ph ground pin corrosion rev 05

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Norcross Corporation White Paper:

pH Ground Pin Corrosion in Corrugating Flexo Applications

Robert Norcross President

10/26/2012

Rev 05


Contents1. How is pH Measured? ......................................................................................................3 2. pH Probes Must be Kept Clean ........................................................................................4 3. How Long Do pH Probes Function? ................................................................................5 4. What is Ground Fault Current? ........................................................................................6 5. Dissection of pH Probe ....................................................................................................8 6. Chemicals Used in Wash Solutions ...............................................................................14 7. What Does this Mean? ...................................................................................................15 8. Who Needs pH Control? ................................................................................................16


1. How is pH Measured? In a pH probe small electrical current flows from the white ring to the central glass or in reverse, depending upon the pH. At a pH of 7.0 there is no current flow. Shown above, behind the glass, is a silver wire that carries electrons to the central core of the coaxial cable. The electrons flow through the white ring, passing through the shield of the coaxial cable. In tanks of ink or other coatings there are often stray electrical currents and/or electrical potential. These are most likely leaking from motors and/or generated through static generation. These will interfere with the flow of electrons in the pH-measuring circuit and thus they need to be drained away. This draining is done via a Ground Fault Pin (shown above), which carries such undesired currents off to earth ground, thus allowing the pH probe (which is basically a simple battery) to perform its measurement of the ink.

Ground fault pin

Silver wire


2. pH Probes Must be Kept Clean Here is a probe that has been in use and is stained with ink but clean and still functioning. This probe has collected ink build-up and will not read the pH of the ink.


3. How Long Do pH Probes Function? pH probes have both a shelf life and a use life. They are not stocked for long periods of time (over 3 months) and their typical useful lifespan can run from a few months to a year. Probes measure electron flow and as the electrons flow through, the pH probe slowly consumes internal charges that were placed in the units at the time of their construction. There is also a slow blockage of the pores in the white ring. Just like a hand-held pH tester, pH probes need regular calibration and must be kept clean and wet, at all times. However, unlike a hand-held tester, pH probes are immersed into the actual process fluid which makes them susceptible to electrical interference and/or improper cleaning. The probes in this study were all installed directly in a corrugated ink line, as opposed to being suspended on the side of a tank. We are checking on a customer with the same installation on a flexo film printer whose equipment has been in operation for one year, as of this release, with no reports of problems. We have reached out to them to confirm. We have sold a number of tank mount systems which do not have the same problems as the in-line probes. This may be due to being used fewer hours per week and/or because it is very easy to lift off the bucket and check it for any disturbances. It also means the probe is not being exposed to cleaning solutions.


4. What is Ground Fault Current? In hot water heaters, there is an anode. An anode is used in an attempt to absorb stray electrical currents, which can come from many sources. Should the anode be completely dissolved, then the galvanic action will attack the metal parts of the heater directly. In the case of machinery, there are many more sources for electrical potential to enter the fluid stream and it may vary from deck to deck. The probe (photograph on the left, below) was in operation for only one month and, aside from the buildup, you may see a small hole in the probe. The photo on the right shows the placement of a pen tip into the space where the ground fault pin used to be! This pin was electro-etched/corroded out of existence. The speed of this process can vary radically.


We have seen one plant with five probes in place for over 18 months. The probes are run for 12 hours per day 5-6 days per week, with some stations being used more often than others. Of the five probes, only one showed a corroded ground pin. This probe is shown later in this paper, in a dissected format. We do know that this probe was on a press without automatic soap injection and the anilox roll wash was done with soap added by hand into the doctor blade chamber while the lines were flushed with water. The probe shown below came from a machine that is run 24/7, with automatic washing, soap injection and regular pumping of soap from a tank. In corrugated printing there are often one or two stations that are used more often than others. Thus, the pH probe may be exposed to more regular washing. This may be the cause of the corrosion or it might be that probes, on stations not in use, may be left in pipelines with cleaning solutions.

