SurTec Technical Letter

Transcription

1 8 SurTec Technical Letter Hydrogen Embrittlement Rolf Jansen and Patricia Preikschat May 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.

2 page 2 1 Formation of Hydrogen 3 1. Ways of the Hydrogen 3 2 Test for Hydrogen Embrittlement and Testing Parts 5 3. Methods to avoid the Penetration of Hydrogen 7 1. Mineral Acid Pickling 7 2. Electrolytic Cleaning 8 3. Zinc Bath 9 4. Reworking 9 4 Heat Treatment to Release the Hydrogen Basics of the Heat Treatment Usual Two Step Method (Old DIN) One Step Method New: Protected Heat Treatment Diffusion Ability of Zinc Layers Practical Research: Property of Effusion of Zinc Layers out of Different Electrolytes Evaluation 15 5 Conclusion 17 Important Normatives DIN , BOSCH-Norm N67F ISO/DIS-9588

3 page 3 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 H 2. In case of electrolytical cleaning or zinc plating, the counter reaction is the formation of O 2, taking place separately at the anodes: in acid medium 2 H 3 O e H H 2 O in alkaline medium 2 H 2 O + 2 e H 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: H 3 O + + e H + H 2 O 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 H ad. The adsorbed atomar hydrogen can react to molekular hydrogen if combined with another atomar hydrogen. H 2 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 H 2 : 2 H ad H 2 or: H ad + H 3 O + + e - H 2 + H 2 O diffusion into the bulk: H ad H mat 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.

4 page 4 diffusing hydrogen atoms adsorbed, oriented water dipoles contact adsorbed anion, partially hydrated hydrated cation adsorbed hydrogen atoms adsorbed inhibitor molecule outer Helmholtz plane inner Helmholtz plane hydrogen molecules picture. 1: metal surfaces in electrolytes are always covered with electrolyte components

5 page 5 According to the diffusion laws, the transport of atomar hydrogen gets faster the more the concentration of H ad 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).

6 page 6 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. Afterwards 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). picture. 3: test rings while a pickling trial

7 page 7 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: Fe 2 O HCl 2 FeCl H 2 O 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. Fe H + Fe 2+ + H 2 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.

8 page % defects Additive 1 (Promotor) HCl 1:1 (without additives) 80 Additive SurTec Additive 3 Additive 4 Additive 5 Additive pickling time in min SurTec 424 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.

9 page 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 H 2 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 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.

10 page 10 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). hydrogen concentration t 0 t 1 t2 t depth picture. 5: Progress of the hydrogen concentration inside the workpiece at different times t 0 : directly after plating t 1 : short time after plating t 2 : several hours after plating t : equilibrium

11 page 11 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 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 C.

12 page 12 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):

13 page 13 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.

14 page 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, mm, HV, with a deliberately too short annealing pretreatment: zinc plating: test electrolytes: 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. 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. 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: test: At 180, 210 and 225 C for 2, 4, 6, 8, 10 and 22 hours 50 rings of each treatment sequence (electrolyte, heat treatment and duration of heat treatment) were put onto a glass rod, see also chapter 2.

15 page 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 deembrittled. 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. % defects C picture. 6: heat treatment in h 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

16 page 16 % defects C picture. 7: heat treatment in h 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 % defects C heat treatment in h Abb. 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)

17 page 17 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 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 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.

18 page 18 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.