Guidelines For the Use of Copper Alloys In Seawater: Page 2

• Page 1
• Effect of Sulfides
• Marine Biofouling
• Parting Corrosion
• Stress Corrosion Cracking
• Galling and Seizing Resistance
• Desalination Environments
• Summary
• References

Effect of Sulfides

In considering the effect of sulfides, it is useful to distinguish several conditions:

1. Exposure to polluted sulfide waters during ship outfitting and extended power plant start-up periods;
2. Exposure to sulfides in well-aerated waters in which sulfides may occur temporarily during dredging, red tides, or other transitory, nonequilibrium conditions;
3. Exposure to sulfides generated by sulfate-reducing bacteria beneath sediment and other deposits that are allowed to remain unremoved in tubing;
4. Long-term presence of sulfides in polluted waters; and
5. Alternate exposure to aerated and sulfide-containing waters as occurs with each change of tide at some power plants.

Syrett noted that sulfides in saline waters are not particularly detrimental to copper alloys unless dissolved oxygen is also present or unless exposure to oxygen-free, sulfide-polluted waters is followed by exposure to aerated, unpolluted waters. The author found that in the complete absence of oxygen, corrosion rates remained low up to sulfide concentrations as high as 55 g/m ³ and at flow velocities up to 5 m/s. The corrosion rate remains low in the absence of oxygen, despite the more active potential, since the only cathodic reaction is hydrogen reduction (Point 1, Figure 8). In polluted waters with both oxygen and sulfide present, the cathodic reaction reverts to oxygen reduction with a much higher corrosion rate, as indicated by the large increase in current despite the more noble potential (Point 2, Figure 8). The potential at Point 2 is somewhat below that at which a stable cuprous oxide film can form, thus permitting the high corrosion rate to persist until either the oxygen or sulfide is consumed in the corrosion process.

Figure 8. Influence of sulfide and oxygen on the corrosion current in a copper-nickel alloy exposed to flowing seawater.

Syrett also showed that the typical black sulfide film that forms during exposure to sulfide-polluted seawater will eventually be replaced by a normal oxide film when the sulfide-polluted seawater is replaced by clean, aerated seawater. However, substantially higher corrosion rates persist for some time during the transition period. These conditions commonly arise when vessels are fitted out in polluted harbors and later operate in the open sea. Experience indicates that once the vessel begins regular operation, the normal protective film forms and persists during subsequent harbor visits. Syrett found that the normal protective film replaced the sulfide film in ~9 days.

Some coastal power plants have encountered sulfide corrosion problems during extended start-up periods when little or no attention was given to condenser lay-up procedures when the unit went down. once the condenser was again placed in operation, a normal film gradually replaced the sulfide film. When properly done, chemical cleaning with inhibited hydrochloric acid removes the sulfide film and expedites formation of the protective film.

Coping with sulfides

Start-up periods: Keep the seawater circulating. Aerate and keep the pH neutral or above. Drain and air-blow dry for standby periods of more than 3 to 4 days.

Transitory sulfides in normally aerated waters: Eliminate the source of sulfides. Return to normal clean water operation as soon as possible. Normal vessel turnaround times in polluted harbors are routine and have seldom led to significant corrosion problems.

Unremoved deposits: Clean on a regular schedule by water flushing, water lacing, and/or shooting with nonmetallic brushes to remove deposits, restore heat transfer, and prevent corrosion under the sediment. Cleaning intervals of 2 to 6 months are normal.

Long-term exposure to deaerated sulfide-containing waters: Not generally recommended.

Alternate exposure to sulfide-polluted and aerated waters: Where occurring daily with tide changes, copper alloys are not recommended. Where alternative exposure seldom occurs, the film formed on the tubes during normal operation provides adequate protection to all but the most prolonged exposures.

