Copper Alloys For Corrosive Downhole Applications
Copper and copper alloys are widely used in many environments and applications because of their excellent corrosion resistance, which is coupled with other desirable properties, including superior electrical and thermal conductivity, ease of fabricating and joining, and resistance to biofouling. Copper corrodes at negligible rates in unpolluted air, water, and deaerated, nonoxidizing acids. Copper alloys resist many saline solutions, alkaline solutions, and organic chemicals. Copper and its alloys are susceptible to rapid attack in oxidizing acids, oxidizing heavy-metal salts, and sulfur and ammonia and their compounds. Air or dissolved oxygen in nonoxidizing acids accelerates otherwise low corrosion rates. Additionally, most coppers and copper alloys possess velocity limitations in flowing, aqueous liquid media
The most common way to discuss and catalog copper and its alloys are to divide them into the following primary families: coppers, high copper alloys, brasses, bronzes, copper nickels, and nickel silvers. The nominal composition of the various coppers and copper alloys included in the study are shown in Table 1
Coppers and high copper alloys
Pure copper is metal that has a designated minimum copper content of 99.3% or higher and is essentially pure copper. Wrought, high copper alloys, are those with a designated copper content less than 99.3% but more than 96% that do not fall into any other copper alloy group. The high copper alloys include cadmium copper, beryllium copper, chromium copper and leaded copper.
Pure copper and the high copper alloys can be considered to exhibit similar resistance to most corrosive environments. They possess excellent resistance to atmospheric environments and seawater corrosion and biofouling within certain velocity limits, where, if exceeded, they are susceptible to erosion-corrosion. As a group they possess limited resistance to sulfur environments, including moist H2S and SO2. As their corrosion resistance is generally less than that of the copper alloys, the coppers and high copper alloys are primarily used in applications that require enhanced mechanical properties, often at elevated temperatures, with good thermal and electrical conductivity. Beryllium copper is a good example of a high copper alloy that is becoming popular for downhole service as drill collar material due to its high strength, good resistance to dry H2S and chloride stress corrosion cracking, and low magnetic permeability
Brasses contain zinc as the primary alloying element, but may contain additional alloying elements including iron, aluminum, nickel and silicon. They are the most widely used and least expensive of the copper alloys. The brasses possess relatively good corrosion resistance, moderately high strength, and, in some compositions, exceptional ductility and forming characteristics, i.e., cartridge brass. They possess generally better resistance to non aqueous crude and sulfur compounds, compared with the coppers and other copper alloys; however, their resistance to CO2 and the nonoxidizing acids is not as good. The addition of lead to brass (leaded brass) increases machineability without sacrificing corrosion resistance
The brasses can be divided into two sub categories, those with less than 15% zinc and those with more than 15%. Brasses with more than 15% zinc are susceptible to dezincification in slightly acid or brackish waters containing sulfur compounds, and can stress crack upon exposure to ammonia and ammoniacal compounds. Stress corrosion cracking of high zinc brasses has occurred in stagnant, recirculating cooling water where ammonia was generated from decaying organic matter
Tin basses are brasses with small additions of tin. Tin significantly increases the corrosion resistance of some brasses, especially to dezincification, although it does not impart immunity. An example of a well-known tin brass is inhibited Admiralty brass
Bronzes are copper alloys where the principal alloying element is not zinc or nickel. There are several categories of bronzes; however, for downhole applications, this study was limited to the aluminum bronzes, based on their superior corrosion resistance, especially to produced waters and brines. The aluminum bronzes possess excellent resistance to impingement corrosion and high temperature oxidation. In most corrosive applications, the resistance of the aluminum bronzes is directly related to their aluminum content. The addition of nickel increases the resistance of the higher alloyed, ß-phase aluminum bronzes to cavitation, dealloying, and embrittlement. However, it should not be necessary to resort to the use of the nickel aluminum bronzes for downhole applications.
