Given rising concern over elevated lead in drinking water in the aftermath of the Flint Water Crisis, forthcoming revisions to the U.S. EPA Lead and Copper Rule (LCR), and federal funding designated for replacing lead service lines, lead-tin solder corrosion control will become increasingly important. Lead-tin solder is often a dominant source of lead in drinking water for homes built before 1986 and has been the source of several recent high-profile water lead contamination events. This dissertation advances fundamental understanding of lead-tin solder corrosion by demonstrating that 1) elevated nitrate in water can trigger severe solder corrosion associated with very high LCR action level exceedances, 2) spallation of metallic solder to water is a source of lead contamination, 3) zinc orthophosphate offers superior corrosion control to mitigate nitrate attack, and 4) free chlorine can inhibit solder corrosion by electrochemical reversal. These principles were also applied to an exemplary related problem of lead contamination of food stored in tin cans.
The conventional understanding is that lead-tin solder corrosion is worsened by low pH, low alkalinity, and elevated chloride relative to sulfate, but a utility that recently switched to a source water previously classified as non-corrosive suffered severe contamination from lead solder. The incident was characterized by the detachment of large chunks of metallic, lead-bearing solder particles from copper pipe joints that sometimes clogged aerators of consumers' faucets. It also caused a 90th percentile lead level of 131 ppb, which was much higher than reported for the 2001-2004 Washington D.C. lead crisis (79 ppb) or the 2014- 2016 Flint, MI water lead crisis (29 ppb). An exhaustive investigation of possible causes eventually revealed a strong correlation (r2=0.79) between seasonal fluctuations in surface water nitrate levels and the 90th percentile lead. The association of high lead with nitrate was unambiguously proven in bench-scale experiments using both copper coupons with new 50:50 lead-tin solder and harvested pipes with aged solder (extracted from a home with ongoing lead release issues) that replicated the characteristic spallation of solder particles (up to 7-mm in length) to water. Lead levels were occasionally >1,000 ppb in homes and >100,000 ppb in the bench experiments with harvested pipe after exposure to high nitrate above 8 mg/L. This finding is especially concerning given that nitrate is not currently identified as a factor affecting solder corrosion in EPA corrosion control guidance and source water contamination by nitrate is increasingly problematic.
It was critically important to identify the mechanism by which nitrate caused solder spallation.
Analysis of lead-tin solder surfaces in the bench-scale tests and harvested pipes indicated that nitrate preferentially attacked tin in the solder alloy. Nitrate severely detinned solder alloys > 40% tin by weight, causing cracking and detachment of metallic chunks of lead-tin solder from copper surfaces in a matter of weeks. Pure lead and alloys with less than 30% tin corroded more uniformly in the presence of nitrate and were not subject to spallation.
Nitrate is reduced to a combination of ammonia and other nitrogenous compounds via reduction reactions that drove lead-tin solder corrosion at the anode. Nitrate also caused 1.3 times more metal weight loss by corrosion than could be explained by Faraday's law even in short-term 32-hour experiments, consistent with a previously identified "chunk effect" and anomalously high tin anode weight loss in nitrate solutions. This severe solder spallation mechanism has never been reported previously in drinking water environments and seems to be unique to nitrate for high tin-content alloys. This discovery also raises concerns about the possibility of pipe joint failures using lead-free tin-based solders that became more commonplace after the federal ban on lead solder in 1986.
Common corrosion control strategies, including the use of phosphate corrosion inhibitors, failed to adequately reduce 90th percentile lead levels at the utility affected by runoff water with high nitrate after 6 months of application. Studies using new lead-tin solder and harvested pipes with aged solder demonstrated that zinc orthophosphate outperformed orthophosphate in controlling corrosion in high nitrate water and reduced lead release by 82-90% compared to phosphate alone or no inhibitor. The benefits of zinc orthophosphate improved with time and the dose of zinc delivered to the pipes. When zinc orthophosphate was applied at the water treatment plant, the 90th percentile lead levels in the affected community fell below the action level within 6 months. Analysis of the pipe scale demonstrated that zinc orthophosphate works by coating the interface usually subject to intense galvanic corrosion between copper and solder.
Disinfectants may also play a role in controlling lead contamination from solder. Two water utilities in the Pacific Northwest experienced lead action level exceedances for decades due to solder corrosion while using the same source water with chloramine disinfectant. After one utility switched to a similar water source using free chlorine disinfectant, lead release dropped to low levels within months. This was consistent with laboratory experiments conducted at the second utility more than three decades ago that indicated much lower lead release using free chlorine versus chloramine using the water utility's source water. There was previously no explanation for the benefits from free chlorine, but it was recently demonstrated that chlorine can cause electrochemical reversal of a copper-lead pipe galvanic cell, which dramatically reduced lead release to water. It was hypothesized that a similar reaction could occur for lead-tin solder as well. This was confirmed when lead-tin solder and copper connections exposed to 4 mg/L free chlorine in circulating rigs experienced electrochemical reversal in some waters over a period of weeks. The electrochemical reversal was accompanied by dramatic decreases in lead release, concomitant with the formation of insoluble lead (IV) oxide scale. In some situations where traditional corrosion inhibitors are not effective, it is possible that electrochemical reversal due to free chlorine may control lead solder corrosion, either unintentionally or purposefully.
