ENGINEERING MATERIALS 65 Granada Drive Pagosa Springs, CO

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1 ENGINEERING MATERIALS 65 Granada Drive Pagosa Springs, CO Desalination Corrosion Study Municipal Water District of Orange County Final Report Submitted To: Richard B. Bell, PE Principal Engineer Municipal Water District of Orange County Ward Street Fountain Valley, CA Submitted By: Joseph A. King, Ph.D, P.E. Engineering Materials 65 Granada Dr Pagosa Springs, CO May 29, 2012

2 EXECUTIVE SUMMARY Alloy 316L, 2205, 2507, AL6XN, CDA630 and CDA715 were evaluated for possible use in the Dana Point Desalination source water. Corrosion test specimens included wrought materials with autogenous weld beads and heat affected zones in the presence of a severe crevice. Specimens were tested for 3, 6, 16 and 22 months. At the end of each exposure period, the specimens were examined optically, with scanning electron microscopy and energy dispersive spectroscopy, monitored for weight loss and pitting/crevice corrosion. The results are summarized below. Alloys CDA715 and CDA630 were aggressively attacked by the source water. These alloys exhibited extensive pitting, general corrosion and significant weight loss. These alloys are not be suitable for use in this desalination environment. Alloy 2205 experienced crevice attack on the weld and heat affected zones. No localized attack was noted in the base material. The crevice attack appeared to worsen over the test duration. Therefore, welded 2205 is not recommended for the desalination environment. Alloy 316L exhibited crevice attack in the parent metal, heat affected zone and on the weld. The crevice attack in this alloy after 22 months was the same as at 3 months. This suggests that either the water chemistry changed and/or the material repassivated after 3 months exposure. Caution should be exercised in using Alloy 316L in this desalination environment. Alloys AL6XN and 2507 were largely immune to pitting and crevice attack and demonstrated no general corrosion. These alloys are acceptable for use in the desalination environment. ENGINEERING MATERIALS Page 2

3 INTRODUCTION The Municipal Water District of Orange County (MWDOC) wishes to exercise due diligence in the design and construction of water desalination facilities. The equipment in these facilities is exposed to water compositions that can produce premature failure due to aggressive corrosive attack. To this end, MWDOC sponsored field testing of various materials which could be used in construction of future installations. A request was issued to provide metallurgical assistance in the selection, exposure and evaluation of field test coupons. This report documents the extent of corrosion after 3,6, 16 and 22 months exposure. The following materials were tested. 316L 70Cu-30Ni (CDA715) 2507 duplex stainless 2205 duplex stainless Ni-Al-Bronze alloy (CDA630) AL6XN Samples were purchased and prepared in accordance with ASTM D2688 Standard Test Methods for Corrosivity of Water in the Absence of Heat Transfer (Weight Loss Method). An autogenous weld bead was included on all samples. A typical specimen is presented in Figure 1, below. Figure 1 Typical specimen The specimens were not passivated. All materials were in the fully annealed condition except for the metallurgical changes produced by the autogenous weld bead. A crevice corrosion test fixture described in ASTM G78 was included with all specimens. The crevice corrosion fixture was mounted across the weld bead. ENGINEERING MATERIALS Page 3

4 A view of the test manifold is presented in Figure 2. Six sample tubes were available for environmental exposure. Figure 2 Test manifold All of one sample type was placed in a single tube to prevent potential cross contamination. A total of 15 samples were placed in each tube. The specimens were stacked in rows of 3 at 5 locations inside the sample chambers. Serial number 16 of all of the material types was set aside to provide a comparison group. A view of one such sample stack is shown in the Figure 3, below. Figure 3 Sample stack ENGINEERING MATERIALS Page 4

