Abstract

A standard 1% Ni weld consumable typical of that used in the Oil industry was utilised to investigate the factors which influence the efficiency of corrosion inhibitors in preventing weld corrosion. Testing was conducted on a 5-segment weld electrode in standard brine under sweet conditions. Pre-corrosion times of between 2 hours to 5 days were employed to assess the affect of different levels of synergist in the inhibitor and what affect this had on corrosion inhibition. Electrochemical techniques including linear polarisation resistance, galvanic current and potential measurements were made to assess the influence of pre-corrosion time and the chemistry of the inhibitor. The results indicated that the length of pre-corrosion had a significant influence on the time required for the corrosion inhibitor to reduce corrosion. It was also concluded that the rate of inhibition could be controlled by application of the appropriate amount of synergist for a particular system.

Introduction

Recent research into the field of corrosion inhibitors in the oil industry has focused on the control and prevention of preferential weld corrosion1. This resulted from a number of cases where, after prolonged service, nickel containing welds were identified to be prone to localised attack, with the weld preferentially corroding while the Heat Affected Zone (HAZ) and parent metal remained unaffected2. This type of attack occurs due to the complex mix of metal phases in the weldment, which are formed due to the heat applied during the welding process and have no fewer than 9 different zones3. The driving force for the corrosion process in weldments is the parent metal. This has a larger surface area and therefore a larger reaction rate will occur at this site. In order to maintain equilibrium, the reactions at the other surfaces need to balance this reaction. It is known that the main effects are seen from the coupling of the weld with the HAZ and the parent metals, therefore the testing of the weldment is normally simplified into looking at these three zones; the weld bead, HAZ and parent metal4. Previous work in the 1980's had identified 1% nickel welds as the most corrosion resistant consumable, while maintaining the required strength for applications such as seawater injection lines, topsides piping and wet gas flowlines5.

Case studies and joint industry projects have identified wet gas systems to be more commonly prone to preferential weld attack than crude oil production systems, although instances in these systems have also been cited. It has also been reported that the low salinity brine in these systems, under sweet conditions, could cause preferential corrosion by limiting galvanic protection afforded to the weld consumable6. Of particular significance was the finding that corrosion inhibitors used in the systems, rather than slowing this process, could accelerate the corrosion process.

Recent studies have shown that if a corrosion inhibitor is applied in the correct manner, the preferential weld corrosion can be controlled7. Generally ‘conventional’ inhibitors have been considered to control preferential weld corrosion, with some success but at higher dose rates than would normally be applied to ensure satisfactory protection. As a consequence of the problems associated with controlling preferential weld corrosion and the limited efficiency of conventional corrosion inhibitors, a study was conducted to look at key factors influencing their performance. The length of pre-corrosion a weld consumable is exposed to has been suggested to have a significant effect on the performance of a corrosion inhibitor. This study aimed to evaluate corrosion inhibitors under identical test conditions but with varying pre-corrosion times ranging from a few hours to several days. 1% Nickel weld consumables are generally cathodic to the parent metal and will therefore be protected from preferential weld corrosion. However due to environment changes and the use of some types of inhibitors the weld can become anodic to the parent metal and localised corrosion of the weld can take place. It is therefore vitally important that evaluation of corrosion behaviours on weldments is carried out where the weld, HAZ and parent metal are considered collectively.

This content is only available via PDF.
You can access this article if you purchase or spend a download.