Electrochemical Determination of Hydrogen Entry to HSLA Steel during Pickling

Powered by TCPDF (www.tcpdf.org) This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Aromaa, Jari; Pehkonen, Antero; Schmachtel, Sönke; Galfi, Istvan; Forsén, Olof


Introduction
e number and the applications of new high-strength lowalloy (HSLA) steels with enhanced formability are constantly increasing. It is generally accepted that the transition from mild steel to HSLA steel occurs at yield strength of about 300 MPa. ese steels have the potential to introduce weight savings while improving performance. Depending on design, the higher strength can evolve into better fatigue and crash performance, while maintaining or even reducing required material thickness [1]. However, HSLA steels may also give rise to some risks. For example, under certain circumstances, hydrogen introduced into steel during its manufacture, subsequent component fabrication, or in service may result in brittle failures at applied stresses far below the yield strength or the nominal design stress for the materials [2].
In this paper, we have studied hydrogen intrusion to HSLA steels during acid pickling to remove surface films before hot-dip galvanizing. e electrolytically produced hydrogen that penetrates into metal lattice can originate from corrosion, pickling, or electroplating [3]. In the pretreatment of the hot-dip galvanizing process, hydrogen may be absorbed in the steel during the pickling stage through contact with the hydrochloric acid which oxidizes the metal and forms adsorbed hydrogen that partly enters the metal lattice. After hot-dip galvanizing, the zinc layer prevents or delays the exit of the hydrogen from the steel [4]. Steels with tensile strength at the level of 800-1000 MPa or higher are considered to be the most susceptible ones to hydrogen embrittlement [5,6]. e majority of steels have generally tensile strength in the range of 200-450 MPa, and they are apparently not subject to hydrogen embrittlement problems. Special consideration must be given to high strength steels, if they are required to be pickled [3].
Inhibitors are used to prevent pitting of steel surface in case of overpickling. ey also minimize the dissolution of iron, decrease the acid consumption, and lower the formation of harmful fumes. Normally, inhibitors provide good protection against hydrogen absorption during acid cleaning. It is known that inhibitors such as benzotriazole obstruct or decrease the diffusion of atomic hydrogen into the steel. In addition, some nitrogen bearing organic compounds such as imidazoline have been found to be highly effective inhibitors of hydrogen penetration into low carbon steels [2,7]. e Devanathan-Stachurski method is the most frequently utilized technique to measure the hydrogen permeation in metals. Hydrogen is produced on the cathodic side of the double cell using cathodic polarisation. Part of this hydrogen di uses through the sample, and on the anodic side exiting hydrogen atoms are oxidized by applying an anodic current. ickness of steel samples in hydrogen permeation experiment is typically 0.3-1.0 mm. Galvanostatic current to produce hydrogen has varied from 0.05 to 100 mA/cm 2 [8][9][10]. e aim of this investigation was to measure the permeation of hydrogen through HSLA steels using the Devanathan-Stachurski method with modi ed anodic surface treatment and hydrogen production using acid. e permeation is commonly studied using external polarisation to produce the hydrogen, but we used the pickling acid as the hydrogen source. e samples had no Pd coating on the exit surface, but the exit surface was passivated potentiostatically using NaOH before permeation measurements. Passivation was done to produce a steady background current on the steel. A schematic diagram on the reactions and phenomena during the measurements is shown in Figure 1. In the anodic compartment, steel is rst dissolved and passivated at a constant potential. e cathodic compartment is empty during the passivation stage. During the permeation tests in the cathodic compartment, steel is corroded by acid at the free corrosion potential, and part of the produced hydrogen enters the steel di using to the exit surface in the anode compartment. When hydrogen di uses through the steel, hydrogen evolution begins on the anodic surface and this is detected as current increase.
Usually the anodic side of the steel sample is coated with palladium to ensure a uniform surface and to minimize the anodic dissolution of the metal during permeation current measurements. Palladium lm favours the kinetics of hydrogen oxidation. Without Pd layer, the oxidation of hydrogen atoms could be incomplete due to the presence of an oxide lm that could build up on a metal sample when anodically polarised in an alkaline medium. e oxide lm acts as a barrier against hydrogen release. According to Manolatos et al. [11], without a palladium coating on the exit side of the sample the surface phenomena are not controlling and the passive layer on the exit side changes with time preventing stabilization of the hydrogen concentration on the exit side, and therefore stationary conditions cannot be obtained. is is the reason why most of the investigations have been carried out using palladium coating. e variation in hydrogen concentration is observed as the nonsteady state of the charging current curve. However, it is possible to obtain reproducible results by controlling the parameters related to the formation of the passive layer [11]. Our approach was to use long passivation times and subtract the steady passive current from hydrogen oxidation transient currents.
e charging of steel with hydrogen is usually done electrolytically or in hydrogen-containing atmosphere (cf. [12]), and the gaseous charging and electrolytic charging are considered equivalent. Identical setup with freely corroding charging side has been used, for example, in [13] to study the e ect of pickling inhibitors for hydrochloric acid and in [14,15] to study corrosion of pipeline steels. In [13], the e ect of inhibitors on hydrogen permeation was calculated by comparing permeation current with and without inhibitor. In [14], the permeation current was used to evaluate the e ect of additives and corrosion lms on hydrogen absorption. In [15], the permeation current increased when the H 2 S concentration in hydrogen entry environment increased. ese examples indicate that use of freely corroding surface can be used to charge steel with hydrogen. In our work, the permeation of hydrogen was investigated using 1-4 mm thick steel samples, and several analysis methods were applied for estimating the di usion coe cients.

