Elsevier

Talanta

Volume 125, 1 July 2014, Pages 257-264
Talanta

An electrochemical calibration unit for hydrogen analysers

https://doi.org/10.1016/j.talanta.2014.02.008Get rights and content

Highlights

  • An electrochemical unit for hydrogen analysers has been developed and tested.

  • Hydrogen as well as Deuterium can be produced in-situ as calibration standards.

  • Current and time allow controlling amount and release kinetics of hydrogen.

  • Effusion transients of hydrogen analysers can be simulated using I/t-programs.

  • The setup is inexpensive and can be easily coupled with existing analysers.

Abstract

Determination of hydrogen in solids such as high strength steels or other metals in the ppb or ppm range requires hot-extraction or melt-extraction. Calibration of commercially available hydrogen analysers is performed either by certified reference materials CRMs, often having limited availability and reliability or by gas dosing for which the determined value significantly depends on atmospheric pressure and the construction of the gas dosing valve. The sharp and sudden appearance of very high gas concentrations from gas dosing is very different from real effusion transients and is therefore another source of errors. To overcome these limitations, an electrochemical calibration method for hydrogen analysers was developed and employed in this work. Exactly quantifiable, faradaic amounts of hydrogen can be produced in an electrochemical reaction and detected by the hydrogen analyser. The amount of hydrogen is exactly known from the transferred charge in the reaction following Faradays law; and the current time program determines the apparent hydrogen effusion transient. Random effusion transient shaping becomes possible to fully comply with real samples. Evolution time and current were varied for determining a quantitative relationship. The device was used to produce either diprotium (H2) or dideuterium (D2) from the corresponding electrolytes. The functional principle is electrochemical in nature and thus an automation is straightforward, can be easily implemented at an affordable price of 1–5% of the hydrogen analysers price.

Introduction

High strength steels [1], [2], [3] and steels for critical applications, such as pipe lines [4], [5], [6], or parts in aggressive aqueous environments [7], [8], [9] are highly susceptible to hydrogen embrittlement. The sources of hydrogen are different in nature, some being uptake during acid pickling and electroplating [10], [11], welding [12], [13], corrosion [14], galvanic coupling [15], [16], cathodic protection [17], tribochemical reactions [18], and others more. Higher hydrogen concentrations enhance the risk of hydrogen embrittlement resulting in the continuous or sometimes sudden degradation of the mechanical properties, such as strength and ductility. This leads to a shorter lifespan of steel products and possible critical failures of the material. Quantification of hydrogen in metals for industrial applications and in laboratory tests is one the key parameters to predict the risk or probability of hydrogen induced embrittlement. Reliable analysis of the hydrogen concentration inside the steel is therefore of great importance.

The melt extraction technique is being widely used for the analysis of total hydrogen contents of steels [19], [20]. In this method a sample is heated up to 2500–3000 °C inside pyrocoated graphite crucible using an electrical impulse furnace. Released gases such as hydrogen and oxygen are fed into the thermal conductivity detector or infrared detector [19] by means of a carrier gas (usually nitrogen). The melt extraction with carrier gas has found widespread industrial application due to the uncomplicated design making routine hydrogen analysis at ambient pressure fast and cheap as compared to (ultra high vacuum) UHV-based techniques, like TDA (thermal desorption analysis) [21], [22], [23]. Most steel plants have some kind of online analysis system in which hot samples from the production process can be analysed during production.

Typically the sample is put into the inner one-way crucible situated inside the outer reusable crucible, placed between two dc electrodes. The sample is being molten during the analysis and analytes are transported by the carrier gas into the detector. This determination technique can be also employed in analysing coated steel sheets. A large number of influencing factors such as sample preparation, cleaning, treatment, coating removal and others more were thoroughly investigated and compared using various Round Robin tests in a European multi partner project that was recently concluded [24]. Parts of this work have been made public in the course of this comprehensive project [25], [26].

The instruments used in melt extraction analysis are usually being calibrated either by gas calibration or with certified reference materials (CRM’s) of stable and precisely known hydrogen concentrations. To perform the gas calibration, a gas dosage valve adjusted to some fixed volume and filled with helium is used. Helium, having a very similar thermal conductivity as hydrogen, is normally used due to the cost reasons to save CRM’s and security restrictions and explosion danger attributed to the work with hydrogen gas. Certified reference materials, usually titanium alloys pre-charged with known amounts of hydrogen, which are stable over long periods of time, are being proposed on the market from different companies. A number of certified hydrogen concentrations are available. Both calibration methods have their pros and cons.

Disadvantages are

  • difficulties with an exact measurement of the calibration gas’ volume

  • coefficients are needed to correct the differences in the thermoconductivity

  • wide ranges of possible hydrogen concentrations are not covered with the CRM’s

  • time stability of CRM’s

  • high purchasing costs of CRM’s

  • irregularities with their charging

  • strongly pronounced tailing of hydrogen transients from metallic samples to name only a few

To overcome these limitations an electrochemical calibration method for hydrogen analysers was developed and employed in this work. Exactly quantifiable amounts of hydrogen can be produced in an electrochemical reaction and detected by the hydrogen analyser. The amount of hydrogen is exactly known from the transferred charge in the reaction following Faradays law; and the current time program determines the apparent hydrogen effusion transient.

Section snippets

Setup

All of the measurements were carried out using either an Eltra OH 900 gas analyzer (Fig. 1) or a Leco RH 402 gas analyzer both equipped with a thermal conductivity detector. The samples are molten inside the graphite crucible at approximately 2200 °C. Hydrogen and oxygen diffusing out from the melt are transported to the detector with a nitrogen carrier gas. Carrier gas is pre-cleaned using a packed bed of sodium hydroxide and with magnesium perchlorate, respectively, to remove CO2 and any

Results and discussion

A typical sample weight in hydrogen determination in metallic samples is 1 g. Calibration standards are often used to cover a range of up to e.g. 10.0 ppm, which is equal to 10.0 μg g−1 of hydrogen. For calculating the amount of electrical charge required to evolve 10 µg of hydrogen, Eq. 1 can be used:Q=m(H2)×z×FM(H2)where: Q is electrical charge, z is the stoichiometric equivalent number and F is the Faraday constant, m(H2) is the weight of hydrogen formed and M(H2) is the molar mass. Computing

Conclusions

Accurately defined quantities of hydrogen and deuterium were produced by consumed charge control using an electrochemical setup connected to the hydrogen analyser. These amounts of gas can be used for the direct calibration of the detector. The electrochemical calibration is superior to the gas calibration, being precisely controllable, multipoint, linear calibration method. Among the major advantages is also the shape of the signal which is similar to that obtained during melt extraction of

Acknowledgment

The financial support of the European Commission within the Framework Research Fund for Coal and Steel through the project Hppm is gratefully acknowledged.

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