Microanalysis of Craters in Aluminum Cans

At a Glance

Ball Packaging International, a metal and plastic packaging provider, came to McCrone to troubleshoot a recurring defect in its aluminum cans, which threatened significant production and profit losses. Through a series of microanalytical tests and consultation with Ball Packaging, McCrone determined that corrosive elements from a cooling tower water leakage into the body maker lubricant solution were causing the problem.

Situation

Ball Packaging International is a provider of metal and plastic packaging for beverages, foods, household products and aerospace to commercial and government customers. Founded in 1880, the company employs 15,000 people in 90 locations worldwide.

Issue

Quality control operators at a Ball Packaging North America production plant noticed a recurring “dewetting” defect in the interior coating of its aluminum cans. Dewetting happens when liquid beads up on a flat surface like drops of water on a waxed car. As a result of the dewetting, small craters formed on the interior sidewalls of Ball Packaging’s cans. The defects caused Ball Packaging to pull hundreds of thousands of cans off the production line.

The cratering problem first occurred nine months after Quaker Chemical, a provider of process chemicals and chemical management, installed new lubricant chemicals for its body maker machinery, which was used to produce can bodies. Through a series of internal tests, including flushing the system and restarting it with fresh chemicals, the plant correctly hypothesized the source of the cratering as the lubricant and cooling system of the body makers.

To test their suspicion that some form of contamination in the lubricant and cooling system was responsible for the cratering defect, Ball Packaging and Quaker Chemical brought the case to McCrone Associates, the analytic division of the McCrone Group, a leader in materials analysis and characterization. An expert in the field, McCrone Associates has worked with every major U.S. can manufacturer and supplier of aluminum, steel, coatings, inks and lubricants. Senior Research Scientist, Wayne Niemeyer—a veteran of the packaging industry for over 20 years and can manufacturing quality control expert—spearheaded McCrone’s efforts.

Solution

Ball Packaging submitted several cratered cans to McCrone Associates’ laboratory for microanalysis to determine if any contaminants were causing the cratering. Normally, craters form when a contaminant on the metal’s surface causes the organic coating, sprayed near the end of the can manufacturing process, to “pull away” from the material. The result is a crater (or a circular ridged ring) with the contaminant in the center. Using polarized light microscopy (PLM), McCrone scientists saw deep corrosion pits at the crater’s center rather than a contaminant mound. This led them to believe the defect was not caused by typical surface contaminants.

To confirm this line of reasoning, McCrone scientists examined the craters using a scanning electron microscope (SEM), where they discovered each pit was actually a double pit separated by an unusual “bridge” of metal (Figure 1. High magnification SEM micrograph of bridge). Observation of the bridge indicated extremely severe and rapid corrosion was causing the defect, not a contaminant. Niemeyer concluded that the defect occurred prior to the can washer stage because this kind of corrosion was physically impossible during can washing.

Figure 1 – High magnification SEM micrograph of bridge.

“I had never seen anything like it,” said Niemeyer. “With all my experience troubleshooting production problems in the can industry, I thought I had seen everything.”

McCrone’s scientists then looked at the corrosion products—the microscopic traces of chemical reaction products left in the crater pits—for clues about the nature of the corrosive agents. Using energy dispersive x-ray spectrometry (EDS) on the SEM, they found only trace levels of sulfur and no fluorine. This finding eliminated the can washer as the contaminant source because the can washer chemicals contained these elements and do not cause such severe corrosion. Separate infrared microspectroscopy (IR) analysis also confirmed this conclusion, which left only the lubrication system in the body makers as the primary source of corrosion.

Finally, the McCrone team analyzed the samples using secondary ion mass spectrometry (SIMS), a much more sensitive technique than EDS, which can detect all elements on the periodic table. SIMS uses a focused beam of oxygen ions to bombard the surface of a sample and produce secondary ions. The ion fragments are collected and injected into a mass spectrometer system. Results from the SIMS analysis showed unusually high levels of boron and iron in the pits (Figure 2. SIMS ion maps showing boron (B) and iron (Fe) in one of the pits). Ironically, borate salts were used as corrosion inhibitors in the cooling tower water that circulated through the heat transfer piping in the body maker’s lubricant and coolant system. The borate salts inhibited corrosion on the cooling tower’s steel piping but, in conjunction with the highly alkaline water, actually caused corrosion when reacting with the aluminum in cans off the manufacturing line.

Figure 2 – SIMS ion maps showing boron (B) and iron (Fe) in one of the pits.

Through this comprehensive series of microanalytical tests, Niemeyer unequivocally determined that corrosive elements, likely from a cooling tower water leakage into the body maker lubricant solution, were causing the problem and eliminated all other possibilities of common contaminants.

Results

Though McCrone scientists helped Ball Packaging identify the contaminant’s nature and location on the manufacturing line through microanalysis, these techniques alone could not provide the evidence necessary to determine why the defect occurred. Therefore, Ball Packaging, Quaker Chemical and McCrone Associates called a joint meeting to discuss the manufacturing process and determine the cause of the defect.

McCrone scientists determined that the cooling tower water, which is used to cool the lubricant solution, was leaking into the body maker lubricant. The extra water diluted the solution, causing corrosion when it reacted with the freshly ironed metal surface of the aluminum cans. Although the lubricant solution was sent to Quaker Chemical once a week for quality testing, the disproportionate mixing of ingredients was so minute it was virtually undetectable until it caused a defect on the cans. Additional testing, done at Quaker Chemical’s laboratory, confirmed that cooling tower water contamination in the lubricant system results in a very corrosive environment for aluminum.

“We really had a big problem and didn’t know what to do,” said Frank McDonough, Quaker Chemical’s Global Product Manager, Can Division. “Because of McCrone’s industry knowledge and technical expertise, they were specially equipped to discover the unknown element accurately with short turnaround—saving us time, effort and money.”

“This was such an unusual case because individually all the clues did not make sense,” added Niemeyer. “But once we started putting the puzzle together, the boron was the key pointing to the cooling tower water system. We also worked in a true partnership with the client, and were able to solve the case through thorough information gathering, both in the laboratory and outside of it, which is one of the factors that makes McCrone Associates unique.”

Ball Packaging fixed the leakage from the cooling tower into the lubricant solution by replacing the piping system. As expected, the dewetting defect discontinued and Ball Packaging was able to restore its can making to full production.