, it is

getting more brittle. The real damage is caused by tensions in the tensile strength. Basic

faults in the structure of the material support the damaging process and lead to an

explosion like breaking of a critical part.

The penetration of hydrogen into the bulk can be minimized using suitable processes for

the galvanic zinc plating, but it can not be impeded totally. As long as the hydrogen is

dissolved atomar inside the iron, it is possible to drive most of it out by heat treatment.

It has to be considered that directly after the zinc plating the hydrogen is located near

the phase border of zinc and iron. This high concentration grade could assist a quick

release of hydrogen right after the zinc plating. When the concentration grade becomes

smaller, the hydrogen migration will slow down (picture 5).

depth

hydr

ogen

con

cent

ratio

n

t0

t1

t2t¥

picture. 5: Progress of the hydrogen concentration inside the workpiece at different times

t0: directly after plating t1: short time after platingt2: several hours after plating t¥: equilibrium

page 10

On the other hand exists also a concentration grade towards the base material, and

hydrogen penetrates deeper into the iron part. This also equalizes the concentration

grade and slows down the hydrogen migration. Consequently, the heat treatment

should be done as soon as possible, to take advantage of the concentration grade

towards the surface.

4.2 Usual Two Step Method (Old DIN)

Successively, the parts are plated with 3-4 µm zinc, heat treated for approx. 2 hours at

180-200 °C, plated again to reach the final thickness, heat treated for a second time

and chromated.

This two step method bears risks:

• hydrogen may stay inside the bulk if the zinc layer is thicker than calculated and if

the heat treatmet is too short

• the zinc layer may be attacked by the high temperature (> 215 °C, depending on the

electrolyte type), leading to a dull surface and a bad appearance, in worse cases

even blisters can occure

• the zinc layer is too thin, and during the activation after tempering pure iron will get

in contact with the acid. Thus, in successive zinc plating new hydrogen penetrates

into the parts.

The main disadventage of this method, however, is the complicated process sequence

and the long required working time. Therefore, the second heat treatment is not done

always, taking the risk of a too short outdriving time. However, a good appearance of

zinc layer and chromating can be ensured in the successive working steps.

4.3 One Step Method

For some time it is known that the diffusion barrier of zinc is not as impermeable as it

was thought. And using high enough temperatures and long enough times, it is

possible to remove the hydrogen after zinc plating. In the one step method the parts are

plated to their desired layer thickness (sometimes up to 20 µm), dried and heat treated

at 210-240 °C.

page 11

But the temperature is limited: at 225 °C the zinc starts to oxidize in air. Additionally,

zinc has a considerable steampower at these temperatures. So, the heat treatment is

most useful at temperatures below 225 °C.

The risks of this method are:

• the heat treatment is done too short

• a high load of hydrogen in the parts can lead to blisters in the zinc layer, due to a

tailback of hydrogen at the phase barrier iron/zinc. (This can be prevented if the

parts are heated up slowly - in contrary to the economic point of view. The

outdriving process starts more slowly in this case.)

• at temperatures above 220 °C and at long heat teatments even at 200 °C, the zinc

layer is oxidized by the oxygen of the air. This reaction can extend deep into the zinc

layer along pores and impairs the appearance. The parts become dull and grey and,

possibly, the corrosion resistance of a chromating applied successively is not good.

In the one step method, the chromating is done after the heat treatment. The parts

need to be put once again in the plating line for activation and chromating.

Nevertheless, this method is less expensive than the two step method. It has less process

steps, making the process more secure. It is not possible that hydrogen penetrates into

the parts during the second treatment step. Yet, the high temperature may damage the

zinc layer.

4.4 New: Protected Heat Treatment

Since new heat resistant trivalent blue passivations (e.g. SurTec 662, SurTec 664,

SurTec 667) have been developed, it is possible to heat treat the parts following the last

galvanic process step. The chromating layers produced in these blue passivations

withstand a heat treatment up to 215°C for 24 hours and protect the zinc layer below.