This new understanding of nitrate's ability to exacerbate lead contamination from lead-tin alloys in drinking water was then extended to concerns about lead contamination of food stored in tin-plated cans. Fruits and their juices can contain nitrate, and if lead is present in the tin plating, the resulting corrosion is predicted to cause significant contamination.
Twenty-one brands of canned pears from across the U.S. were assessed for lead content, and one brand was found to contain 2-3 times higher lead in the fruit (average=14 ppb, max=38 ppb) and syrup (average=7 ppb, max=15 ppb) than other brands. The brand of cans with higher lead in the fruit also had higher levels of lead in the can materials: surface lead levels in the interior tin-plate was 0.1% by mass on average (max=0.60%) and 7.5% by mass on average (max=29%) in the interior seam, which is up to 146 times the 0.2% value advised in FDA guidelines for lead in food-contact surfaces. Follow-up testing with three brands of canned pears confirmed lead levels in the syrup were also associated with higher levels of ammonia in the juice—ammonia is a reaction product of nitrate corrosion of tin in the can.
To confirm that the can material was the source of the lead contamination, the pear cans were emptied and then refilled with a variety of synthetic solutions containing up to 50 mg/L NO3-N. The higher nitrate levels always formed ammonia and were associated with higher lead release in some cases. The use of lead-tin alloys (either lead-bearing tin-plate or solder) in unlined canned goods with solutions known to contain nitrates can create unnecessary lead exposure for consumers.
This dissertation provides novel insights into lead-tin solder corrosion with profound implications for water lead contamination, the integrity of potable water infrastructure, and corrosion control strategies in potable water. Rising concerns about nitrate contamination of source waters underscore the importance of understanding these effects on lead and public health. As illustrated by the application of these principles to lead contamination of tin-lined fruit cans, the results may also enhance understanding of corrosion of tin-based materials in electronics, museum artifacts, electrochemical water treatment, and in the automotive and aerospace industries. / Doctor of Philosophy / Given rising concern over elevated lead in drinking water in the aftermath of the Flint Water Crisis, forthcoming revisions to the U.S. EPA Lead and Copper Rule (LCR), and federal funding designated for replacing lead service lines, the issue of lead-tin solder corrosion control will become increasingly important. Lead-tin solder is often a dominant source of lead in drinking water for homes built before 1986 and has been the source of several recent high-profile water lead contamination events. This dissertation advances the fundamental understanding of lead-tin solder corrosion by demonstrating that 1) high source water nitrate levels can trigger severe solder corrosion associated with elevated lead release in drinking water, 2) detachment (i.e., spallation) of metallic solder to water is a source of lead contamination, 3) zinc orthophosphate offers superior corrosion control to mitigate nitrate attack, and 4) free chlorine disinfectant can inhibit solder corrosion by electrochemical reversal. These principles were also applied to an exemplary related problem of lead contamination of food stored in tin cans.
It is understood that lead-tin solder corrosion can be affected by water chemistry, but a utility that recently switched to a new source water previously classified as non-corrosive was surprised to discover severe water lead contamination from solder. The contamination was characterized by the detachment of large chunks of lead-bearing solder particles from copper pipe joints that sometimes clogged aerators of consumers' faucets. It also caused a 90th percentile lead level of 131 ppb, a level much higher than reported for the 2001-2004 Washington D.C. lead crisis (79 ppb) or the 2014-2016 Flint, MI water lead crisis (29 ppb).
The presence of such large chunks of lead-bearing solder is contrary to the belief that water lead contamination occurs by the dissolution of lead rust from solder. An exhaustive investigation of possible causes eventually revealed that lead release in this community was strongly related to seasonal fluctuations in surface water nitrate levels. The association of high lead with nitrate was unambiguously proven in experiments using both copper coupons with new 50:50 lead-tin solder and harvested pipes with aged solder that had been extracted from a home with ongoing lead release issues, which replicated the characteristic spallation of solder particles (up to 7-mm in length) to water. Lead levels were occasionally >1,000 ppb in homes and >100,000 ppb in the bench experiments with harvested pipe after exposure to high nitrate above 8 mg/L. These levels of water lead contamination are amongst the highest ever recorded. This discovery is especially concerning given that nitrate is not currently identified as a factor affecting solder corrosion in EPA corrosion control guidance and source water contamination by nitrate is increasingly problematic.
