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Flow Induced-Corrosion and the Effect of Brush Cleaning on Alloys UNS C71500 and UNS NO6022 in Warm Polluted Seawater

A. Al-Sayegh and A. Al-Hashem
Materials Application Department
Kuwait Institute for Scientific Research

email ahashem@safat.kisr.edu.kw

The flow induced-corrosion behavior of copper (Cu)-nickel (Ni) alloy (70% Cu- 30% Ni) (UNS C71500) tubes in warm flowing Arabian Gulf Seawater containing upto 5 ppm sulphide ions was investigated. The effect of brush cleaning was assessed by monitoring the open-circuit potentials of such heat exchanger tube with respect to a saturated calomel electrode (SCE). The open-circuit potentials were measured before and after cleaning the tubes. Visual, optical and scanning electron microscopy examinations of the internal surfaces of the tubes were conducted to determine the susceptibility of this alloy to erosion-corrosion, taking into consideration the nature of the corrosion product film-formed.

Keywords: Flow, Corrosion, Sulphide, Heat Exchanger, Erosion Corrosion, Open- Circuit Potential, Seawater.

INTRODUCTION

An extensive research work related to marine corrosion was made in our laboratory to investigate the flow dependent corrosion behavior of several tubing materials used in the manufacturing of heat exchangers for seawater applications. This work was due to repeated failures of such tubing materials utilizing seawater in the Petroleum refineries in Kuwait (1-5). Failure Investigations (1-3) revealed that the damage was mainly due to erosion-corrosion. In this work, the results of laboratory erosion-corrosion studies made on alloys UNS C71500 and UNS NO6022 will be summarized. The main purpose of this work was to: determine the relative erosion-corrosion performance of the two alloys (2) The effect of sulphide pollutants on the performance of such alloys; and (3) The effect of brush cleaning on the open-circuit potential during erosion-corrosion testing.

Most metals and alloys exposed to moving fluids are susceptible to erosion-corrosion. Although electrochemical processes do occur, the many examples of this corrosion form are attributed to mechanical effects. Typical examples of this type of corrosion can be found in piping systems, particularly bends, elbows and tees, valves and pumps (4).

In failure investigations, the effect of sulphide was of particular interest; sulphide levels of 0.2 ppm polluted the seawater (3). Sulphide in aerated seawater can lead to accelerated corrosion attack of copper-based alloys (6 - 9).

EXPERIMENTAL

A condenser tube test rig was used for the erosion-corrosion tests (10). The test rig consisted basically of a circular manifold for feeding seawater to 10 vertical samples of condenser/heat-exchanger tubes through angled high-velocity nozzles. The velocity at the nozzles was about 8 m/s; in the low-flow areas velocity was about 0.2 m/s.

The nominal compositions of the two alloys studied are listed in Table 1.

The seawater in the tests was collected from an unpolluted Arabian Gulf area. The pH of the seawater was monitored at regular intervals during each experiment, and, if necessary, small additions of sodium hydroxide (NaOH) or hydrogen chloride (HCl) solutions were made to maintain the pH at its natural value of 8.2. Sulfide (S^2-) (5 ppm) was added twice daily as a pollutant in the form of a standardized 0.1 mol sodium sulfide solution. The seawater was replaced every two or three days, with the requisite sulfide additions.

The 200-mm lengths of condenser tube from the test pieces were cut off and the ends machined square with the axis of the tube. The lengths of all the test pieces were the same 1 mm. The samples were washed with acetone to remove oily deposits . The apparatus was assembled, and the rate of flow of water through it was adjusted to 25L/min. The only attention that it required was an occasional check that the water was flowing freely from all 10 outlets and that the heat block temperature was accurately maintained at 45C.

The tube samples were taken off and cut for examination after running the test for eight weeks. This exposure period was deemed sufficient for any protective films, which developed on the interior surface of the tubes, to thicken and blister, which might occur in service with a particular combination of tube material and water.

For potential time measurements, electrical contact wires were spot welded to each tube. The reference electrode used was a saturated calomel electrode (SCE) which was incorporated at the top of the plastic tube chamber.

RESULTS

Visual Examination

Visual observation of the UNSC 71500 and UNS NO6022 alloys after the test runs at flow velocities of 5 1/min and 15 l/min respectively are given in Table 2. At a flow velocity of 5 1/min in the presence of 5 ppm sulphide ions, UNS C71500 suffered superficial erosion damage and some pitting attack. Few shallow pits and little crevice corrosion were observed on the alloy. The alloy developed a reddish-brown corrosion-product film interspersed with greenish products. Black friable deposit developed near the impingement area. No erosion damage, pitting or impingement attack was encountered on the UNS NO6022. The alloy developed a whitish scaled deposit, and a green-tinted film was observed near the crevice area.