Brightness and colour of the layer system are kept.

The one step zinc plating gets secure thanks to the protected heat treatment. The

adventages of this method can be used without any danger for the zinc surface.

If zinc plated and blue passivated parts are tempered, the corrosion resistance always

will decrease, depending on temperature and duration of the heat treatment. To reach

the demanded corrosion resistance of the normative DIN 50961, two things have to be

considered (for barrel application):

page 12

• During tempering, the passivation layer gets thinner. Hence, parts for tempering

should be chromated thicker (longer application time and/or higher bath

concentration, see also SurTec Technical Letter 7). Prior to tempering, these parts are

light greenish to yellow, but after the heat treatment they are clear blue and have a

better corrosion resitance, due to their thicker chromating layer.

• The water content of the chromating layer should be driven out slowly. Best is to put

the parts into the cold oven and heat it up slowly to the desired temperature. If the

parts are put into a hot oven, the water is released too fast and the chromate layer

can be damaged.

The main advantage of the method is the zinc layer being protected against oxidising in

air. The quality of the zinc is superior compared to the unprotected heat treatment,

indeed.

Yellow, olive and black passivations can not be applied before tempering, they are

destroyed at the high temperature. Nevertheless, the adventage of the protected heat

treatment can also be used, because blue chromated parts can be passivated afterwards

yellow, olive or black without problems.

4.5 Diffusion Ability of Zinc Layers

Does the electrolyte type have an influence on the diffusion ability of the zinc layer? In

literature and in praxis different answers are spread. Mostly, it is assumed that hydrogen

in zinc layers from cyanide electrolytes is easy to drive out. In contrary, hydrogen in zinc

layers from cyanide free alkaline electrolytes is difficult to drive out. Hydrogen in parts

plated with acid zinc almost is impossible to be released. Accordingly, the valid

normatives demand a special permission to apply bright layers and layers from acid zinc

electrolytes.

The influence of the electrolyte type surprises, because the electrolytically deposited zinc

is a very pure zinc. By analytical methods extremely low amounts of carbon (coming

from the organic additives) and iron are determined in the layer. Especially in layers from

alkaline electrolytes. The different behaviour can only be explained by their different

morphology. Depending on the electrolyte, there are some differences, indeed.

page 13

4.5.1 Practical Investigation: Effusion Behaviour of Zinc Layers from

Different Electrolytes

To answer the question wether the electrolyte type influences the diffusion of hydrogen,

following tests have been done.

test material: Specially constructed critical test rings according to DIN 471, 5 ´ 0.6 mm,

650-680 HV, with a deliberately too short annealing

pretreatment: For each electrolyte, 950 test rings successively were hot degreased

for 10 min, cleaned electrolytical (cathodic) for 5 min, pickled in HCl

1:1 without inhibitor for 5 min, cleaned electrolytical again, 20 s

cathodically and 20 s anodically and put into an acid dip of 5 % HCl

for 3 min.

zinc plating: Right after this extra strong and hydrogen developing pretreatment

and without heat treatment, the test rings were plated in barrel

process for 2 hours in the zinc electrolyte, reaching a layer thickness

of at least 17 µm.

test electrolytes: The following electrolytes (standard make-up of each) were used:

cyanide free alkaline bright zinc process

low cyanide bright zinc process

weak acid bright zinc process

weak acid technical zinc process

The three bright zinc processes were adjusted to the same grade of

brightness, the technical zinc was matt to semi bright.

heat treatment: At 180, 210 and 225 °C for 2, 4, 6, 8, 10 and 22 hours

test: 50 rings of each treatment sequence (electrolyte, heat treatment

and duration of heat treatment) were put onto a glass rod, see also

chapter 2.

page 14

4.5.2. Evaluation

The higher offer of hydrogen in alkaline electrolytes was visualized by this experiment:

without heat treatment 100 % of the rings broke, whereas only 70 % of the bright acid

zinc plated rings broke. And just 56 % of the technical acid zinc plated rings broke (see

the breaking rate at the time 0 in picture 6).