Anode compartment
Cathode compartment Empty NaOH

Cathode compartment
Anode compartment Acid NaOH

Experimental
A double-compartment test cell was constructed to measure the permeation of hydrogen through steel ( Figure 2). Material of this double cell was Teflon, and the steel sample was installed between the two separate cells. e electrochemical tests were done using Autolab PGSTAT30 Potentiostat with Autolab Software 4.9.
Test materials were high-strength low-alloy (HSLA) steels with yield strength of 355, 500, 700, and 900 MPa. Samples for experiments were cut from rectangular hollow sections 50 mm wide with 6 mm material thickness. e material in these hollow sections is hardened, hot-rolled steel, and corresponding grade carbon steel S235 was used as a reference material. e tensile strength of this steel is 235 MPa. Chemical compositions of test materials are shown in Table 1. CEV (carbon equivalent for estimation of steel weldability) values calculated using CEV � C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 were taken for small material thicknesses used in this study. e analyses and material properties were supplied by the manufacturer and referred to general product qualities. e surface area of the sample that was in contact with solution was 19.6 cm 2 .
e thickness of the sample area exposed to the solutions was reduced by turning ( Figure 3). e thickness of exposed sample area varied from 1 to 4 mm.
After thickness adjustment, the anodic surface of the sample was ground with emery paper (grade 400) followed by washing in citric acid just before electrochemical experiments. No coating was used on the anodic side of the sample. e anodic potential in potentiostatic permeation experiments was determined by potentiodynamic polarisation measurements for carbon steel S235 and S700 in 0.2 M NaOH solution at room temperature. Anodic polarisation curve from the open circuit was measured with a potential sweep rate 100 mV/min.
Hydrochloric acid diluted to 16.5% (5.4 M) was used in the permeation measurements to produce hydrogen on steel surface. e acid was used both without dissolved iron and with Fe 2+ concentrations 14 g/L (0.25 M) and 45 g/L (0.8 M) to simulate a used pickling bath. Iron was added as FeCl 2 ·4H 2 O. Temperature of the solution was 20°C. e sample was installed between the two compartments of the cell, and 1 L of 0.2 M NaOH solution was added into the anodic side of the test cell. ereafter, sample was polarised to a potential of 0 mV versus SCE to form a passive layer on the surface, and the current was measured up to steady state situation (about 15 h). e passive film in alkaline solution is usually magnetite [16,17]. e passivation procedure causes initially an increase in the current due to the corrosion of the steel when the oxide layer is formed, and this current decreases with time as the passive film reaches a steady state. In the passive state, steel dissolves, but at a low and constant rate. When the passive film had reached a steady state, hydrochloric acid (1 L) was introduced in the cathodic compartment. When hydrogen started to diffuse to the exit side a current increase was seen. When the current reached   a steady maximum level, the acid was removed through the bottom valve, and after some time, a decrease of the anodic current was observed. During the whole measurement, the anodic side of the sample was kept at constant potential. In the beginning of the test, the current was due to anodic oxidation and passivation of steel. Oxidation of hydrogen that had di used to exit side was seen as current increase. When hydrogen was stopped by removing the acid, current started to decrease and reached nally the same passive current level as before the introduction of acid. is background current is subtracted from the current measured during the permeation measurements.
us, the e ect of only hydrogen di usion on the current could be observed.
To increase the uptake of hydrogen into the metal, recombination poisons such as arsenic, hydrogen sulphide, and thiourea can be used [18]. ese elements block adsorption sites on electrode surface diminishing the coverage by adsorbed hydrogen. e adsorbed H is an intermediate resulting from discharge of H + , and its recombination produces H 2 in the cathodic hydrogen evolution. No tests were done with hydrogen poisons as they are not used in pickling baths. e use of recombination poisons was not found to be necessary as detectable amount of hydrogen was generated from the concentrated hydrogen chloride solutions used in this work.

