One of the major challenges of running an anodizing process is that it consists of many steps, each of which must be managed and controlled to ensure the quality of the finished product. From initial degreasing and cleaning, to the actual anodizing step, to final sealing, every bath in the anodizing process has a different chemistry that must be managed to ensure consistent, high-quality results. For the anodizing professional, it is crucial to understand what one needs to measure in the baths and how to measure them in order to ensure a consistent, durable, and high-quality finish.
Anodizing Parameters: pH
Different baths in the anodizing process consist of different chemical mixtures that require certain parameters to be maintained to ensure reliable performance and high-quality results. Periodic testing of the baths is necessary to determine if bath composition must be altered or if active compounds in the bath need to be replenished. For almost every bath in the anodizing process, pH is a critically important control parameter.
pH is a logarithmic scale for describing the acidic or basic strength of an aqueous solution. It is equal to the negative base-10 log of the activity of hydrogen ion in solution. This means that a shift in one pH unit corresponds to a tenfold change in the hydrogen ion activity. Solutions below pH 7 are considered to be acidic, and solutions above pH 7 are considered to be basic.
Different baths in the anodizing process need to be maintained at different pHs in order to keep their chemistry working properly. Additionally, pH is a regulated parameter for discharged wastewater. Failure to adequately monitor pH where required can lead to poor quality finishes and fines from regulatory authorities.
To that end it is critical to understand how to properly measure pH so that one can be confident in their measurements. The first step of a proper pH measurement is to calibrate the meter. Calibrating a pH meter corrects the reading for the condition of the electrode. While inexpensive pen testers are easy to use and can be helpful for spot checks, to take defensible pH measurements, a meter that can report measurements in pH millivolts and record calibration parameters is absolutely necessary. The electrode of a pH meter is consumable, and will degrade with time and usage. After a certain point, the electrode will no longer be able to sufficiently be selective towards hydrogen ions to measure pH. At this point, the electrode would have to be replaced.
An ideal pH electrode should exhibit a potential of 0 mV at pH 7 with the potential changing by 59.16mV per pH unit from zero (positive for pH below 7 and negative for pH above 7). If either of these parameters vary too much from their ideal values, the electrode is no longer functioning to accurately measure pH.
Challenges of Measuring pH in an Anodizing Process
The production floor of an anodizer presents unique challenges in measuring pH. When measuring pH with an electrode, the highly conductive nature of the bath and electrical currents present within the tank can cause interference with the reading of the pH electrode. As the sensing element of a pH electrode has very high impedance, a ground loop can form between the reference electrode of the pH electrode and the anodizing tank itself and ambient electrical current can affect the reference potential of the electrode. This can cause inaccurate readings and shortened lifespan of the electrode. To avoid this, when testing in a bath, it is necessary to use a pH electrode with a matching pin. A matching pin is an external ground connection that isolates the pH electrode from its external electrical environment.
Spot-Checking vs. Continuous pH Monitoring
To improve pH monitoring, process monitors and controllers can be installed to continuously monitor pH. Through continuous measurement, you can see if the pH of your bath is drifting before it becomes a problem. In addition, pH controllers and monitors can be connected to and integrated with a central control and monitoring system though a 4-20mA output to allow remote monitoring.
Anodizing Parameters: Composition
While other baths in the anodizing process such as dye, seal, and rinse can be sufficiently controlled by monitoring pH, the actual anodizing bath requires more detailed analysis to adequately control its chemistry. As common anodizing processes are based around chromic acid, boric acid, or sulfuric acid chemistries, knowing the concentration of acid in an anodizing bath is critical to keeping the make-up within specifications. Additionally, the level of dissolved metals in the bath needs to be controlled. To accurately quantify the amount of acid in a bath, it is necessary to test a sample of the bath by titration. As an example, with a sulfuric acid anodizing bath, pH alone cannot determine the sulfuric acid concentration. To determine this value, a sample of the bath is titrated with sodium hydroxide to a methyl orange endpoint and subsequently to a phenolphthalein endpoint to determine the aluminum concentration.
Performing manual titrations can be tedious and difficult to perform, especially if the sample is heavily colored. Differences in color perception and analytical technique can result in different operators obtaining different results when testing the same samples. To that end, automating titration can resolve both of these issues. Potentiometric measurements are less ambiguous than observing the change in color of a chemical indicator, leading to more reliable endpoint detection and automated titration systems can typically dose more accurately and precisely than an operator with a manual burette. As an example, the high precision dosing system on a Hanna Instruments automatic titrator can dose with a minimum increment of 5 microliters.
Titration in Analyzing Anodizing Baths
While the process of measuring pH is virtually the same regardless of the bath, analyzing composition varies depending on the chemistry of your anodizing process. If you are already performing manual titrations of your baths the monitor parameters such as acid, aluminum, and chloride, these titrations can be performed potentiometrically by an automated titration system. Colored pH indicators can be replaced by a pH electrode. Argentometric titrations can be monitored with a silver ion-selective electrode. By replacing a visual indicator with a potentiometric measurement, sources of error such as inconsistent endpoint determination between operators and difficulty of resolving color changes in heavily colored samples can be avoided. Removing ambiguity from visual endpoint detection leads to more consistent and reproducible results both from run to run and from operator to operator.
In addition to removing ambiguity in endpoint detection, automated titrators can dose more precisely than can be achieved through traditional, manual means. The 40,000 step burette pump in Hanna Instrument’s HI931 and HI932 titration systems can add doses as small as 5 µL when using a 10 mL or 25 mL syringe or down to 1 µL when using a 5 mL syringe. This means that an automated titrator can produce results more accurate and consistent than any human operator with a manual burette.
Benefits of Improved QC
Though the costs of improving chemical analysis may appear intimidating, they can easily pay for themselves. More accurate chemical analysis means more accurate control of ones baths. Because of this, one can save money by minimizing chemical usage and reducing scrap and rework. By being able to more accurately assess the levels of one’s bath, one can control how much chemical needs to be used to replenish it. By only adding as much chemical as you need to and only when you need to, you can reduce the costs of maintaining your anodizing baths. Additionally, with the greater analytical consistency allowed by automation, scrap and rework can be minimized by ensuring that your anodizing baths are always in spec.
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