E6) Study on the Corrosion of Steel and Zinc with Flow Velocity and Corrosion Control by the Vacuum Membrane Deaeration System in Tap Water

Transcription

1 E6) Study on the Corrosion of Steel and Zinc with Flow Velocity and Corrosion Control by the Vacuum Membrane Deaeration System in Tap Water Analysis on the corrosion of steel and zinc and galvanic corrosion of gunmetal/galvanized steel pipes Toshihiro YAMATE Technical Research Laboratory, Takenaka Corporation Ohtsuka, Inzai City, Chiba, , Japan Saburo MURAKAWA Graduate School of Engineering, Hiroshima University Kagamiyama, Higashi-Hiroshima City, Hiroshima, , Japan Abstract The comparison experiment on the influence of flow velocity on the corrosion of steel and zinc, and the galvanic corrosion of gunmetal/galvanized steel pipe with the tap water and the vacuum membrane deaeration water were carried out. The tap water and the vacuum membrane deaeration water were separately flowed into the experimental apparatus built with the steel pipe and the galvanized steel pipe. In those experimental pipe lines, the velocity of flow rate was changed in the five levels in accordance with the different diameter of the pipes. The test pieces of steel, the test pieces of zinc and the galvanic pairs of gunmetal pipe/galvanized steel pipe were put in the testing pipe lines, and were exposed for 439 days. A galvanic current was measured through the experiment period for a galvanic pair of gunmetal pipe/galvanized steel pipe. Moreover, the polarization curves of steel, zinc, and gunmetal were measured in the tap water and in the vacuum membrane deaeration water. Based on these results, an electrochemical examination was done. The main results of the study are shown as followings. 1) The corrosion rate of the steel that passes the tap water decreases with an increase at flow velocity in the range of m/s. It is because the surface of the steel is made a passivity. Corrosion is made a localization with an increase at flow velocity and the local corrosion is promoted. 2) The effect on zinc is small though the corrosion controlling effect of steel by the vacuum membrane deaeration is large. 3) A galvanic current of gunmetal/galvanized steel pipe decreases by the vacuum membrane deaeration, and the galvanic corrosion of the galvanized steel pipe/the gunmetal is reduced. 343

2 Keywords Tap Water, Steel, Zinc, Corrosion, Flow Velocity, Vacuum Membrane Deaeration, Galvanic Corrosion 1 Introduction The flow velocity in pipes affects the diffusion rate of dissolved oxygen with respect to the pipe surface, and is a major factor in corrosion. In tap water and other freshwater systems, the corrosion reaction is controlled by the cathodic reduction reaction of the dissolved oxygen. Therefore, dissolved oxygen plays an extremely influential role as a cause of corrosion. The goal of this study was to determine the effect of flow velocity on the basic corrosion behavior of steel pipes and galvanized steel pipes, as well as the corrosion-preventing effectiveness of reducing dissolved oxygen through deaeration. For this purpose, long-term tests were conducted with using piping equipment and test specimens. Furthermore, in order to determine the mechanism by which corrosion occurs, an electrochemical evaluation was also conducted through measurement of the polarization curves for steel and zinc. Moreover, the issue of galvanic corrosion as a major cause of corrosion in piping systems of building equipment was also taken up, and a study of the effectiveness of deaeration in preventing corrosion was conducted for galvanic corrosion in the typical combination of galvanized steel pipes and gunmetal. 1)- 3) 2 Outline of test method 2.1 Test equipment Figure 1 is an outline of the test equipment. The piping system was divided into a noncirculating tap water system and a quasi-transitory deaerated water system. In each of these systems, a steel pipe (JIS G 3452 black pipe) measuring 300 mm in length was used, with the aperture varied between 32 A and 10 A in order to change the flow velocity in the pipe in five levels from 0.18 to 1.42 m/s. In order to compare the corrosion resistance of the pipe material, galvanized steel pipe (JIS G 3452 white pipe) measuring 10 A and 100 mm in length was incorporated into the tap water and deaerated water systems. As shown in Figure 2, transparent PVC columns with different flow velocities were attached to each of these piping systems, and two mild steel (SS 400) and zinc test specimens each (measuring 30 x 10 x 2 mm in the case of the 20 A and 16 A column aperture, and 30 x 8 x 2 mm in the case of the 13 A column aperture; wet #400 polish finishing) were inserted. As the galvanic corrosion sample pipe, two types of galvanic pairs were fabricated as shown in Figure 3, and these were incorporated into the tap water and deaerated water system piping units shown in Figure 1. As the galvanic pairs of pipe for measuring galvanic current, a gunmetal pipe (BC 6) measuring 25 A and 300 mm in length and a galvanized steel pipe (JIS G 3442) measuring 25 A and 300 mm in length were connected in such a manner as to ensure that they did not come in contact with one another, as shown in Figure 3 (top). The gunmetal pipe and galvanized steel pipe were short-circuited with lead wire, and a 344