5 The sample exposure schedule is shown below. May 10, 2010 Samples loaded June 3, 2010 Source water flow began September 27, 2010 First sample set removed December 6, 2010 Second samples set removed October 17, 2011 Third sample set removed April 6, 2012 Fourth and fifth sample sets removed 1 Examination included photo documentation, photomicroscopy, SEM/EDS analysis, weight loss measurements, and pitting/crevice/metallurgical analysis. Data for all specimens is presented in the attached appendices. Typical data is presented below. 1 The fifth sample set was used for microbiological studies. ENGINEERING MATERIALS Page 5

6 Photographic Documentation EXAMINATION AND RESULTS Photographs were taken before and after cleaning. Typical photographs for CDA630 specimens taken prior to cleaning are shown in Table 1. Table 1. CDA630 specimens after different exposure times prior to cleaning--an unexposed specimen is included in each photograph for comparison 3 Months 6 Months Photomicroscopy 16 Months 22 Months Photomicrographs were taken at 10x and 50x. A photomicrograph of the control specimen is shown with each sample group for comparison. 10X and 50X photomicrographs for CDA 630 specimens are presented in Tables 2 and 3, below. ENGINEERING MATERIALS Page 6

7 Table 2. 10X photomicrographs of CDA630 specimens after various exposure times and before cleaning No Exposure 3 Months 6 Months 16 Months 22 Months ENGINEERING MATERIALS Page 7

8 Table 3. 50X photomicrographs of CDA630 specimens at various exposure times and prior to cleaning No Exposure 3 Months 6 Months 16 Months 22 Months ENGINEERING MATERIALS Page 8

9 Scanning Electron Microscopy Selected specimens were examined and photographed using scanning electron microscopy. Scanning electron micrographs for CDA630 specimens are presented in Table 4 below. Table 4. SEM micrographs for CDA630 specimens No Exposure 3 Months 6 Months 16 Months 22 Months ENGINEERING MATERIALS Page 9

10 Energy Dispersive Spectroscopy The elements present on selected areas of selected samples were determined using Energy Dispersive Spectroscopy. The results of an unexposed specimen and the specimen after 22 months exposure for CDA630 specimens are presented below. EDS analysis was done prior to cleaning. Figure 2 EDS results for unexposed CDA630 specimen Figure 3 EDS results for CDA630 specimen after 22 months exposure ENGINEERING MATERIALS Page 10

11 Weight Loss All specimens were first cleaned in an ultrasonically agitated acetone bath for two minutes. The 316L 2205, 2507 and AL6XN stainless steel specimens were then placed in an ultrasonically agitated solution of 100ml HNO 3, 20ml HF and 880ml H 2 O for two minutes. The specimens were then immediately rinsed for two minutes in a refresh rinse bath. A similar procedure was used for the CDA630 and CDA715 specimens except that a 10% solution of sulfuric acid was used as the primary cleaning solution. Percent weight loss was averaged over three specimens in comparison to their original weights. Statistically significant weight loss was only observed in the copper alloys CDA715 and CDA630. The summary data for is presented in Table 5, below. Table 5. Summary of weight losses for all samples ENGINEERING MATERIALS Page 11

12 Localized Corrosion: Pitting and Crevice Attack Pitting analysis was performed using a slight modification of ASTM G46. Specimens were systematically reviewed by subdividing the surface area into 50 square blocks. The specimens were then examined at 20X and rated per the charts presented in ASTM G46. Table 6 summarizes the ratings for the alloys after 6 months. In some cases, the presence of the crevice washers led to localized attack that was difficult to separate from pitting attack. The crevice corrosion is listed as such in Table 6 but is still rated in frequency and severity per ASTM G46 Table 6 Localized corrosion ratings for all alloys after 6 months exposure Alloy 316L Parent Material Heat Affected Zone Crevice Attack Crevice Attack A1, B5* A1, B5 Weld Crevice attack A1, B No attack No attack Crevice attack A1, B No attack No attack No attack AL6XN No attack No attack No attack Al Bronze 70Cu/30Ni *Frequency, Severity Pitting & Crevice A3, B1 Blotchy General Corrosion Pitting & Crevice A3, B3 Blotchy General Corrosion Pitting & Crevice A2, B2 Pitting: A-3, B-3 General Corrosion The localized corrosion ratings did not change significantly for longer exposure times so additional ratings were not recorded. Instead, the worst case pitting/crevice attack was examined for each material type. The results for CDA630 are presented in Table 7, below. ENGINEERING MATERIALS Page 12

