Utilization of Nitrided 13 Chromium Stainless Steels for Oil Patch Valve Internals
Authors: Bednarowicz T.A. , Hartmann M.P. 
Published: October 2008

For various oil patch applications increased susceptibility to corrosive attack can be expected with the addition of a nitrided surface layer on 13 chromium stainless steel. The nitrided materials' vulnerability to degradation was found to be dependent on the type of nitriding process employed, test environment and the galvanic interaction with non-nitrided 13 chromium stainless steel.

INTRODUCTION

In an effort by valve design engineers to economically satisfy and improve the performance capability of internal dynamic sealing surfaces of oil field valves, various materials and surface treatments have been evaluated. The 13 chromium (Cr) stainless steel is commonly considered for applications when oil and gas containing measured amounts of carbon dioxide (C0 2), sea water brine (primarily NaCl), hydrogen sulfide (H 2 S), and possibly oxygen (O 2) are present. Surface treatments which are required to provide the combination of adequate corrosion (in service or in storage prior to service) and sufficient wear resistance include weld overlay, nickel or chrome plating, carbide coatings, and nitriding.

Presently, for many applications the nitrided 13Cr satisfies the mechanical functionality requirement of abrasive and galling wear resistance. Very little data, however, is available on its corrosion performance. It is therefore the purpose of this study to gain insight on the corrosion behavior of this product in various laboratory simulated oil patch type environments.

Two different nitriding processes applied to 13Cr were evaluated - gas (GNI3Cr) and liquid (LNI3Cr) - and for a base of reference, nonsurface treated 13Cr. Evaluation tests included salt fog exposure, electrochemical anodic polarization and galvanic coupling, and coupon exposure weight loss/localized corrosion. Various concentrations of NaCl, H 2 S, CO 2 and 0 2 (air) were utilized.

EXPERIMENTAL

The complete matrix of all tests performed in this study is outlined in Table I. For each series of tests non-nitrided 13Cr, GN13Cr and LN13Cr were evaluated.

Materials and Heat Treatment

The 13Cr alloy utilized in this study was one with a nominal 0.10% carbon content. It was heat treated through an austenitizing, quench hardening and double tempering to Rockwell C22 maximum.

The GN process was a typical commercial treatment which utilized cracked ammonia gas in a nitrogen atmosphere free of oxygen. The free nitrogen diffused into the 13Cr stainless steel matrix to form nitrides. These nitrides resulted in a high hardness phase within the metal matrix surface which increased the materials wear bearing and galling resistance. This process typically occurred at a temperature of 540 °C for a duration of 4 to 10 hours.

The LN process was a proprietary method where liquid salts were used to provide the necessary diffusional matrix nitrogen. The process basically occured in four steps which resulted in a hard compound surface layer in addition to the single nitride phase layer of the GN process. The temperature of the immersion steps ranged from 580 to 400 °C.

Specimens and Preparation

Salt fog tests utilized 150 × 75 × Z mm test panels. Prior to exposure, each was cleaned with acetone to remove grease, oil and other debris found after commercial nitriding.