Chlorine Dioxide Storage Tank

Data-dense robotic inspection technology discovered pinhole-sized damage, avoiding potentially fatal outcomes at a kraft paper mill.


through-wall pit found


million UT readings


corrosion monitoring locations established


  • Chlorine dioxide (ClO2) is used for the paper bleaching process, but it is toxic to humans at ambient pressures and temperatures
  • This vessel was located directly outside of a regularly manned control room
  • ClO2 is highly acidic and will degrade carbon steel very quickly if exposed
  • The storage tank was tile lined on the interior and insulated on the exterior
  • Semi-annual visual inspection of the tile liner revealed no signs of liner failure or degradation
  • Process Safety Management (PSM) audit determined the vessel needed to be stripped of insulation and inspected for corrosion under insulation (CUI)
  • Previous inspection methods only covered corrosion monitoring locations (CML) or drop scans


  • Full-coverage external visual and ultrasonic testing (UT) inspection for CUI
  • Robotic Rapid Automated Ultrasonic Testing (R-AUT) phased array corrosion mapping tool
  • High definition S-scan data visualization and reporting to identify, locate, and size damage


  • Rapid AUT’s millimeter resolution found a small, isolated (1.75” or 44 mm diameter) corrosion cell on the inner diameter (ID) of the fourth shell course with a nearly 80% through-wall pit
  • At the most severe location, Gecko Experts calculated a short term corrosion rate of 0.152” (4mm)/year, driving the removal of a 2’x2’ (61cm x 61cm) patch of shell and installed a flush patch and subsequent liner repairs
  • Gecko Experts identified an additional 20 CMLs of ID corrosion with 20% wall loss or greater, signifying failure or damage to the tile liner
  • Robot localization came within 1” (25mm) of the corrosion cells for all 21 CMLs
  • Traditional A-scan, B-scan, or C-scan inspection methods and data visualization would have missed the insidious corrosion cell, potentially resulting in fatal outcomes
  • The inspection found no CUI on the ClO2 storage tank but found significant manufacturing flaws

Download the full case study