The aluminum bronzes are generally suitable for service in nonoxidizing mineral acids, including phosphoric, sulfuric and hydrochloric, in organic acids such as acetic, citric and formic, neutral saline solutions such as seawater or brines, and in alkalies, including sodium and potassium hydroxide. They possess marginal resistance to aqueous sulfur compounds. In all nonoxidizing acid environments, corrosion rates increase dramatically with the introduction of air or dissolved oxygen, as is true of all coppers and copper alloys. As one might expect, the aluminum bronzes, as well as the coppers and copper alloys, are not resistant to oxidizing acids, such as concentrated sulfuric, nitric and chromic acids. As with most of the copper alloys, aluminum bronzes are susceptible to SCC in the presence of ammonia or its compounds, such as ammonium hydroxide
The copper nickels have nickel as their primarily alloying element and are noted for their excellent aqueous corrosion resistance. They are superior to copper and other copper alloys in resisting acid solutions and are highly resistant to ammonia SCC and impingement attack.
Nickel silvers are essentially high zinc copper alloys with nickel additions. They possess good corrosion resistance; however, for downhole components, they offer no special advantages over the alloys listed in Table 2
Corrosion and Damage Mechanisms
Copper and copper alloys are susceptible to several forms of corrosion in downhole environments including: (1) general corrosion, (2) pitting, (3) dealloying, (4) galvanic corrosion, (5) erosion-corrosion, (6) stress corrosion cracking and, (7) intergranular corrosion. The following is a general summary of the above corrosion mechanisms and the relative resistance of copper alloys to those mechanisms
General corrosion is defined as well distributed metal loss of an ntire surface with little or no localized penetration. It is the least damaging form of corrosion and one that can be monitored through weight loss data. Environments that cause general corrosion at relatively low rates include fresh, brackish, and salt waters, neutral, alkaline and acid salt solutions and organic acids. Environments that cause general corrosion at faster rates include oxidizing acids, sulfur-bearing compounds and cyanides. All of the coppers and copper alloys in this study are susceptible to general corrosion to some degree, especially in oxidizing media, sulfur environments and aqueous amine solutions. Also, contributing to accelerated general corrosion of the coppers and high copper alloys are high sea water velocities
Pitting is one of the more damaging corrosion mechanisms because it can be unpredictable, it reduces load-carrying capacity, and creates stress concentration effects. Pitting is the usual form of corrosive attack on surfaces where there are incomplete protective films, nonprotective deposits of scale, or deposits of dirt or other foreign substances. Copper and its alloys are unique among the corrosion-resistant alloys in that they do not form a truly passive corrosion product film, unlike the chromium bearing, iron-based alloys. Pitting in seawater usually occurs at low water velocities, i.e., less than 2-3 feet/sec. Susceptibility to pitting is greatest when a copper alloy is initially placed in service, before a protective film is formed. The most pit resistant of the copper alloys are the aluminum bronzes with less than 8% aluminum and the low zinc brasses, i.e., red brass. Copper nickels and tin bronzes have intermediate pitting resistance and the high copper alloys and silicon bronzes are the most prone to pitting attack
Dealloying is a corrosion process in which the more active metal is selectively removed from an alloy, leaving behind a weak deposit of the more noble metal. The copper-zinc alloys containing more than 15% zinc are particularly prone to dealloying (with the copper alloys it is called dezincification). In dezincification of brass the selective removal of zinc leaves a weak and porous layer of copper and copper oxide. Dezincification can occur generally over the entire surface or it can occur as random plugs, called plug dezincification. Dezincification of uninhibited, high-zinc brasses typically occurs in waters high in oxygen and carbon dioxide, especially at areas of stagnant or low velocities. Tin additions help to resist dezincification, as well as additions of arsenic, phosphorus or antimony, which are added to Admiralty brass. Other copper alloys resistant to dezincification include red brass, commercial bronze, inhibited aluminum brass and the copper nickels. Aluminum bronzes with less than 8% aluminum are also fairly resistant, although not to the same extent as the copper nickels
Galvanic corrosion is the preferential corrosion of a metal or alloy in electrical contact with another metal or alloy in a conductive solution. Copper and the copper alloys are normally the cathodic member of a galvanic couple, thereby, resulting in accelerated corrosion of the other metal of alloy. Coppers can preferentially corrode when in contact with the high nickel alloys, titanium and graphite. With stainless steels the coppers may be the cathodic or anodic member, depending on exposure conditions. The range of corrosion potentials exhibited by the different coppers and copper alloys within the copper family is small enough that galvanic corrosion between two different copper alloys is normally not a problem.
