The Effect of Ultrasonically Induced Cavitation Conditions on the Behavior of L-80 Carbon Steel in Seawater

Al-Hashem, Abdul Hameed (Kuwait Institute for Scientific Research) | Carew, John A. (Kuwait Institute for Scientific Research) | Abdullah, Abdulmajeed (Kuwait Institute for Scientific Research)

OnePetro 

ABSTRACT
An ultrasonically induced cavitation facility was used to study the cavitation corrosion behaviour of L-80 carbon steel in sea water. The work included measurements of free corrosion potentials, and mass loss in the presence and absence of cavitation. The cavitation tests were made at a frequency of 20 kHz and at a temperature of 50°C. Cavitation conditions caused an active electronegative shift in the free corrosion potential of this alloy. Cavitation also increased the rate of mass loss of this alloy as a function of exposure time. Cavitation made the surface of this alloy very rough, exhibiting large cavity pits in the middle region of the attacked area as revealed by the scanning electron microscope (SEM). Mechanical factors were determined to be the leading cause of metal loss,

INTRODUCTION
It has been planned to use seawater from the northern Arabian (Persian) Gulf for the purpose of secondary oil recovery for some Kuwaiti oil fields to enhance production and maintain reservoir pressure 1. Initially, 1 million barrels per day of seawater from a seawater injection plant will be injected into the reservoirs of high flow rate 2. Carbon steel L-80 alloy has been considered for this waterflooding scheme because of its corrosion resistance under sweet conditions. Therefore, the aim of this study is to characterize the effect of cavitation conditions on the corrosion behaviour of L-80 carbon steel utilizing Arabian Gulf seawater.

EXPERIMENTAL METHOD
Apparatus
The vibratory apparatus used for this test method works at a frequency of 20 kHz and an amplitude of 25 ~m. This test method produces axial oscillations of a test specimen inserted to a specified depth in the test liquid. The vibrations are generated by a magneto strictive or piezoelectric transducer, driven by a suitable electronic oscillator and power amplifier. Figure 1 shows a schematic view of such apparatus.

Material
Specimens were made from a tubular section of L-80 carbon steel pipe. Each specimen had a diameter of 1.59 cm and a thickness of about 0.27 cm. Before experimental testing, specimens were mechanically polished with silicon carbide papers up to 1200 grit. For morphological examination, some specimens were etched before testing to reveal their microstructure. The test specimens were fixed on a special holder which was placed at a distance of 0.125 cm from the apparatus horn. At the end of each cavitation test, detailed morphological examinations were carried out on the specimens. Optical and scanning electron microsocopy (SEM) were used to identify the initiation and mode of damage in addition to the role played by the constituent phases of the alloy.

Test Solution
The chemical composition of the seawater used in this study is shown in Table 1. The seawater was contained in an open 600 ml glass beaker surrounded by a copper coil in a water bath. Inside the beaker, the seawater was maintained at 50_+1°C

RESULTS
Mass-Loss of L-80 Carbon Steel
Figure 2 shows the rate of mass loss of alloy C-80 carbon steel specimen exposed to the seawater at 50°C. It is evident that the rate of mass loss under cavitation conditions increases initially until it reaches a maximum value after about 1 hr of testing. Then it gradually decreases up to 4 h of cavitation after which it jumps to another value of 1.6 mg/hr.cm 2 after 14 h of testing. The rate of mass loss again begins to decrease after 15 h of testing, reaching a steady state value of about 1.0 mg/h cm 2 after 40 h of cavitation testing.

Electrochemical Measurements

To study the effect of cavitation on the free corrosion potential of L-80 carbon steel exposed to seawater, potential measurements were