3 nonresistance ammeter was incorporated in order to measure the galvanic current flowing between the two pipes. As shown in Figure 3 (bottom), a sample pipe was created by screwing a galvanized steel pipe (JIS G 3452 white pipe) into the upstream and downstream sides of a gunmetal socket to form another galvanic pairs. Figure 1 - Outline of test unit Figure 2 - Inserting the test specimen Figure 3 - Galvanic pair pipes 345

4 2.2 Measurement/analysis methods and test conditions The test was conducted for 439 days. After the test had concluded, the sample pipes were split lengthwise and the inner surface was observed on the corrosion status. The depth of the corrosion pits produced on the inner surface of the pipe was also measured. For the test specimens, the corrosion rate (mm/y) was calculated by determining the reduction in weight (the difference in weight before and after the test). An analysis of the corrosion product on the sample pipes and test specimens was also conducted. With regard to the galvanic pairs, the galvanic current was measured once per hour for 439 days by means of automatic measurement. After the test had concluded, the corrosion status on the inner surface of the pipe was examined, as in the case of the other sample pipes. In addition, in order to investigate the mechanism by which steel and zinc corrosion and galvanic corrosion in the galvanic pairs of gunmetal/steel galvanic pipes occurred from an electrochemical standpoint, the polarization curves for the steel, zinc and gunmetal in tap water and deaerated water were measured. These measurements were conducted at room temperature and at a state of rest in water, from the natural potential of each material in the anode direction and cathode direction at a potential sweep rate of 20 mv/min. 2.3 Water quality Table 1 shows the average water quality of the tap water and deaerated water during the test period. The average concentration of dissolved oxygen in the deaerated water is approximately 1/25 that in the tap water. With deaerated water, the decarboxylation action resulting from deaeration makes the ph slightly higher as compared to tap water, and the free residual chlorine is slightly lower due to the deaeration action. However, in terms of other water quality indicators, no differences between the two were observed. Table 1 - Average Water Quality During Test Period Note: figures in parentheses ( ) indicate fluctuation range. Analysis Item Unit Tap Water Deaerated Water Temperature C 18.1 ( ) 19.4 ( ) Turbidity <1 (<1) <1 (<1) Color <2 (<2) <2 (<2) ph 7.2 ( ) 7.4 ( ) Dissolved oxygen mg/l 8.14 ( ) 0.33 ( ) Electric conductivity ms/m 26.0 ( ) 26.1 ( ) Free residual chlorine (Cl 2 ) mg/l 0.91 ( ) 0.86 ( ) M-alkalinity (as CaCO 3 ) mg/l 41 (39-43) 40 (38-44) Total acidity mg/l 3.3 (3-4) 3 (3-3) Calcium hardness (as CaCO 3 ) mg/l 50 (49-53) 52 (49-54) Total hardness (as CaCO 3 ) mg/l 72 (69-77) 79 (67-91) Total dissolved solids mg/l 203 ( ) 205 ( ) Chloride ion (Cl - ) mg/l 29 (27-30) 29 (27-30) Sulfate ion (SO 2-4 ) mg/l 34 ( ) 34 ( ) Soluble silicic acid (as SiO 2 ) mg/l 18 ( ) 18 ( ) Total iron (Fe) mg/l <0.1 (<0.1) <0.1 (<0.1) Zinc (Zn) mg/l <0.05 (<0.05) <0.05 (<0.05) 346

