Cycled hydrogen permeation through Armco iron – A joint experimental and modeling approach

Highlights

• Cycled permeation experiments are evaluated with physically sound diffusion models.

• Use of Armco iron eliminates the role of defects due to alloying and precipitation.

• Hydrogen trap sites are identified and their densities and energies are determined.

• Fully parametrized diffusion and trapping model is provided.

• Microstructure-property relations for trap density evolution are presented.

Abstract

Understanding hydrogen embrittlement in steels requires research in hydrogen diffusion and trapping at microstructural defects. The present paper deals with hydrogen permeation and trapping at defects in the base material, Armco iron, eliminating effects coupled with alloying and precipitation. Cycled permeation curves are recorded and evaluated by using sound diffusion models to identify hydrogen trap sites as dislocations, grain boundaries and vacancies and assign their trapping energies. Furthermore, trap densities are evaluated and used together with the trapping energies as parameters in an adapted diffusion equation for hydrogen, interpreting the experiments significantly better than simple use of classical Fick’s laws.

Introduction

Hydrogen embrittlement in ferritic steels is a permanent research topic already since several decades. This rather complex problem includes understanding of both the fracture mechanisms in the presence of mobile hydrogen in steels and the transport mechanisms together with trapping of hydrogen at defects. The current paper is devoted to the second part of the problem.

State of the art concerning the fracture mechanism occurring due to hydrogen embrittlement is published in nearly numerous articles, see e.g. [[1], [2], [3], [4], [5], [6]].

The transport of hydrogen in ferritic low alloyed steels occurs by interstitial diffusion in the lattice due jumping of hydrogen atoms from one tetragonal interstitial site to another [7]. The low barrier for jumping is reflected in low activation energy of diffusion of hydrogen in steels. Thus, hydrogen diffusion is active also at temperatures significantly below 0 °C. Low alloyed ferritic steels in industrial products are systems with a thermomechanical history reflected in complex microstructures including several microstructural objects as:

• dislocations,

• grain boundaries and subgrain boundaries,

• precipitates,

• atoms of alloying elements and their aggregates.

All these microstructural objects may act as traps of different density and depth. If one wants to understand the role of the individual traps, it is necessary to limit the number of microstructure features and vary their densities. This can be realized by using pure iron with no precipitates and no alloying elements as object of a study. Moreover, the dislocation density and the grain size can be controlled in rather wide ranges by proper thermomechanical treatment. There are some recent investigations aiming in a similar direction in literature [[8], [9], [10]] but none of them provide such a systematic and complete variation of combined crystal defects in pure iron as in the present contribution.

This paper is based on the evaluation of experiments by using the diffusion and trapping concepts developed by some of the authors in the last decade. The theoretical works concerning general aspects of the trapping concept can be found in [[11], [12], [13]]. Its application to hydrogen diffusion is described in detail in [[14], [15], [16]]. Of course, we appreciate the pioneering and valuable literature published on this rather subtle topic in the last six decades. Relevant references are placed in the following sections.