Sulfuric and hydrochloric acids are among the most common chemicals produced.in the process industry. Nickel alloys have been a traditional material of choice. This paper will review the alloys available for this service as well as identify the temperature limits and other conditions that should be considered when selecting an alloy.
Titanium does not show the required mechanical strength for high temperature high pressure applications and it can only be used to form liners for an SCWO apparatus. Therefore, pressure tubes made of alloy 625 were lined with titanium grade 2, Additionally corrosion tests with coupons made of titanium grades 2, 5, 7, 12 and P-C were performed.
The high performance alloy UNS N06058 contains about 21 wt.-% chromium and at least 18.5 wt.-% molybdenum. It is the first member of the Ni-Cr-Mo alloys intentionally strengthened with nitrogen. Based in the combination of its alloying constitutes this alloy is highly resistant to oxidizing as well as to reducing corrosive media. Evidenced by the high Pitting Resistance Equivalent (PRE) number of about 86 the alloy UNS N06058 shows an outstanding resistance to localized corrosion as well.
The objective of this paper is to outline the characteristics of alloy UNS N06058 and therefore to highlight the successful use of it in demanding corrosive applications in the process industry and other sectors.
Main characteristics such as microstructure, mechanical properties and fabrication will be discussed. The very good weldability of alloy UNS N06058 has been verified by the German TÜV1. A matching filler metal, which is officially qualified according to AWS 5.14 (American Welding Society)2, is available. In addition, prior work shows that alloy UNS N06058 can be reliably explosion clad. Furthermore, corrosion resistance determined under laboratory conditions as well as in field studies will be presented not only for the base material but also exemplary on welded and cladded samples. Examples of applications will complete the comprehensive overview of this work.
Coiled tubing is defined as a continuous tubular product that is used for oil and gas well interventions. Its popularity continues to grow due to its versatility and speed of operation. Though superior grades of metal alloys exist in terms of corrosion resistance, coiled tubing operations primarily employ high-strength low-alloy steels because of their availability, lower cost and weldability. The low-alloy steel can also be thermo-mechanically controlled to elicit specific material properties, such as yield strength and ductility. These coiled tubing steels are often introduced into potentially corrosive downhole conditions, therefore proper testing must be completed to ensure adequate corrosion protection prior to job execution. Downhole corrosive conditions often encountered include; oxygen saturated fluids, elevated temperatures, exposure to oxidizing agents, hydrochloric acid and highly concentrated brines. Often these fluids will be recirculated in a closed loop system, consistently re-exposing equipment to potentially damaging conditions. Frequently, these challenging conditions faced are tested individually with pressurized mass loss coupon testing at bottom hole conditions. However, due to a recent coiled tubing incident in which the coiled tubing pipe had completely parted downhole, the post-job incident investigation involving SEM and metallographic analysis revealed pitting corrosion throughout the tubing, despite the pre-job testing performed indicating adequate acid corrosion protection for the entirety of the job. A literature review indicated very little research was available involving the possible interaction of brine solutions and diluted acid on coiled tubing carbon steels. This paper aims to investigate the possible corrosive interactions between salt brines and inhibited acid blends at elevated temperatures on high grade coiled tubing coupon samples through metallographic examinations and mass loss tests in pressurized heated cells. Coiled tubing coupons will be exposed to a variety of acid blends diluted with a 10% brine (8% wt NaCl and 2% wt CaCl2) or fresh water to investigate the possibility of corrosion enhancement between saline fluids in a diluted acid system.