Marine Biofouling

Biological organisms abound in the open sea, coastal estuaries, and rivers. They are found on piers and pilings, boat hulls, offshore oil platforms, other marine structures, and within piping and condensers. Copper and copper alloys are more resistant to the attachment of biofouling organisms than steel and most of the other common materials of construction. Efird documented the resistance of C70600 to biofouling in quiet seawater over an 18-month period, as shown in Figure 9. ¹³ In the absence of wave action or velocities above 0.5 fps (0.15 m/s), the slime layer gradually thickens to the point at which biofoulers begin to attach to the thick slime layer after ~18 months. With the wave action on offshore platforms or with normal flow velocities in cooling water systems, the slime layer never reaches a thickness that permits biofouler attachment.

Figure 9. Progression of biofouling from 3 to 18 months on Alloy C70600 in seawater.

The normal condition of copper alloy surfaces in seawater is illustrated by the clean condition of the C70600 hull of the copper mariner after 52 months of sea time, as shown in Figure 10. A few pin head-size barnacles have been able to attach, but most have been removed by wave action underway. Those remaining could be removed easily by finger pressure. Operation had kept the hull almost as clean and smooth as it was originally. It is both the inherent resistance to biofouler attachment and the poor adherence of the occasional biofouler able to attach that made copper so useful as hull sheathing for sailing vessels and copper-nickel so useful for the piping, water boxes, and other components of marine cooling water systems.

Figure 10. Small barnacles at the waterline on the bow area of Copper Mariner after 52 months of sea time.

Copper alloys are also resistant to the microfouling that occurs within condenser and heat exchanger tubing. Lewis measured the time required for microfouling to significantly reduce the heat transfer of C70600 tubing in clean seawater at 6 to 8 fps (1.8 to 2.4 m/s) over a 180-day period ( Figure 11). ¹⁴ While not immune to microfouling, the indicated 90- to 110-day interval between cleaning for C70600 is an order of magnitude greater than the 10-day cleaning interval found necessary for other than copper alloy condenser tubing in this study.

Figure 11. Resistance of Alloy C70600 to heat transfer resulting from the growth of a microfouling film on the inside wall of tube in clean seawater.

The resistance to biofouling is an inherent characteristic of copper alloys and appears to be associated with copper ion formation within the corrosion product film. Coupling to steel or less noble materials or cathodic protection, which suppresses copper ion formation, allows biofouling to occur on copper alloys as readily as on other materials.

Biocides

Chlorine is a widely used and effective biocide when injected continuously so that a 0.2 to 0.5 ppm residual is maintained at the outlet tubesheet of a power plant condenser. Intermittent injection, as usually practiced, is only partially effective at best. Barnacles, mussels, and other hard-shelled organisms possess the ability to tightly close their shells when first sensing a toxic substance such as chlorine in the water, to remain closed until it passes, and then to reopen and resume feeding. Copper alloy tubing is resistant to chlorination at concentrations required to control biofouling. 0verchlorination can damage copper alloy tubing, however.

Parting Corrosion

Dezincification

Brass alloys with more than 15% zinc, such as Admiralty, aluminum brass, Muntz metal, naval brass, and manganese bronze, were susceptible to a specific form of corrosion in seawater termed dezincification (or parting corrosion). A porous, spongy layer or plug of copper devoid of zinc developed on the metal surface when in seawater service or stagnant environments. In some cases, the layer was superficial in depth; in others, it extended completely through the wall.

Dezincification is rarely encountered today except in the case of yellow brass, which is not normally produced with an inhibitor. The addition of 1% tin to Admiralty, naval brass, and manganese bronze reduces the tendency toward dezincification. The further addition of a few hundredths of a percent of arsenic effectively prevents dezincification in high-zinc alloys, such as C44300, C68700, C36600, and C46500. Antimony and phosphorus are also effective as dezincification inhibitors but are less commonly used. Other than the 1% tin content, which is not fully effective alone, the cast manganese bronze propeller Alloy C86500 is produced without an inhibitor. The steel hull and its cathodic protection system provide considerable protection for the propeller except at the outer portion of the blades, where velocity is highest.

