Sep-2020

Types of corrosion and materials to combat them

A review of corrosion problems in fluid systems and how to prolong their life in one of the oil and gas industry’s most challenging environments.

GERHARD SCHIROKY
Swagelok Company

Article Summary

Large offshore platforms feature thousands of installed fluid system components and miles of tubing. Valves, tubing, and fittings are used in process facilities, seawater systems, and utility lines. These components face many corrosive threats, which can come from internally contained fluids or externally from seawater that forms chloride-rich deposits on equipment upon drying. Corrosion, if not detected in its early stages, can lead to costly shutdowns, repairs, and, in worst case scenarios, accidents. Therefore, regular inspections should be performed to assess the integrity of fluid systems. Systems should be constructed with readily available materials, have optimal corrosion resistance to the particular environment in which they are used, and be cost effective.

Historically, 316/316L stainless steel (SS) has been the preferred choice for constructing typical fluid systems. However, as more assets began operating in hot and humid climates, the limitations of this material became increasingly evident — most notably in the form of pitting corrosion on tubing, which can lead to perforations and leaks. When the advent of deep water well injection technology required fluid systems to operate at higher pressures, it became evident that alloys with better mechanical properties than 316/316L SS became preferred candidates for the material of construction of components. Finally, the production of oil and gas from increasingly sour reservoirs has led to the use of nickel alloys as preferred materials of construction.

Stainless steels and many nickel alloys consist of roughly 10 different elements, each of which provides the material with a distinct characteristic or property. The most important alloying additions to iron are nickel, chromium, and molybdenum — and, in some cases, nitrogen. The importance of chromium is illustrated in Figure 1a, which shows how this element reacts with ambient air to form a very thin, adherent, and protective oxide layer on stainless steel.

Various forms of corrosion, which are described in this article, threaten the integrity of materials. To select optimal materials of construction for fluid systems on offshore platforms, it is important to first understand the internal and external environments the components will see. Then, candidate materials can be identified and evaluated against different corrosive threats.

Identifying types of corrosion
Corrosion is a natural phenomenon that takes place as a metal physically degrades due to interactions with its environment. It occurs when a metal atom is oxidised by a fluid, leading to a loss of material in the metal surface (see Figure 1b). This loss reduces the wall thickness of a component and makes it more prone to mechanical failure. Many types of corrosion exist and several are detailed in this article, with each posing a threat that must be evaluated when selecting optimal materials for platform applications.

General (uniform) corrosion
General, or uniform, corrosion (see Figure 1c) is the easiest to spot and predict. It forms relatively uniformly across the surface of carbon or low alloy steel and appears as iron oxide scale, or rust, as the surface begins to break down. General corrosion rarely leads to disastrous failures — but it is not unheard of — and therefore is often regarded as an eyesore rather than a serious problem.

When protective paint no longer protects the structural carbon steel components of a platform, steel oxidises quickly. Water may transport rust and redeposit it on stainless steel parts. In such cases, stainless steel parts themselves may appear to have corroded, but they actually have been contaminated with the corrosion products of carbon steel. Excessive contamination may ultimately initiate corrosion of stainless steel.

Localised corrosion: pitting and crevice corrosion
Localised corrosion occurs in a concentrated area where the metal’s protective layer breaks down when exposed to fluids that contain chlorides. It is common in chloride containing acidic environments and in installations with crevices between metals or between a metal and a non-metal. Localised corrosion may occur in the form of pitting corrosion (see Figures 1b and 1d) or crevice corrosion (see Figures 1b and 1e), both of which are more difficult to detect than general corrosion. As a result, these types of corrosion can be more challenging to identify, predict, and design against.

Materials with higher critical pitting temperatures (CPT) and critical crevice corrosion temperatures (CCCT) are more resistant to localised corrosion. Potential solutions to combat both pitting and crevice corrosion include using 6-moly stainless steel, 2507 super duplex stainless steel or nickel alloys 625, C-276, or 400 (see Figure 2).1

Pitting corrosion
Common in high-chloride environments at elevated temperatures, pitting corrosion causes small cavities, or pits, to form on the surface of a material (see Figures 1b and 1d). It begins when the passive oxide layer on the metal’s surface breaks down, making the metal susceptible to the loss of electrons. When an electron from the metal escapes, iron in the metal dissolves into a solution in the bottom of the pit, diffuses toward the top, and ultimately oxidises to rust. As the pit gets deeper, the iron chloride solution concentration in the pit can increase and become more acidic, which in turn accelerates the pit growth. Eventually, the corrosion may lead to perforation of tubing walls and leaks.

Pitting corrosion is best prevented by selecting alloys with higher pitting resistance equivalent number (PREN) values. Different metals and alloys can be compared using their PREN, which is calculated from the chemical composition of the material (see Figure 3). PREN values increase with higher levels of chromium, molybdenum, and nitrogen, and higher values indicate greater pitting corrosion resistance.
 
Crevice corrosion
In a typical fluid system, crevices exist between tubing and tube supports or tube clamps, between adjacent tubing runs, and underneath dirt and deposits on component surfaces. The breakdown of the material’s protective oxide layer in these areas leads to the formation of small pits, which grow larger and deeper until they cover the surface of the entire crevice (see Figures 1b and 1e). Crevice corrosion can occur at far lower temperatures than pitting corrosion.