ABSTRACT

The nickel-based alloys UNS N06625 and N08825 are today commonly used in the Oil & Gas industry for piping or pressure vessels applications. The material selection is mainly driven by the corrosivity of the environment in terms of chloride concentration, temperature, pH and H2S partial pressure. For these applications, several mechanical characteristics are generally required. Clad plates represent therefore a cost-effective solution, combining the corrosion resistance of a nickel alloy layer (thickness around 3 mm - 1/8 inch) to the mechanical properties of a carbon-steel backer material.

This paper will be dedicated to the corrosion resistance of roll-bonded UNS N06625 and N08825 clad materials. Special attention will be paid to their resistance to stress corrosion cracking which is the primary cracking mechanism that shall be considered for nickel alloys as per NACE MR 0175 / ISO 15156 part 3. Results regarding their pitting corrosion and intergranular corrosion resistance will also be provided.

INTRODUCTION

The nickel-based alloys UNS N06625 and N08825 are today commonly used in the Oil & Gas industry for piping or pressure vessels applications. The material selection is mainly driven by the corrosivity of the environment in terms of chloride concentration, temperature, pH and H2S partial pressure. For economic reasons, it is now common to use clad plates, consisting of a thin layer of corrosion-resistant alloy combined to a thicker carbon-steel backing material. The cladding alloy ensures corrosion resistance properties while the backer steel provides mechanical strength and toughness.

One way to produce clad plates is to co-laminate both steel layers by means of the roll-bonded process. The surfaces are assembled together by combining high temperature and hot-rolling. Atoms from both sides interact to create a new bond. In order to obtain the required mechanical properties of the carbon-steel backer, an appropriate heat treatment has to be performed. It has to be carefully selected not to have any significant detrimental impact on the corrosion resistance of the clad material caused by the formation of chromium carbides at the grain boundaries.

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