5 3 Analysis of corrosion status on steel/galvanized steel pipes and steel/zinc test specimens 3.1 Sample pipe corrosion status Figure 4 shows the corrosion status of the sample pipe and the section shape after derusting (shown in silhouette). For the sample pipe through which tap water flowed, arrow-shaped tubercles that widen on the downstream side were observed at m/s. As the flow velocity increased, however, these became spindle-shaped, and at m/s, they changed to granular tubercles. After derusting, corrosion pits that followed the shape of the tubercles formed. At low flow velocity, the corrosion pits were broad and shallow; as the flow velocity increased, these became localized deep corrosion pits. However, on the 1.42 m/s galvanized steel pipe, only pockmarked corrosion was evident, and the corrosion was minor compared to the steel pipe at the same flow velocities, indicating that zinc has a higher corrosion resistance than steel. Figure 4 - Corrosion on inner surface of sample pipe (top: before derusting/bottom: after derusting) Conversely, in the sample pipes through which deaerated water flowed, there was less of a rust layer than in the case of tap water, and overall corrosion was minor, consisting only of roughening of the surface in the case of 0.18 m/s. As the flow velocity increased, corrosion became more visible, and at m/s, small pits similar to insect bites were produced in the flow direction. At 1.42 m/s, however, conversely there was less corrosion, with only shallow pockmarked corrosion evident. The corrosion was even less evident in the case of the 1.42 m/s galvanized steel pipe, consisting of small number of corrosion pits that gave the pipe a pockmarked appearance. Figure 5 shows the depth measurements for the 20 deepest corrosion pits from among those corrosion pits produced on half of the surface of the sample pipe. In the tap water system, the corrosion depth generally increased as the flow velocity increased. In the deaerated water system, at m/s the corrosion depth increased as the flow velocity increased. Conversely, however, at 0.90 m/s the depth decreased slightly, and at 1.42 m/s the depth decreased greatly. The corrosion depth for the galvanized steel pipe through which deaerated water flowed was generally equal to that of the sample pipe in the tap water system at the same flow velocity (1.42 m/s). 347

6 1.0 [Deaerated water] [Tap water] Corrosion depth (mm) Flow velocity m/s Type of pipe Steel pipe Galvanized steel pipe Steel pipe Galvanized steel pipe Figure 5 - Depth of corrosion produced on steel pipe and galvanized steel pipe 3.2 Test specimen corrosion status In the case of the steel test specimen in the tap water system, the tubercles became localized small particles as the flow velocity increased, in the same manner as in the sample pipe. With the test specimen in the deaerated water system, no tubercles were observed, and corrosion was produced in a uniform manner. After derusting, the test specimen in the tap water system showed irregular corrosion on approximately 70% of its surface area at 0.62 m/s. At m/s, however, the corrosion area appeared to be localized and the corrosion became pitting corrosion. The test specimen in the deaerated water system exhibited uniform corrosion at m/s, and the corrosion was very minor in comparison to the tap water system. As the flow velocity increased, however, the surface condition gradually became rougher. In both the tap water system and the deaerated water system, there was much less corrosion produced on the zinc test specimens as compared to steel, and in each case corrosion was produced in a uniform manner on the test specimen. The corrosion status of the zinc test specimens was similar for both the tap water system and the deaerated water system, and unlike the steel test specimens there were no significant differences in corrosion resistance. Moreover, for both the tap water system and the deaerated water system, fine corrosion tended to progress on the surface of the test specimen as the flow velocity increased. Figure 6 shows the corrosion rate for steel and zinc using the weight reduction method. The corrosion rate for steel in tap water decreased as the flow velocity increased. Conversely, however, the corrosion rate for steel in deaerated water increased slightly as the flow velocity increased. Corrosion was reduced for steel in deaerated water as compared to tap water, to approximately 1/6.5 in the case of 0.62 m/s and to approximately 1/3 in the case of 1.50 m/s. The corrosion rate of zinc in both tap water and deaerated water tended to increase as the flow velocity increased. The corrosion rate of zinc in deaerated water was slightly lower as compared to zinc in tap water. 348