Results and Discussion
e open circuit potential of carbon steel S253 was −490 mV versus SCE and a wide passive range up to the potential of +400 mV versus SCE was found in 0.2 M NaOH solution ( Figure 4). Based on these results, the potential of 0 mV versus SCE was chosen for the permeation measurements. e selected potential is on the passive range of tested steels, and it is higher than the equilibrium potential of H + /H 2 reaction so oxidation of di used hydrogen was guaranteed.
e current density at this potential is less than 0.1 µA/cm 2 . e dissolution rate of steel is at very low level, approximately 1 μm/year. A typical current density versus time curve in an experiment on steel S355 is shown in Figure 5. e current is related only to the reaction rates on the steel surface in the anodic compartment containing NaOH. A high current density is observed at the beginning of the experiment, when the steel surface is still in active state, and much lower current density of about 0.3 µA/cm 2 was reached when the steel passivates. Hydrochloric acid was added on the cathodic side of the cell after 18 h when the anodic surface had reached the passive state. After a delay of over two hours, an increase in the current was seen in the anodic compartment.
is current increase is due to hydrogen that had di used through the steel and was oxidized on the anodic side. A steady maximum was observed after about 5 h from the acid addition indicating steady ux of hydrogen through the steel sample. At this point, the hydrochloric acid was removed from the cathodic side of the cell. e anodic current density started to decrease as no more hydrogen was produced on the acid side, and hydrogen charged into the steel was consumed. At the end of the test, the current was approximately same passive current as before acid addition.
Results for hydrogen permeation experiments for high strength steel are shown in Figure 6. e curves show only the part after adding HCl in the cathodic compartment. e hydrogen penetration time through the sample is similar for all steels with similar thickness, as shown in Figure 6. is is seen as an increase in the current density after HCl addition. e maximum current density is lowest for the steel S900 and highest for steel S500. e grade of the steel does not have direct correlation with the maximum current. e maximum current density gives the maximum penetration rate of hydrogen through the steel. eoretically, integration of current versus time curve gives the amount of hydrogen penetrated through the sample.
Introduction of iron into the pickling acid has only a minor e ect on the current versus time curve (Figure 7). As an example, the in uence of ferrous ion concentrations 0 g/L, 14 g/L, and 45 g/L on hydrogen entry for S700 steel is  shown in Figure 7. Dissolved iron had only a minor e ect on the permeation of hydrogen in the studied steels. e hydrogen di usion coe cient was calculated by using four di erent methods given in [19,20]: and for the decay transient (both ends at zero concentration and linear initial condition), the following solution is obtained: (2) Figure 8 shows the principles for determination of characteristic times for calculation of di usion coe cients.
In the time lag method, analysis is done by integrating the rising curve to calculate the quantity of hydrogen, which has permeated through the sample as a function of time. An extrapolation of the plot of quantity against time gives the time lag t L which is related to the di usion constant by e t L in (3) can be obtained by determining the time at which the rate of permeation is 0.63 times the steady-state value described by current density j ss [19].