Dealuminification

Dealuminification in the 5 to 8% aluminum bronzes is occasionally reported, but it is not a significant problem until aluminum reaches the 9 to 11% range. The cast aluminum bronzes containing 1 to 5% nickel have been widely used as pumps, valves, and propellers in marine service, with varying degrees of dealuminification-related problems. Ferrara and Caton reported on an extensive Navy study of aluminum bronze components from Naval vessels. ¹⁵ The authors concluded, "Dealloying in NAB (nickel-aluminum bronze) castings containing 4% minimum nickel appears to be limited to approximately 6 mm (1/4 in.) for service times up to 15 years." The composition of Alloy C95800 has been adjusted in accordance with their findings. The Navy specification for C95800 pump and valve components (MIL-B-24480) requires a 6-h final anneal at 1250 F (677 C). The C95500 composition changes and the 1250 F (677 C) anneal are designed to minimize dealuminification. The propeller alloy (C95500) and the Navy specification Mil-B-21230 remained unchanged. Weld repairs are permitted and routinely made on the nickel-aluminum bronze propeller alloy without sensitizing the alloy to stress cracking or dealuminification in later service.

Denickelification

Denickelification is occasionally reported in the higher-nickel content copper-nickel alloys in refinery overhead-condenser service, where hydrocarbon streams condense at temperatures above 300 F (149 C). The problem appears to be associated with hot spots that develop in the tubing as a result of fouling and thermogalvanic differences that arise. Although extensive studies have not been undertaken, the solution is more frequent cleaning to remove deposits that could lead to hot spots and/or increasing flow rate to avoid deposits.

Stress Corrosion Cracking

The higher-zinc content brasses are subject to stress corrosion in the presence of air (oxygen), water vapor, and traces of ammonia if the alloy is in the stressed (tempered) condition. These conditions can arise in marine atmospheres, in storage (the ammonia coming from such sources as rat and sea gull waste), as well as from operations. While it is rarely encountered in feedwater heaters and condensers because of their low oxygen content, air inleakage during downtime periods should be minimized.

Thompson studied the resistance of copper-base alloys to stress corrosion cracking (SCC). ¹⁶ Table 8 from his work shows the relative resistance of copper-base alloys to ammonia SCC. The aluminum bronze alloys exhibit a resistance greater by an order of magnitude. Copper and copper-nickel alloys are practically immune in service. There are some reports of the stress corrosion of copper-base alloys by sulfur dioxide, sulfamic and nitric acids, copper nitrate, industrial atmospheres, mercury, and other media, but the principal causative agents, if the other prerequisites are present, are ammonia or mercury.

Although aluminum bronze is resistant to ammoniacal SCC, it is susceptible in steam or hot water. The 0.2 to 0.5 tin addition in Alloy C61300 is sufficient to suppress the SCC tendency in steam. Some investigators believe that the tin addition also improves the resistance to ammoniacal SCC, but this has not been well established, probably because the inherent resistance is adequate for most applications.

When welded with a matching filler metal, aluminum bronze becomes susceptible to SCC in marine and desalination environments. Microfissuring of both the weld metal and heat-affected zones leaves these areas prone to parting corrosion and SCC. The remedy is to weld with a higher-aluminum content (10%) duplex filler metal. The final pass is made with nickel-aluminum bronze to minimize the possibility of galvanic corrosion of the higher-aluminum content filler metal. ¹⁶

Nickel-aluminum bronze propellers are routinely and successfully welded with nickel-aluminum bronze filler metal without increasing the SCC susceptibility in service. Manganese bronze propellers are stress relief annealed after weld repair to reduce the possibility of SCC.

Galling and Seizing Resistance

Copper-base alloys are well known for their wear resistance, but less known for their outstanding resistance to galling and seizing. The tin bronzes have been used for components in the sliding mechanisms of naval ordnance, field artillery, and machine guns, which must resist galling and seizing under the extreme impact loadings of recoil. Lubrication is often marginal or nonexistent.