7 Moreover, at flow velocities of 0.62 m/s and 1.50 m/s, the corrosion rate was reversed for steel and zinc in tap water. 0.3 Tap water Deaerated water Tap water Deaerated water Corrosion rate (mm/year) Flow velocity (m/s) Figure 6 - Effect of flow velocity and deaeration on steel and zinc corrosion behavior 3.3 Corrosion product In observations made using a scanning electron microscope (SEM), no clear differences between tap water and deaerated water could be determined in the particle diameter and shape of the microscopic rust. The rust components on the steel pipes and steel test specimens were made up primarily of magnetite Fe 3 O 4 and goethite FeO(OH) in the case of tap water and goethite in the case of deaerated water. 3.4 Examination Figure 7 shows the measurements for the polarization curves for steel and zinc in tap water and deaerated water. In the cathode polarization curve for steel in Figure 7, the diffusion limiting current density of deaerated water is approximately 1/3 that of tap water. Judging from this result, the fact that steel corrosion was greatly reduced in the deaerated water system as compared to the tap water system in Figure 6 is thought to be because the dissolved oxygen concentration was reduced, preventing the cathodic reduction reaction (shown below) from occurring. 1 O2 H 2 O2e 2OH 2 In Figure 6, there is little difference between the deaerated water system and the tap water system in terms of the corrosion rate of zinc. In the comparison of the diffusion limiting current in the cathode polarization curve in Figure 7, however, the value for deaerated water was reduced to approximately 1/10 that of tap water. This is thought to be because the polarization curve indicates the initial corrosion status with respect to the 349

8 polished surface, and this was different from the average corrosion rate over a long period of time. In Figure 6, as the flow velocity in the deaerated water system increases, the corrosion rate of steel gradually increased. This is thought to be because the diffusion velocity of dissolved oxygen onto the surface of the test specimen increased. In the tap water system, the corrosion rate of steel decreased as the flow velocity increased, and this is thought to be because the supply of highly concentrated dissolved oxygen caused the surface of the test specimen to become passive. In Figure 4, the corrosion of the steel pipe in the tap water system became localized as the flow velocity increased, and the corrosion depth increased as shown in Figure 5. This is thought to be due to corrosion defects produced on the pipe surface that had become passive. These results suggest that, in the case of steel pipes, the surface becomes passive as the flow velocity increases, and localized corrosion progresses even if the overall amount of corrosion is reduced. As shown in Figure 6, the corrosion rate of zinc in the tap water system is less than 1.0 m/s, lower than that of steel, and the difference tended to be greater at lower flow velocities. In addition, the corrosion tended to be uniform corrosion. Based on these results, it is thought that the corrosion prevention effectiveness of galvanizing with respect to steel is particularly great in the low flow velocity range. Figure 7 - Polarization curves for steel and zinc 4 Analysis of corrosion of galvanic pairs of gunmetal pipe/galvanized steel pipe 4.1 Measurement of galvanic current Figure 8 shows the results of galvanic current measurement. In both the galvanic pairs through which tap water flowed and the galvanic pairs through which deaerated water flowed, the galvanic current decreased over time. However, the reduction of galvanic current in the case of tap water was greater than that for deaerated water. When measurements were initiated, the galvanic current flowing through the galvanic pairs in the tap water system was approximately double that for the galvanic pairs in the deaerated water system. However, the difference between the two decreased over time. The fact that the galvanic current decreased over time is thought to be due to 350

9 polarization. In particular, the corrosion potential of the steel and zinc that function as anodes shifted to noble direction, decreasing the difference with the gunmetal that functions as a cathode and thereby reducing the galvanic current Galvanic current (ma) Gunmetal/galvanized steel Tap water Deaerated water Elapsed time (days) Figure 8 - Changes over time in galvanic current in galvanic pair pipes 4.2 Sample pipe corrosion status Figure 9 shows the corrosion status on the inside of the galvanic pairs of galvanized steel pipes. Figure 10 shows the degree to which the zinc plating layer remains in the area near the joint with the gunmetal pipe in the case of the 25 A sample pipe used for galvanic current measurement. Figure 9 - Corrosion status of galvanized steel pipe in area of contact between dissimilar metals 351