At a flow velocity of 15 l/min, the UNS C71500 tube suffered from some crevice and impingement attack, and broad shallow pits formed in the low flow area. Thick, nonadherent, greenish-brown product film formed around the impingement area. A light-brown product film developed in the low flow areas and impingement area. UNS NO6022 was completely unaffected except for the deposition of a thin, loosely adherent, whitish layer of probably calcareous deposits from the seawater.

In general, the UNS C71500 heat-exchanger tubes suffered increased pitting and, crevice and erosion attack with increased flow velocity. The pitting which occurred in the flow zone was more pronounced closer to the crevice region. There was a gradually decreasing turbulence in the heat-exchanger tube test rig. from the impingement zone to the region further up the tube.

Corrosion Potential Measurements

The potential-time curves of UNS NO6022 and UNS C71500 tube specimens in polluted seawater at 45C under 5 1/min and 15 l/min flow velocities are shown respectively in Figs. 1 and 2. The measured corrosion potentials as function of time of the UNS NO6022 alloy exposed to warm (45C) seawater polluted with 5 ppm dissolved sulphide ions and flow velocities of 5 1/min and 15 l/min are shown respectively in Fig. 1. The corrosion potentials under the flow velocity of 5 1/min were more noble than the corrosion potentials of tubes under the 15 1/min flow velocity, with the potentials varying from 25 to 200 mV SCE. The potential-time behaviour, Fig. 2, depicted by the UNS C71500 tubes followed a trend similar to that of the UNS NO6022. The potentials at the 5 1/min seawater flow velocity were slightly more noble than those obtained at the flow velocity of 15 1/min.

After 8 weeks of testing, the tubes were cleaned mechanically, with nylon brush to remove any loose corrosion products. Retesting under similar conditions showed no adverse effects. The potentials of the tubes gradually attained a steady state after a variety of time intervals.

DISCUSSION

At 45C in the presence of 5 ppm sulphide, UNS NO 6022 was unaffected except for the deposition of thin, loosely adherent, light-brownish layer, probably calcareous deposits from the seawater as described in Table 2. The fluctuation of the open-circuit potential as a function of time for UNS NO6022 might be attributed to the non uniform deposition of the loosely adherent light-brownish layer on the surface of the tube or due to the destruction action of the moving fluid of the loosely adherent layer exposing fresh metal surface to the electrolyte.

Layered films of UNS C71500 have been identified as inner oxide layer of nickel oxide (NiO) and Copper oxide (CuO), doped with sulphur and oxides of iron, and an outer sulphide layer of predominantly copper sulphide (Cu₂S), (11,12). Broad shallow pitting and some crevice corrosion surrounded by thick bluish-green corrosionproducts were also observed for the UNS C71500. These differences in behavior could be attributed to the nature of the surface film. With this alloy, a surface film inhibits mass transfer of Cu ions (Cu⁺), and inhibits corrosion if the oxide is not disrupted (13). Incorporating nickel ions (Ni⁺²) into the cuprous oxide (Cu₂O) film decreases electron and ion conductivities. Crevice corrosion surrounded by thick bluish-green corrosion products also was observed for the UNS C71500. Shallow pitting attack was noted for this alloy.

The fluctuations in potential with time for UNS C71500 may be due to crevice and pitting attacks or the destruction of the surface layer specially at higher flow velocities.

After cleaning, fresh metal surfaces are exposed to the electrolyte and this action leads to increase the electrochemical activity of the two alloys. However, due to the presence of Ni in both alloys, passivation occurred as soon as the tubes were exposed to the aerated seawater. The passivation process was faster for the UNS NO6022 and was gradual for UNS C71500 alloy.

Figure 1. Potential-time for UNS NO6022 alloy in seawater polluted with 5 ppm sulphide at 45C and flow velocity of 5 and 15 l/min.

Figure 2. Potential-time for UNS C71500 alloy in seawater polluted with 5 ppm sulphide at 45C and flow velocity of 5 and 15 l/min.

CONCLUSION

In general, UNS NO6022 was more resistant to pitting, impingement, and erosion corrosion than UNS C71500 in warm sulphide polluted seawater.

ACKNOWLEDGMENT

The authors acknowledge the financial support of Kuwait Foundation for the Advancement of Sciences (KFAS), Kuwait National Petroleum Company (KNPC), Petrochemical Industries Company (PIC), and Abu Dhabi National Oil Company (ADNOC).

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