One popular galling test is made in a Brinell hardness tester. A small polished button of one material is slowly rotated 360 degrees against a flat polished plate of the second under increasing deadweight loads. The threshold galling stress is defined as the maximum load before a distinct weld junction can be observed at 10 times magnification. This usually occurs at the end of a wear track. This test method provides a relative rating (in psi) for the two materials in contact. The test has been used to develop ratings for SSs, nickel-base alloys, several steel alloys, and silicon bronze. ¹⁸ The data are given in Table 9.

While the threshold galling stress is low for both silicon bronze and AISI 304 SS when tested against themselves, there is an order of magnitude improvement when silicon bronze is run against AISI 304 SS. The applicability of these test data is probably limited to slowly moving, heavily loaded, poorly lubricated components.

Semenov studied and defined the resistance of copper-base alloys to galling and seizing as a function of their composition. ¹⁹ Figure 12 shows the relative resistance to galling of copper alloys with varying amounts of Zn, Mg, Al, Si, Sn, and P at 25 and 150 C. Small additions of P and Sn effect the greatest improvement, closely followed by Si, then Al and Mg, and finally Zn. Semenov's relative ratings indicate why the tin bronzes are so widely used in ordnance and appear to be in agreement with general experience.

Figure 12. Resistance to seizing at 150 C as a function of alloying elements.

Table 10 lists some of the principal applications for copper-base alloys in which resistance to galling and seizing in seawater is a principal consideration.

Desalination Environments

Copper alloy tubing has made desalination economical and practical in the arid areas of the world. Distillation processes predominate, but there is an increasing usage of reverse osmosis, especially with brackish waters. In the distillation processes, raw seawater is pumped through the heat reject section, where it is warmed to 105 to 110 F (40 to 43 C) for easier deaeration. The heat reject section of the plant is similar to a power plant condenser and requires ~10% of the total tubing. Tubing performance has been similar to that in coastal power plant condensers, except in those desalination plants that draw their intake water from shore side wells rather than the open ocean. Water from shore side wells lacks oxygen and contains hydrogen sulfide, with 5 to 20 ppm being typical. Copper alloy tubing can be used successfully in sulfide-bearing waters free of oxygen, provided that care is taken to remove all debris and sediment that would damage the protective sulfide film. However, in practice, operations have not been up to the control and skill level required. Thus, titanium and SSs have been preferred for plants that are fed from shore side wells or are otherwise laden with hydrogen sulfide in the incoming seawater.

After deaeration, the seawater is fed through tubing in the heat recovery section, where it is gradually heated in a series of stages to either 180 F (82 C) in the plants using polyphosphates or 220 to 230 F (104 to 110 C) in the plants using acid or chemical additives. This section of the plant requires ~87% of the total tubing. Oxygen contents range from 20 to 100 ppb, and corrosion rates are low. Alloys C68700 and C70600 are used in the heat recovery section of the plant with outstanding success. Figure 13 illustrates the continuing downward trend of the corrosion rate of Alloy C70600 with time as compared to the more constant corrosion rate of C68700.20 The tendency for the corrosion rate of Alloy C70600 to decrease with time as it does in seawater partly accounts for its increasing preference for use in the heat recovery section.

Figure 13. Variation in corrosion rate for copper alloys from 5 to 30 months in the heat recovery section of an experimental desalination plant.

From the heat recovery section, the seawater is fed to the brine heater, where it is heated approximately another 10 F (6 C) before being fed to the first flash chamber. The brine heater requires ~3% of the total tubing. The principal problem in the brine heater is the precipitation of calcium carbonate and/or calcium sulfate. Scale formation occurs from overheating and/or inadequate polyphosphate, acid, or additive treatment. Even in well-operated units, it is often necessary to rod the tubes using a power-driven rotating tool. Since the tube walls must be scraped in this manner, both the protective film and possibly some metal are lost. For this reason, somewhat heavier-wall 16-gage tubing is preferred in the brine heater. Figure 14 shows the same decreasing corrosion rate with time for C19400, C70600, and C71500 under brine heater service conditions that characterized C70600, C71500, and C61300 under heat recovery service conditions. ²⁰ Although both C70600 and C71500 are used in brine heater service successfully, Alloy C71500 seems to be preferred.