10 Figure 10 - Residual zinc plating (25 A) at cathode end of galvanized steel pipe Galvanic pairs of 25A gunmetal pipe/galvanized steel pipe At the end of the galvanized steel pipe on the gunmetal pipe side in the tap water system, the surface in the circumferential direction is khaki-colored and a brown rust layer has formed on the pipe interior up to approximately 35 mm (in general, approximately 30 mm) from the end of the pipe, as shown in Figure 9. After rinsing, the zinc plating layer disappeared down to a maximum length of approximately 34 mm from the pipe end, and the bare steel is exposed. As shown in Figure 10 (top), more of the zinc plating layer is gone at the locations nearest to the pipe end. After rinsing, corrosion progressed in the area approximately mm from the end of the galvanized steel pipe connected to the gunmetal pipe, and corrosion progressed to the greatest extent in the areas nearest to the pipe end. At the end of the galvanized steel pipe on the gunmetal pipe side in the deaerated water system, there is brown discoloration over the entire surface in the area approximately mm from the pipe end, as shown in Figure 9. However, there was much less rust as compared to the sample pipe in the tap water system. In addition, there were pitting with the color of burnt umber in the area approximately 30 mm from the pipe end. As shown in Figure 10 (bottom), the zinc plating layer still remains over a wide area even near the end of the pipe. With regard to the condition after derusting by means of a water rinse or acid cleaning, slightly more corrosion had progressed in the sections that correspond to the brown discolored areas than in other areas, as shown in Figure 9. Up to 23 mm from the end of the pipe on the gunmetal pipe side, minor surface roughness was observed, and the sections with surface roughness became somewhat more numerous within 10 mm from the pipe end. 352

11 A galvanic pairs of gunmetal socket/galvanized steel pipe With regard to the galvanized steel pipe in the tap water system, the section cut in the pipe axial direction revealed a tendency toward metal loss in the area approximately 10 mm from the pipe end on the gunmetal socket side. This was particularly noticeable in the area approximately 3 mm from the pipe end. After derusting, corrosion had progressed in the circumferential direction at the joint with the gunmetal socket approximately 1.5 mm from the pipe end, and fine defects were observed at the pipe end. Moreover, approximately 5-7 mm from the pipe end, corrosion tended to increase, and this tendency was more pronounced at the locations nearest to the pipe end. With regard to the galvanized steel pipe in the deaerated water system, there was less rust overall as compared to the sample pipe in the tap water system. On the downstream side pipe, at the joint with the gunmetal socket, a rather thick layer of brown rust had formed in the circumferential direction up to a length of approximately 4 mm from the pipe end. In the section cut in the axial direction at the joint with the gunmetal socket, on the upstream pipe the metal loss was slightly greater approximately 10 mm from the pipe end, while on the downstream pipe the metal loss suddenly became greater at approximately 3 mm from the pipe end. After derusting, the overall corrosion status of the galvanized steel pipe was minor as compared to the sample pipe in the tap water system. At the joint with the gunmetal socket on the upstream side pipe, the length up to approximately 0.5 mm from the pipe end was missing as a result of corrosion. At the joint with the gunmetal socket on the downstream side pipe, corrosion over the entire circumference increased up to a length of approximately 2 mm from the pipe end. 4.3 Polarization curves In galvanic pairs of gunmetal/steel and gunmetal/zinc, many cathodic reactions are thought to occur on the surface of the gunmetal. For this reason, it is possible to make a general determination of galvanic corrosion from the current density at the intersection between the gunmetal cathode polarization curve and the steel anode polarization curve, and at the intersection between the gunmetal cathode polarization curve and the zinc anode polarization curve. Figure 11 shows the polarization curves for gunmetal, steel and zinc in tap water and deaerated water. At the intersection between the gunmetal cathode polarization curve and the steel anode polarization curve, the galvanic current density was approximately ma/cm 2 in tap water and approximately ma/cm 2 in deaerated water. At the intersection between the gunmetal cathode polarization curve and the zinc anode polarization curve, the galvanic current density was approximately ma/cm 2 in tap water and approximately ma/cm 2 in deaerated water. In each case, the current density was lower in deaerated water. This suggests that deaerating tap water lowers the diffusion limiting current, reducing the galvanic current in the galvanic pairs of gunmetal/steel and gunmetal/zinc. This corresponds to the aforementioned corrosion status in the sample pipes. 353

