What Is A Hotspot Corrosion Survey?
Hotspot, Bottle to Bottle, Cell to Cell - they're all interchangeable names for the type of corrosion survey I will be discussing in this delicious article. The overall premise of a Hotspot Survey is to pinpoint and identify anodic/cathodic areas on buried metallic structures. These regions typically have accelerated corrosion occurring compared to their surrounding regions. Hotspot surveys can be extremely beneficial in locating this areas which can prevent failures, leaks, and extend the structure's life. The picture below represents what I am/will be discussing.
What Do I Need to Complete This Saucy Survey?
Calibrated High Impedance Voltmeter - Fluke or Allegro
Two Reference Electrodes
A 10ft lead, typical test lead, and clips.
Pipeline Locator
Flag/Paint/Stakes
Proper PPE per Company's Guidelines
Chain Tape (optional but not really)
A Positive Attitude (I usually forget this)
How Do You Complete This Survey?
I am glad you asked! At RCS, we have a standard procedure to completing these surveys. Follow your company's SOPs. Our procedure is as follows:
Reference electrodes being used must be compared to a standard reference electrode, and then compare the two field electrodes together. We aim for below 3mV compared to each other. This number seems very miniscule, but when performing this task, the magnitude being displayed is the gradient between the two cells.
The pipeline needs to be thoroughly located and marked every TEN feet by paint, stakes, flags, sticks, boulders, tables - whatever. There just needs to be some distinct marking for these intervals.
While using the voltmeter of your choice, RCS uses American Innovations Allegros, put your 10ft lead into the positive port and clip onto one reference cell. Install the other test lead into the negative port and clip onto the other reference cell.
To make things easier here, we use MCMiller's survey reference cells, so we just plug the long lead from the cell directly into the voltmeter.
Keep the negative ref cell stationary at the beginning location for the survey. Place the positive cell directly 10ft in front of you.
ALWAYS stay directly over the line for these surveys. Do not stray off the path.
Collect and record the voltage measurements displayed on the voltmeter.
Polarity is extremely important. Actually, it's one of the most crucial bits of information being collected in this survey.
Move the positive cell 10ft forward, and put the negative cell where the positive cell just was.
Always make sure the negative cell is going EXACTLY where the positive cell was.
Continue recording these measurements. Once the polarity changes from positive to negative, a side drain reading must be collected. All a side drain reading is a lateral read collected 10ft left then right of the pipeline.
Keep the negative cell stationary and place the positive cell 10ft left. Collect the reading. Move the positive cell 10ft to the right. Collect the reading.
Every company is different, but in our spec, anything over 40mV is considered significant and will be marked at a possible anode location.
Keep repeating steps 6-7 until the line is completed.
I Got All This Data. What Does It Mean?
The reason side drain readings need completed where measurements flip polarity from positive to negative is because that's where the current is discharging.
As you survey down the line, with the cells in the corresponding positions listed above, you notice all the readings are positive. This indicates the current is traveling down the line in the same direction. As you pass an ANODIC area, or discharge point, the readings will go negative. This indicates current is traveling backwards in the direction of your negative cell. Side drain readings need completed to show the magnitude of the discharge point. Think of it like locating an anomaly in DCVGs. I know how much everyone loves those surveys. Review with your company what the threshold needs to be in magnitude to justify being a possible anode installation location.
Important Tips/Things To Remember:
ALWAYS stay directly over the line. I already mentioned this, but placing the cell left or right of the buried structure will skew the readings.
The hotspot survey mentioned above is strictly for BARE buried metallic structures - typically pipelines.
Soil contact is essential. Kick away leaves, debris, rocks, or whatever else to achieve the best soil contact. Saturate the cells with water if conditions deem it essential. This should be a rule of thumb for collecting any measurements with a reference cell.
Hotspot surveys can be an extremely essential tool to learning more about a system or structure's integrity. Action can be taken from the findings to ensure safety, longevity, and keeping your boss impressed and happy.