Clean, unused probe


5. Dissection of pH Probe Shown below is a probe (mentioned above) that came from a deck where it had been installed in-line for nearly two years. The probes on the other four decks were showing readings but were not removed for inspection. This probe showed a reading of zero. The debris on the table was gently removed from the end of the probe, where it had gathered around the ground pin. The photo shows a hole in the probe where the ground pin used to protrude.


Here, we took the probe to a vise and cut it open, on the side away from the ground pin, short of the central glass chamber. The gel shown in the center of the probe is charged with ions and serves as one half of the pH probe “battery”.


Next we cut through the glass center of the probe and there you can see the silver core wire. Here you can see the silver wire being pulled out from the center of the sensor.


Next we cut on the other side of the glass center, removed it, and exposed the plastic sleeve which protects the ground wire.

Clean, unused probe


Here you can see the sheath is empty at the top. Here you can see the sheath being cut open.


Here you can see how far back into the sheath the ground pin has been “eaten” or corroded.

Clean, unused probe

End of pin


6. Chemicals Used in Wash Solutions We obtained MSDS sheets from three customers detailing cleaning solutions used in corrugated presses. We then Googled the chemicals and found the following information: Ethoxy-alcohol Appears okay (Ethoxylated Alcohol) Butyldiglycol Incompatible with aluminum and alloys, copper, copper alloys, neoprene, natural rubber, strong oxidizing agents Diphostphate terapotassium Compatible with SS Potassium Hydroxide Reactive with metals. Corrosive of SS304 Sodium Metasilicate Compatible with SS Nonylphenoxypolyethoxyethanol No information was found for this chemical Tetrasodium Solutions are corrosive to metal Triethanolamine Reactive with reduced agents, organic materials,

metals. Corrosive in presence of steel, aluminum, zinc, copper. Non-corrosive in presence of glass.

Perhaps the use of corrosive materials, combined with ground fault currents, create a situation that is damaging to in-line pH sensitivity systems in corrugating printing.


7. What Does this Mean? It appears that in the corrugating world, the in-line pH sensors will have a propensity to corrode and such corrosion will also cause some buildup. Likewise, buildup itself may intensify corrosion. In-line pH sensors need to be regularly removed, inspected, replaced and/or recalibrated. Tank mount sensors can easily be used on all corrugated presses and are easier to inspect. Staff operating pH probes must be trained on how to remove, replace and recalibrate the pH sensor. It is also important to keep hand-held testers current and calibrated. We advise daily calibration. While this paper does not specifically discuss tank mount sensors, our experience of nearly 20 years has never shown the ground pin being destroyed on a tank mount sensor. The death of a pH probe can be slow or fast. This means that you must have spare units on hand. In our experience, most plants do not have protocol for hand-held tester calibration. This is an important aspect of pH control. It also means that you must have a good protocol to calibrate your hand-held sensor, most plants do not. One company we encountered at the 2012 SuperCorr show, from Oregon, calibrates their hand sensor daily. This shows that they understand the importance and sensitivity of pH control. Norcross has a program available where customers can subscribe and receive small samples of certified pH solutions along with test recording sheets. These can be scheduled for regular shipment and contain instructions for the press managers to test hand-held probes. Then, if they are found to be out of calibration a simple e-mail will have us send along two solutions of 7 and 10 buffer solution for calibration. This same system can be used for the in-line sensors. We maintain records and report to the company what we are doing and responses we are receiving thus giving our customers traceability.


8. Who Needs pH Control? North & South America The pH adjuster used in the Americas is a type of ammonia compound that evaporates and interacts with the acidity of corrugated stock. Thus the pH shifts over time thus proving there is a need for both pH and viscosity control. Europe In Europe, a different pH adjuster is used with a different type of ammonia compound that does not evaporate like it does in in the Americas. The European adjuster apparently does not interact with the acidity in corrugated stock. Therefore in Europe we have not seen a demand for pH control, just viscosity control. Asia We are currently researching the pH needs in Asia. Others If you are located in a country outside of the locations listed above, one way to determine whether or not you might need pH control is to ask yourself if your press operators are having to hand test, with one of many devices on the market (one of which is shown below) the pH of the Ink and add a pH adjuster on a regular basis.

ph ground pin corrosion rev 05

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ground fault pin silver