12 Figure 11 - Polarization curves for steel, zinc and gunmetal in tap water and deaerated water 4.4 Examination For both the galvanic pair created through external short-circuiting (25 A) and the galvanic pair created through socket connection (10 A), the progress of corrosion was less in the deaerated water system than in the tap water system. This is thought to be because deaeration reduced the dissolved oxygen concentration, preventing the cathodic reduction reaction from occurring as discussed in 3.4. Regarding the galvanic pairs in the tap water system, the galvanic current was lower than in the galvanic pairs in the tap water system, indicating a tendency for galvanic corrosion to be reduced by deaeration. Judging from the scope of galvanic corrosion in the galvanic pair of 25 A sample pipe, the actual anode length in both the tap water system and the deaerated water system was seen to be approximately 30 mm from the joint. Assuming an anode length of 30 mm, the average current density as judged from the galvanic current (the integral value for galvanic current during the test period) for the galvanic pairs in the tap water system was calculated as mA/cm 2, corresponding to a steel corrosion rate of approximately 0.15 mm/y. This corrosion rate generally corresponds to the corrosion status of the anode section of the sample pipe. With regard to the galvanic pairs in the deaerated water system, the average current density determined from the galvanic current was mA/cm 2, and the corrosion rate was calculated as approximately 0.11 mm/y. However, the zinc plating layer at the end of the sample pipe remained, as shown in Photo 5, and there was less corrosion as compared to the calculated value. This is thought to be partly because the galvanic current was drained off at ranges exceeding 30 mm in length, but the reason is not yet clear. 5. Conclusion 1) In the steel pipes through which tap water flowed, the surface became passive as the flow velocity increased within the range of m/s, with the result was 354

13 that the amount of corrosion was reduced. However, the corrosion became localized and local corrosion progressed. 2) At flow velocities of 1.0 m/s or less, the corrosion rate for zinc was lower than that for steel. The difference between zinc and steel tended to be greater as the flow velocity decreased. In addition, the corrosion tended to be uniform. Judging from these results, the corrosion prevention effect of galvanizing with respect to steel is thought to be particularly great in the lower flow velocity range. 3) Vacuum membrane deaeration of tap water greatly increases the corrosion prevention effect of steel. However, the effect with respect to zinc is low at flow velocities of approximately 1.0 m/s or less. 4) With regard to the galvanic pairs of gunmetal/galvanized steel pipe, the galvanic current was reduced by the vacuum membrane deaeration of tap water, and the galvanic corrosion status of the galvanized steel pipe was less than in the tap water system. It is thought that, as a result of deaeration, the cathodic reduction reaction on the surface of the gunmetal near the joint was reduced, preventing galvanic corrosion. 5) With regard to the galvanic pairs of gunmetal/galvanized steel pipe, for both tap water and deaerated water, the range affected by galvanic corrosion was a length of approximately 30 mm from the joint. The corrosion was greatest in the areas nearest to the cathode side. References 1. Yamate, Toshihiro and Murakawa, Saburo: "Preventing Corrosion in Building Equipment Pipes by Means of Vacuum Membrane Deaeration" Proceedings of the Architectural Institute of Japan (Planning), No. 539, pp , January Yamate, Toshihiro: "The Relationship of Flow Velocity and Vacuum Membrane Deaeration to Corrosion Behavior of Steel and Zinc" Library of Academic Conference of The Society of Heating, Air Conditioning and Sanitary Engineers of Japan, pp , September Yamate, Toshihiro: "Effectiveness of Vacuum Membrane Deaeration in Preventing Galvanic Corrosion in Galvanized Steel Pipes" Library of Academic Conference of The Society of Heating, Air Conditioning and Sanitary Engineers of Japan, pp , August 2005 Presentation of Author Toshihiro Yamate is a senior researcher in the Environmental Planning Department of the Takenaka Research & Development Institute at Takenaka Corporation. His area of expertise is research into the prevention of corrosion in building equipment piping. Recently he has been engaged in the study of water treatment for equipment and sanitation. 355

14 356