Failure Analysis

Corrosion Failures

Corrosion is one of the greatest problems encountered with produced fluids and accounts for about one-half of all sucker rod failures. Corrosion is the destructive result of an electrochemical reaction between the steel used in making sucker rods and the operating environment to which it is subjected. Simply put, corrosion is nature's way of reverting a man-made material of a higher energy state (steel), back to its basic condition (native ore) as it is found in nature. The elemental iron in steel combines with moisture or acids, to form other compounds such as iron oxide, sulfide, carbonate, etc. Some form and concentration of water is present in all wells considered corrosive and most contain considerable quantities of dissolved impurities and gases. For instance, carbon dioxide (CO2) and hydrogen sulfide (H2S) acid gases, common in most wells, are highly soluble and readily dissolve in water which tends to lower its pH. The corrosivity of the water is a function of the amount of these two gases that are held in solution. All waters with low pH values are considered corrosive to steel, with lower values representing greater acidity, or corrosiveness.

All downhole environments are corrosive to some degree. Some corrosive fluids may be considered non-corrosive if the corrosion penetration rate, recorded as mils of thickness lost per year (mpy), is low enough that it will not cause problems. However, most producing wells are plagued by corrosion problems and no currently manufactured sucker rod can successfully withstand the effects of this corrosion alone. While corrosion cannot be completely eliminated it is possible to control its reaction. All grades of sucker rods must be adequately protected through the use of effective chemical inhibition programs (reference current editions of API Specification 11BR and NACE Standard RPO195). Some sucker rod grades, due to different combinations of alloying elements, microstructures and hardness levels, are capable of longer service life in inhibited corrosive wells than other grades of either low or high tensile strength.

Why do new sucker rods seem to corrode faster than older rods in the same string? Two sucker rods with the same chemical analysis will form a galvanic corrosion cell if the physical condition of one is different from the other. Physical differences in a sucker rod may be caused from poor care and handling practices (i.e. surface damage resulting in bruises, nicks, bends) and/or corrosion deposits. Since new sucker rods go into the well without corrosion deposits, they often corrode preferentially in relation to rods that are coated with corrosion deposits. Corrosion on steel starts very aggressively but often slows down as soon as an obstructive surface film of corrosion deposit (scale) is formed upon the metal surface. For example, CO2 generates iron carbonate scale as a by-product of its corrosion. This scale coats the sucker rod and retards the corrosion penetration rate, which tends to slow down corrosion. However, if this deposit is continuously cracked by a bending movement or removed by abrasion, aggressive local corrosion continues on the area with the scale removed, and results in deep corrosion pitting.

Can high tensile strength sucker rods be used in a corrosive environment? Generally soft rods tolerate corrosion better than hard rods and, as a rule of thumb, you should always use the softest rod that will handle the load. However, if load requirements dictate the use of high tensile strength rods then it is important to protect the rods with an effective surface film of corrosion inhibitor. In most cases, if you can adequately protect downhole equipment from corrosion, you should be able to adequately protect high tensile strength rods from corrosion by increasing the application frequency of the corrosion-inhibitor program. In other words, if you effectively batch treat once a week with 40 parts per million (ppm) of corrosion inhibitor for D class rods, you will need to batch treat twice weekly with 40 ppm of corrosion inhibitor for high tensile strength rods. Treatment volumes vary and are dependent upon many factors too numerous to list. Always consult with a corrosion control specialist prior to the installation of every rod string, especially when stress corrosion fatigue is suspected as the failure root cause.

Figure 18 is an example of stress-corrosion fatigue from CO2 corrosion. The size of the pit, as far as when it becomes detrimental to the rod, depends on two factors-material type and hardness. Class K sucker rods may develop deeper and larger pits than a Class D sucker rod before it becomes detrimental to the rods. Class D sucker rods may develop deeper and larger pits than a high tensile strength rod before it becomes detrimental to the rods. Softer materials with lower rod stress tolerate larger pits than do harder materials with higher rod stress. Therefore, small pits can be detrimental to higher tensile strength sucker rods as opposed to a softer rod that does not have as much rod stress.