13 Table 7. Metallurgical evaluation of the worst case pitting/crevice attack for CDA630 ENGINEERING MATERIALS Page 13

14 Optical evaluation DISCUSSION Visual examination clearly demonstrates that the copper based CDA alloys are not suitable for this application. Extensive localized and general corrosion were noted on all specimens. Significant localized attack was observed on the weld bead and in the heat affected zones on Alloy This attack worsened over time. Localized attack was noted on the weld bead, heat affected zone and parent material of the 316L material. However, this localized attack did not worsen after 3 months. No evidence of corrosion was noted on any parts of the 2507 or AL6XN materials. Weight loss Weight loss measurements are consistent with the results from visual examination. Significant weight loss was measured in the CDA alloys. The weight changes measured for the other alloys were within experimental error. The minimum detectable corrosion rate in this study can be calculated from the following formula: 2 where: Corrosion Rate (mils per year) = 22.3 W/(d a t) W = weight loss, mg ( 2 mg accuracy) d = density of the metal, g/cm 3, (8.0 g/cm 3 ) a = exposed area of coupon, in 2, (3.343 in 2 ) t = time, days (668 days) With the values shown above, the minimum detectable general corrosion rate for this study is determined to be mils per year. The corrosion rate for the iron base alloys in this study is less than this value. SEM/EDS Scanning electron micrographs show significant contamination build-up on the specimens. The EDS analysis identified the chemical composition of the contamination. Na, & Cl were detected on all samples. This is consistent with exposure to natural seawater. Increased amounts of Ca, Mg, Si, and O were found when the analysis field included particulates. These particulates are most probably sand particles deposited on the surface. Large amounts of sulfur were detected in multiple regions on selected specimens. Fe contamination was also present but it can only be judged as contamination when analyzing the Cu-based samples. 2 ASTM D2688, Equation 1. ENGINEERING MATERIALS Page 14

15 Sulfur was detected on all samples after all exposure conditions. The sulfur peak overlaps the Molybdenum peak in the EDS spectra so the amount of sulfur present in the Mo bearing alloys is difficult to quantify. This is not the case for the CDA alloys. The presence of sulfur could be indicative microbiological induced corrosion activity. It is noted that the amount of sulfur on the CDA715 samples decreased dramatically in the interval between 3 and 6 months of exposure time. Predicting Alloy Performance The pitting and crevice corrosion resistance is sometimes reported as the Pitting Resistance Equivalent Number or PRE. This number is sometimes calculated from the following formula. PRE = % Cr x %Mo + 16 x %N The PRE values for the alloys examined here are presented in Table 8. Table 8. PRE values for the alloys tested in this study %Cr %Mo %N PRE 316L AL6XN The rank performance of the alloys examined in this study are consistent with the calculated PRE's. This number can be used to judge the performance of other alloys considered for use in this desalination environment. Disclaimer It is important to note that these specimens were tested in source water. The return water would have higher Cl - concentration. Different results would be expected if these alloys were tested in the return water. Also, due to the low temperatures involved, the stress corrosion resistance of these alloys was not investigated. ENGINEERING MATERIALS Page 15

16 CONCLUSIONS & RECOMENDATIONS 1. The copper base alloys, CDA630 nor CDA715 are not suitable for this desalination application. Both of these alloys experienced severe pitting, crevice corrosion and general corrosion after just 3 month exposure L and 2205 are marginal for this application. Alloy 2205 experienced crevice attack on the weld and the heat affected zone. This localized attack increased with time. The 316L material experienced crevice attack that appeared to get no worse after 3 months exposure. 3. No corrosive attack was noted on the 2507 or AL6XN alloys. ENGINEERING MATERIALS Page 16