In the breakthrough time method, the time t b is obtained by drawing tangent in the rising part of the curve. e tangent crosses the time axis at point t b [19]: In the half-rise time method, the characteristic time t 0.5 corresponds to the current density that is 0.5 times the current density at steady-state situation j ss . Di usion coe cient can then be calculated as follows [19]: In the decay transient analysis, a characteristic time constant t D is determined using the slope of current decay after the steady state. e obtained current versus time is corrected for offsets and divided by its maximum value at the steady state. Taking the logarithm of the obtained normalized current, one obtains for the beginning decay transient a linear section from which the slope is the inverse decay time constant [20]: When the sample thickness increases, the diffusion coefficient value in replicate tests seems to become more reproducible. At sample thicknesses of 1 mm, the diffusion coefficient values were from 10 −9 to 10 −6 cm 2 /s. When the sample thickness was 1.5-2 mm, the range was from 10 −8 to 10 −6 cm 2 /s and with sample thicknesses of 4 mm from 10 −7 to 10 −6 cm 2 /s. is effect was clearer with 500 MPa and 700 MPa samples.
is effect agrees with the finding of Charca et al. [21] by using Armco iron. It is generally easier to attain solubility limits with thinner metallic samples than thicker ones, which contain more material defects and hence trapping sites. e diffusion coefficient was not dependent on the analysis method. e coefficient decreased slightly with increasing strength. e results presented in Table 2 show that both charging and discharging transients have similar diffusion coefficients. is is important for the argumentation, whether deep traps are significantly involved and filled during the charging.
Filling of traps would lead to an internal accumulation of hydrogen, and this hydrogen cannot be detected in the charging transient anymore. Since deep traps are preferentially filled before the hydrogen can diffuse on, the penetration of the hydrogen is delayed, which would show up in the analysis as changed diffusion coefficients. For the decay transient, when all the deep traps were already filled, the diffusion of the hydrogen would proceed normally. us, whenever there would be deep traps present, this should be visible as a significant difference in the diffusion coefficient determination methods for charging (3)(4)(5) compared to the discharge method (6).
at the diffusion coefficients obtained from the decay transient are similar to the other method shows, thus, that an involvement of traps for the tested steels can be neglected. at iron-based samples do not have a significant number of traps has been already mentioned in [22].
To evaluate a concrete problem case, the here obtained diffusion coefficients can be used to simulate the penetration of hydrogen during pickling and the release of it during the time between pickling and hot dip galvanizing. is, however, is very much depending on the dimensions of the steel product and on the degree of deformation within that product. All this we leave for future investigations.

Conclusions
e purpose of this paper was to study the intrusion of hydrogen during pickling of steel. Hydrogen permeation can be caused by pickling acid, such as HCl, instead of cathodic hydrogen production that is mainly used in investigations found in the literature.
No protective coating, for example, palladium on the anodic side of the sample, is required. Permeation measurement needs to be carried out after passivation of steel, and this low background current is to be subtracted from the measured current before the calculation of the diffusion coefficient.
e permeation rate of hydrogen into the steel and desorption rate from the steel are slightly lower for higher strength steel. is means that if these steels are plated soon after pickling, for example in hot-dip galvanizing, hydrogen has no time to leave steel and hydrogen-induced failures may occur.
Iron dissolved in pickling acid has no essential effect on the permeation of hydrogen.

Conflicts of Interest
e authors declare that they have no conflicts of interest.