At the low tempering temperature, the cyanide zinc process was a little better than the

others, even though the layer was deposited with the same grade of brightness.

Generally, 180 °C had been too low as a heat treatment: even after 22 hours, none of

the test rings was completely de-embrittled. After 22 h at 210 °C, all test rings were de-

embrittled.

At 225 °C all of the test rings were de-embrittled already after 6-8 hours. However, the

zinc layers had been damaged strongly by exposing to this high temperature. Pores of

approx. 1 µm diameter were formed (picture 8). Through these pores, the hydrogen

could penetrate easily, but they are not tolerable.

0 3 6 9 12 15 18 21 24

100

80

60

40

20

0

180 °C% defects

heat treatment in hpicture. 6: At 180 °C, the alkaline electrolytes are better than the acid ones, but all four of them are

not enough diffusible 19 µm cyanide free alkaline bright zinc plated 17 µm weak acid bright zinc plated 19 µm cyanide bright zinc plated 17 µm weak acid matt zinc plated

page 15

0 3 6 9 12 15 18 21 24

100

80

60

40

20

0

210 °C

heat treatment in h

% defects

picture. 7: At 210 °C the behaviour of effusion is nearly the same for all electrolytes; all processes need clearly more than 10 hours for a breaking rate of 0

0 3 6 9 12 15 18 21 24

100

80

60

40

20

0

225 °C% defects

heat treatment in hAbb. 8: At 225 °C the zinc layers are perforated by the effusing hydrogen. The breaking rate is 0

after a short time, but the zinc layers are strongly damaged (see REM picture)

page 16

Obviously, 225 °C is too high as heat treatment. At 210 °C a duration of about 12

hours is sufficient for the 3 bright zinc processes. In case of the matt zinc layers it is very

difficult to drive out the hydrogen. Generally, the bright zinc processes are more able to

effuse hydrogen than the mate zinc layers. Hence, hardened parts should be plated with

bright zinc and heat treated at 215 °C for at least 10-12 hours.

5 Conclusion

Hardened parts can be plated with zinc without fearing embrittlement. The described

tests lead to the following recommenations:

• The surface of the critical parts has to be free of oilcarbon and should allow short

pickling times and a mild pretreatment.

• Suited inhibitors and pickling additives must be used; SurTec 424 was developed

especially for this application. However, the pickling should not exceed 5 minutes.

• A cathodic electrolytical cleaning must not be applied.

• There is no special type of zinc electrolyte demanded; all four tested processes are

suited. The diffusibility of the cyanide process tended to be the best, whereas the

hydrogen offer in the weak acid technical zinc process was the lowest.

• The zinc layer should be chromated with a heat resistant blue passivation (e.g

SurTec 662, SurTec 664, SurTec 667), to allow a protected heat treatment.

• After the plating process the parts should be tempered as soon as possible (within

the next 4 hours) to prevent damages. The one step method is more secure than the

two step method; the layer thickness should be thinner than 16-20 µm, though.

• The optimal temperature of tempering weak acid zinc processes is 220 °C, for

alkaline plated parts it is 215 °C. The period of the heat treatment has to be adapted

to the type of parts, but must last at least 6-8 hours.

• It is possible to chromate in succession with a yellow, oliv or black passivation.

page 17

The described test is an excellent method to test on hydrogen embrittlement. The critical

test rings are perfectly suited as test material on account of their more sensible

behaviour (hardness, dimension, annealing etc.) than any other part. That means: If

there are no defects of the overcritical test rings, none of the normal critical parts should

be damaged. Maybe the plating process is done "too well", but now with a definite

value.

The tests can be carried out by everyone and a test kit (including the critical rings, tongs,

adapted glass rods and the description of the method) is available from SurTec GmbH at

cost price.

page 18