01/30/2023 | Electroplating | 9 MINUTE READ

The Importance of pH Measurement in Surface Finishing

metal surface

When you measure anything in an industrial setting, the type of measurement you perform is just as important as the process the measurement is being applied to. Among the most commonly used measurements in industrial settings involves pH, which determines the alkalinity or acidity of a liquid solution.

When it comes to metal finishing applications, pH is an essential measurement for everything from the pretreatment process to the plating and wastewater treatment processes. The results you obtain from this measurement can help you identify the current plating throughput and rate. When performed for environmental regulation purposes, pH is necessary for making sure that an industrial facility maintains regulatory compliance.

Despite how useful pH measurements can be in an industrial setting, many of the individuals who are tasked with maintaining product quality, regulatory compliance, and resource efficiency overlook pH testing. Even some of the professionals who perform these measurements don’t fully understand what they mean.

If you use the result from pH testing correctly, you can improve cost control, environmental compliance, and product quality. This guide offers an in-depth look at surface finishing and how pH measurement enhances it.

bad surface finish

What is Industrial Metal Finishing?

Metal finishing is a common industrial process that’s used to describe the process that occurs when a metal coating of some type is placed on a metallic part’s surface. This surface is known as a substrate. In many cases, metal finishing will incorporate the electroplating process, which involves metal ions being deposited on a substrate through an electric current.

While metal finishing is often used to describe plating, it covers many unique processes, each of which comes with its own advantages. The many benefits of using industrial metal finishing include:

  • Improving the substrate’s strength and reducing wear resistance
  • Improving a part’s appearance
  • Increasing how well a product can be soldered
  • Polishing, cleaning, and getting rid of surface defects
  • Boosting chemical resistance
  • Making certain surfaces electrically conductive
  • Lowering the effects of friction
  • Reducing the impact that corrosion has
  • Being used as a primer coat for better paint adhesion

Understanding Metal Finishing

Finishing involves any process that changes the surface of a metal. It’s mainly used to improve certain features or enhance the metal’s appearance. When you perform industrial metal finishing, you can improve a metal’s:

  • Chemical resistance
  • Electrical resistance
  • Torque tolerance
  • Solderability
  • Hardness
  • Tarnish resistance

The Metal Finishing Process

There are numerous methods that are used to implement metal finishing, which include:

  • Welding
  • Metal coating
  • Metal plating
  • Metal anodizing
  • Heat treating
  • Galvanizing
  • Soldering
  • Brazing

The different types of finishes that you can apply to metals include coatings, surface treatments, lacquer, and paint. Surface treatments include everything from metal and chemical coatings to case hardening and chemical conversions. Metal can be cleaned with acids.

Why is Metal Finishing Important?

Industrial metal finishing can provide you and your facility with many benefits, the primary of which is that the finished material will be highly resistant to corrosion, wear, and tear. Additional benefits include:

  • Metal finishing boosts the appearance and presentation that metal products have
  • Finishing can enhance the ability a metal has to bond with rubber, adhesives, or coatings
  • This process can enhance a metal product’s electric properties
  • It can form components according to the exact needs of an application or client
importance of metal finishing

pH and Process Control

The main applications for controlling and measuring pH in metal finishing revolve around consistency. Precise pH measurements are necessary to make sure that the right aqueous solution is created for specific processes. Obtaining consistent pH measurements allows you to replenish water chemistry components on an hourly or daily basis. Proper pH monitoring of a surface finishing solution allows you to avoid such issues as adding too many chemicals into the solution at once.

The importance of maintaining a specific pH differs depending on the finishing operation that’s being performed. When it comes to electroless nickel, pH must be controlled to a specific number plus or minus one-tenth of a unit. If pH values are too low or too high in the rinse tank for most plating operations, the end product could be spotty or have an uneven color, texture, or finish.

Metal Plating and Wastewater

Modern regulations require that any toxic substances that are produced in an industrial facility need to be removed from the plant effluent before being discharged into the public water treatment system or environment. To do so, industrial facilities may need to perform oxidation of cyanide as well as the removal of chrome and other heavy metals.

Among the most important areas of concern involves treating chemical solutions that are comprised of hexavalent chromium. This substance is toxic to humans, plants, and animals, which is why it needs to be removed from any wastewater stream.