It was critically important to better understand the mechanism by which nitrate caused solder spallation. Analysis of lead-tin solder surfaces in the bench-scale tests and harvested pipes indicated that nitrate preferentially attacked tin in the solder alloy. Nitrate is reduced to a combination of ammonia and other nitrogenous compounds while contributing to lead-tin solder corrosion at the anode. Nitrate severely degraded solder alloys with >40% tin by weight, causing cracking and detachment of metallic chunks of lead-tin solder from copper surfaces in a matter of weeks. Pure lead and alloys with less than 30% tin corroded more uniformly in the presence of nitrate and were not subject to spallation. Nitrate corrosion also caused 1.3 times more water contamination than predicted by conventional chemical reactions that do not consider spallation, even in short-term 32-hour experiments. This severe solder spallation mechanism has never been reported previously in drinking water environments and at present seems to be unique to nitrate for solder alloys with high tin content.
This discovery also raises concerns about the possibility of pipe joint failures, and associated pipe bursting and water damage, when using lead-free tin-based solders that became more commonplace after the federal ban on lead solder in 1986.
Common corrosion control strategies, including the use phosphate corrosion inhibitors, failed to adequately reduce 90th percentile lead levels at the utility affected by runoff water with high nitrate after 6 months of application. Studies using new lead-tin solder and harvested pipes with aged solder demonstrated that zinc orthophosphate outperformed orthophosphate in controlling corrosion in high nitrate water, reducing lead release by 82-90% compared to phosphate alone or no inhibitor. The benefits of zinc orthophosphate improved with time and the dose of zinc delivered to the pipes. When zinc orthophosphate was applied at the water treatment plant, the 90th percentile lead levels in the affected community fell below the action level within 6 months. Analysis of the pipe scale demonstrated that zinc orthophosphate works by coating the interface usually subject to the most intense galvanic corrosion between copper and solder.
Disinfectants used to kill bacteria in drinking water may also play a role in controlling lead contamination from solder. Two water utilities in the Pacific Northwest experienced lead action level exceedances for decades due to solder corrosion while using the same source water with chloramine disinfectant. After one utility switched to a similar water source using free chlorine disinfectant, lead release dropped to low levels within months. This was consistent with results from a laboratory study conducted more than three decades ago at the second utility that indicated much lower lead release using free chlorine versus chloramine using this water utility's source water. There was previously no explanation for the benefits from free chlorine, but it was recently demonstrated that chlorine can control lead pipe corrosion by reversing normal electrochemical reactions which dramatically reduced lead release to water. It was hypothesized that chlorine could have a similar effect for lead-tin solder as well. That hypothesis was confirmed when lead-tin solder and copper connections exposed to 4 mg/L free chlorine in circulating rigs experienced electrochemical reversal in a synthesized version of this water within weeks. The electrochemical reversal was accompanied by dramatic decreases in lead release, along with the formation of protective lead (IV) oxide pipe scale. These unexpected benefits of free chlorine may help explain why some water utilities with water normally considered corrosive have not been experiencing lead solder corrosion problems or lead action level exceedances.
This new understanding of nitrate's ability to exacerbate lead contamination from lead-tin alloys in drinking water was then applied to concerns about lead contamination of food stored in tin-plated cans. Fruits and their juices can contain nitrate, and if lead is present in the tin plating, the resulting corrosion is predicted to cause significant contamination.
Twenty-one brands of canned pears from across the U.S. were assessed for lead content, and one brand was found to contain 2-3 times higher lead in the fruit (average=14 ppb, max=38 ppb) and syrup (average=7 ppb, max=15 ppb) than other brands. The brand of cans with higher lead in the fruit also had higher levels of lead in the can materials: surface lead levels in the interior tin-plate was 0.1% by mass on average (max= 0.60%) and 7.5% by mass on average (max=29%) in the interior seam, which is up to 146 times the 0.2% value advised in FDA guidelines for lead in surfaces that contact food. Follow-up testing with three brands of canned pears confirmed lead levels in the syrup were also associated with higher levels of ammonia (a reaction product formed by nitrate corrosion of tin in the can) in the juice. To confirm that the can material and high levels of nitrate in the original food contributed to the lead contamination, the pear cans were emptied and then refilled with a variety of synthetic solutions containing up to 50 mg/L nitrate. The higher levels of nitrate always formed ammonia and were associated with higher lead release in some cases. The use of tin alloys (either lead-bearing tin-plate or solder) to package acidic food containing nitrate can create unnecessary lead exposure for consumers.
This dissertation provides novel insights into lead-tin solder corrosion with profound implications for water lead contamination, the integrity of potable water infrastructure, and corrosion control strategies in potable water. Rising concerns about nitrate contamination of source waters underscore the importance of better understanding these effects on lead and public health. As illustrated by the application of these principles to lead contamination of tin-lined fruit cans, these results may also enhance understanding of corrosion of tin-based materials in electronics, museum artifacts, electrochemical water treatment, and in the automotive and aerospace industries.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/116546 |
| Date | 25 October 2023 |
| Creators | Lopez, Kathryn G. |
| Contributors | Civil and Environmental Engineering, Edwards, Marc A., Grant, Stanley, Cai, Wenjun, Pieper, Kelsey Janette, Widdowson, Mark A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | Creative Commons Attribution-NonCommercial 4.0 International, http://creativecommons.org/licenses/by-nc/4.0/ |
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