Figure 14. Variation in corrosion rate for copper alloys from 5 to 30 months in the brine heater section of an experimental desalination plant.

Oxygen content affects copper alloys differently, as shown in Figure 15. ²⁰ The corrosion rates of C70600 and C71500 remain low in the 20 to 200 ppb oxygen range, whereas those of C68700 and C19400 increase significantly as oxygen content increases. The lower rates maintained by C70600 and C71500 account for the preference of these alloys in desalination applications.

Figure 15. Corrosion rate as a function of low oxygen concentrations at 220 F (104 C) in an experimental desalination plant.

After the hot seawater leaves the brine heater, it is fed to the first flash chamber, where pure water is removed as steam. Above the demisters that gather and return water droplets, the steam is condensed on the outside of the tubing, also heating the incoming seawater. Provided that the noncondensible gases are properly vented from the flash chambers, the condensing steam has no effect on the outside of the tubing, which remains bright and clean.

The flash chamber floor, walls, and overhead must be protected from the more corrosive brine. The first several chambers are lined with Alloy C70600 or constructed from C70600 clad material. Organic linings and coatings are often used as an alternate to C70600 linings in the subsequent lower-temperature flash chambers to reduce initial cost. The maintenance of organic linings varies widely. Little or no maintenance is reported on C70600-lined flash chambers. AISI 316L SS linings are also used as an alternate to C70600, but have suffered from corrosion during shutdown and standby periods.

Waterboxes are normally constructed of C70600. Brine heater piping is also normally C70600. While some plants use C70600 piping throughout the heat recovery section, others use fiber-reinforced plastic and coated steel below 168 to 180 F (76 to 82 C) with varying degrees of success.

More detailed data are available in a Dow Chemical Company report, ²⁰ from which Figures 13 through 15 were taken, as well as the A.D. Little survey of performance reports. ²¹

Summary

• The initial unfilmed corrosion rate of copper alloys in seawater decreases ~100 times in the first hour of exposure at 60 F (16 C) and continues to decrease during the first several years of exposure.
• Long-term steady-state corrosion rates for most copper alloys in seawater range between 0.5 and 1.0 mpy (0.01 and 0.025 mm/y); for copper-nickel, the most resistant alloys approach 0.05 mpy (0.001 mm/y).
• Copper alloys vary widely in their resistance to velocity, with C95500 being the most resistant of the cast alloys, C72200 the most resistant of the condenser and heat exchanger alloys, and C70600 the most resistant of the ship hull plate alloys.
• Copper alloys have an inherent resistance to biofouling that makes a major contribution to their usefulness as condenser tubing, piping, and other components of seawater cooling systems.
• Although copper alloys generally may be coupled to each other without serious acceleration of galvanic corrosion, close attention to galvanic effects will considerably enhance performance. The use of SS or titanium tubes in copper alloy systems will generally require cathodic protection to prevent the accelerated corrosion of copper alloys that would otherwise occur.
• Copper alloys are resistant to occasional excursions into polluted waters, but they are not recommended for frequent or continuous exposure to polluted, sulfide-bearing waters.
• Parting corrosion has been largely overcome in high-zinc alloys by alloying and in the aluminum bronzes by proper welding techniques and heat treatment.
• Copper-nickel alloys exhibit outstanding resistance to the ammoniacal SCC that occurs in the high-zinc alloys and to a lesser extent in aluminum bronze alloys.
• Copper alloys exhibit outstanding resistance to seizing and galling under impact and rotating loadings.
• Copper alloys have made modern desalination plants possible and practical for arid areas of the world.