Erosion-corrosion of coppers is characterized by cut grooves, gullets, elongated pits, ruts or gullies. Erosion-corrosion is most often found in waters containing low levels of sulfur compounds, and with polluted, contaminated, brackish or seawater. The coppers and high coppers are most susceptible to erosion-corrosion, with increasing resistance being conferred by the tin brasses, aluminum bronzes and copper nickels, in that order. Maximum velocities for the various copper alloy categories have been developed in order to avoid erosion-corrosion. The corrosive notes to Table 1 list published velocity limits for several copper alloys of interest.
Stress corrosion cracking
Stress corrosion cracking (SCC) is cracking of an alloy resulting from contact with a corrosive environment while under a tensile stress. Cracking appears to be spontaneous and can be intergranular or transgranular, depending on the environment and specific alloy. Cracking of coppers occur in environments where general corrosion rates are low and where the corrosion is localized.
Ammonia and ammonium compounds are the substances most often associated with SCC of copper alloys. For cracking to occur in ammonia, both moisture and oxygen must be present. Other environments that have been documented as also causing SCC of coppers include SO2, nitrites (sodium nitrite), nitrates, mercury, amines, oxides of nitrogen (nitric acid vapors), sulfates and steam. A chart listing cracking environments vs. copper alloys is contained in the Appendix to this report. With respect to cracking in ammonia compounds, a relative susceptibility ranking of the coppers and copper alloys considered in this study, from least to most resistant is as follows: (1) high zinc (>15%) brasses, (2) aluminum bronzes, (3) copper and high copper alloys and, (4) copper nickels.
Intergranular corrosion is infrequently encountered, occurring in applications involving high-pressure steam. The copper alloys most susceptible to intergranular corrosion include Muntz metal, admiralty brass, aluminum brasses, and silicon bronzes.
Environment vs. Corrosives Compatibility Chart
Corrosive Environment Definitions and Descriptions
Table 1 contains a list of alloys and their relative resistance to various, anticipated downhole corrosives. The list was compiled from various literature sources (see References) and is intended to be used as a guide for ranking copper alloys and selecting candidate materials for further testing. Where the corrosion resistance of an alloy differed between sources, a conservative approach was taken and the worse case data was used. Corrosion resistance considerations took in to account cracking mechanisms, as well as general and pitting corrosion.
The corrosion resistance category's E, G, F, and P are qualitative and are not necessarily associated with specific corrosion rates. For example, in several instances, a poor rating could be based on susceptibility to stress corrosion cracking, as opposed to general corrosion rates. However, alloys with Good to Excellent ratings should exhibit acceptable corrosion rates. The only caveat to the Good and Excellent ratings is that the associated environments are not well defined. The following are descriptions and definitions of the downhole environments listed in Table 2
Sweet Crude. Hydrocarbons containing low ppm levels of elemental sulfur or H2S concentrations defined as non sour by MR0175. Sulfur compounds in the absence of moisture are not particularly corrosive.
Sour Crude. Hydrocarbons with H2S contents meeting the definition of sour in MR0175, or those contain appreciable levels of elemental sulfur.
Sweet crude with water. Hydrocarbons with no sulfur and entrained or second phase produced water low in chlorides (<1000)
Sweet crude with water and dissolved gases. Crudes low in sulfur or sulfur compounds, with entrained or second phase water containing dissolved carbon dioxide.
Sour crude with water and dissolved gases. Sulfur containing hydrocarbons containing entrained or second phase water containing dissolved carbon dioxide.
Acids. Any of the inorganic or organic acids commonly used for acidizing or well stimulation, including hydrochloric, hydrofluoric, citric, acetic/formic or sulfamic acids at low to moderate concentrations and mild temperatures.
Alkalies. Aqueous solutions of potassium or sodium hydroxide at moderate concentrations and temperatures
Inhibitors. Any of the aqueous, amine-based inhibitors
Vapors. Condensed vapors of SO2, H2S, CO2 or acids.
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