Removing chrome from wastewater can be done with specific chemical reactions, which will allow the chromium to be separated from the water. Once the separation has occurred, the chrome can be taken out of the water via filtration or the flocculation process, after which the wastewater will be discharged directly into the environment. In the past, reducing the hexavalent chromium to its trivalent state was enough. Modern regulations consider trivalent chromium to be toxic as well.

Properly removing chrome from water requires accurate pH control. When pH levels are correct, the chrome will be able to react immediately. However, pH levels being too low can result in the chemical reaction occurring at such a slow rate that the wastewater will need to be kept in a larger tank before it’s discharged. Since metal finishing facilities can be considered liable for how much chemicals are in their effluent, it’s highly recommended that you monitor pH readings on a continuous basis.

metal plating and wastewater

Measuring pH

There are multiple ways you can choose to measure pH for metal finishing, which include everything from using monitoring sensors to performing electrometric testing.

Monitoring Sensors

Sensorex offers pH sensors that allow you to monitor and manage pH levels for metal finishing. When you use one of these pH sensors, you’ll be able to precisely measure pH levels on a continuous basis. Keep in mind, however, that each pH sensor comes with its own guidelines. While some sensors require cleaning on a monthly basis, other sensors should be cleaned daily.

The performance and lifespan of a sensor are dependent on the exact environment in which the device is used. If you’re using a pH sensor for metal finishing applications, you may be tasked with monitoring conditions in highly acidic and caustic environments.

If a pH sensor is placed in high-coating wastewater, it might need to be cleaned almost constantly. In comparison, a sensor that’s used in a rinse water application doesn’t need to be cleaned as often. However, this application could involve heavy metal ions that are able to substantially reduce the sensor’s lifespan. It’s possible for plant operators to boost performance by choosing the right sensor for the specific application.

Regardless of the application, the reference junction being coated is the main challenge that must be overcome when performing pH measurements in dirty applications. Coated sensors are unable to determine how the application is progressing, which might result in a slow response or a complete lack of a response. When it comes to metal finishing, the sensor can be coated when placed in a wastewater tank alongside heavy substances like grease.

Electrometric Testing

Electrometric testing is currently the most common method of measuring pH since it avoids the issues that come with manual testing. One option with this form of testing is to implement continuous on-line measurement, which involves an analyzer and on-line sensor that help you maintain pH control in various metal finishing applications. It provides the best accuracy and delivers data that ensures environmental standards are adhered to.

These sensors are comprised of a reference electrode, a temperature element, and a pH-sensitive electrode. The reference electrode is able to maintain a voltage potential constantly and is able to complete the measuring circuit by delivering a reference potential for the electrode. The exact difference in the total voltage potential between the reference and pH electrodes allows for a millivolt signal to be created based on the solution’s pH.

While this type of sensor doesn’t measure pH directly, it’s able to interpret a voltage that should mirror any changes that have occurred to a solution’s pH. Keep in mind that these sensors should be recalibrated regularly to maintain accuracy of pH testing.

Traditional Grab Sample Analysis

Many finishing processes measure pH by obtaining a grab sample and sending this sample to an internal or external lab. At this stage of the process, the chemists working in the laboratory use pH papers that will change color based entirely on the stream pH. The resultant hue will then be compared to a color chart.

In the event that a sample is highly concentrated, the chemist may need to perform titration to identify how much acid is in the solution. This process involves a caustic being added slowly to a grab sample until the acid has fully reacted. Once all acid has reacted, you’ve reached the equivalence point, which is identified by measuring the solution’s pH while titration is ongoing and determining when the most substantial change takes place.

There are a few limitations to analyzing a solution with this technique. For one, the response time is much slower when compared to a standard pH sensor. In the event that the process solution is found to not meet specifications, you can’t determine when this problem developed or how long it’s been going on.

Environmental Issues

There are also some environmental challenges that must be taken into account when measuring pH in surface finishing applications. For instance, the reference in a sensor can be contaminated during pH measurement. This reference is usually comprised of a chloride-based electrode that may be damaged if iron, lead, chrome, and other poisonous ions get into the reference and react directly with the silver wire.

environmental water sensors

Metal parts can contain these ions, which is why reference poisoning is a common issue. The latest technologies help slow down the reference poisoning process by making it more difficult for these substances to get into the reference.

Surface finishing involves many distinct applications and processes, which is why pH levels must be monitored and maintained at all times. When you use a high-quality pH sensor, you can obtain accurate measurements that allow you to make quick changes to the solution as required.

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Posted by Dominic O'Donnell on January 30, 2023

Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.

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