In a Vertical Titanium Heat Rod Operating at 150°C in a Phosphoric Acid Sludge, How Does the Periodic Submergence Depth (30% vs. 70%) Change the Location of Maximum Pit Depth?

In phosphoric acid sludge – a viscous mixture of phosphoric acid (30-50%), precipitated gypsum (CaSO 4 .2H 2 O), silica and metal sulfates – vertical titanium heat rods experience non-uniform corrosion. It is periodically filled , drained and settled and the sludge level is variable . The position of the maximum pit depth is determined by two conflicting factors, namely the depth of submergence, i.e. the proportion of the rod length submerged in sludge. At 30% submergence (mostly exposed to vapor) oxygen concentration cells dominate, and the maximum pit depth is at the sludge-air interface (the meniscus). At 70% submergence (mainly submerged), sludge accumulation is the dominant process and the maximum pit depth occurs at the bottom tip where settled materials accumulate. Understanding this change is critical to planning targeted inspections and maintenance.

The mechanism of pit location shift as submergence depth changes

If the rod is partially immersed (30%), the sludge line is close to the upper part of the rod. The thin film of sludge on the meniscus evaporates, concentrating phosphoric acid and fluorides to very high levels. Differential aeration renders the sludge-air interface a very severe crevice environment and pitting starts from 200 to 500 hours. With 70% of the rod immersed, the sludge line is close to the top and the bottom of the rod has extended contact with settled gypsum and metal sulfate particles. At the tip, under-deposit corrosion is dominant, generating pits beneath the heavy sludge layer where oxygen is absent and chlorides or fluorides concentrate. The corrosion rate at the tip is also increased by the galvanic action between the submerged and vapor zones.

Quantitative pit depth distribution at various submergence depths

Controlled testing has been conducted in phosphoric acid sludge (40% P2O5, 20% solids, 1,500 ppm fluoride) at 150°C over 1,000 hours, showing unique pit depth distributions. At 30% submergence (70% of rod in vapor zone), the maximum pit depth is 0.6-1.2 mm at sludge line (meniscus at 30% of length from bottom). Pits less than 0.1 mm are observed in the vapor zone above the meniscus and 0.2–0.4 mm pits are observed on the bottom tip. At 50% submergence (equal immersion) a bimodal distribution is observed with pits of 0.4-0.8 mm at the sludge line and the bottom tip. For 70% submergence (30% in the vapor zone), the highest pit depth is at the bottom tip (0.8-1.5 mm) with the sludge line showing 0.3-0.6 mm pits. At 100% submergence (totally submerged), pits are evenly distributed on the lower half with depths of 0.5–1.0 mm and the meniscus effect disappears completely.

Pit location influenced by submergence cycling and sludge composition

The frequency of cycling and the solids content of sludge influence the region of maximum pitting. The meniscus has pits of 0.8-1.2 mm with constant 30% submergence and 20% sludge solids with 1000 ppm fluoride. Under the same conditions, with a steady submergence of 70%, the bottom tip has pits 0.9–1.5 mm. At daily immersion cycles between 30 and 70 % the meniscus and bottom tip are heavily attacked (pit depths of 1.2–2.0 mm) at both locations. The bottom tip pits increase to 1.5-2.5 mm at the higher solids concentration (40%) with constant submergence (70%). With lower solids (10%) and consistent 30% submergence, meniscus pits are reduced to 0.3-0.6 mm as less sludge collects at the interface. The effect of fluoride concentration on pitting is an accelerating one at all places and does not affect the location of maximum depth of pitting for a particular pattern of submergence.

Submergence Depth Control to Control Pit Location

The following table contains guidelines for regulation of submergence depth to regulate placement of pits and increase heater life in phosphoric acid sludge at 150C.

Recommended Submergence Depth Operation Strategy RationaleService Life Expectancy (1.5 mm wall)
Fixed submersion 100% (fully submerged)No meniscus attack, corrosion uniformly distributed 1-2 years
Fixed submergence with 100 percent blowdown of sludge with bottom removal of sludgeDecreases bottom tip under-deposit corrosion 2–3 years
Periodic draining completeDo not partially immerseDraining forms meniscus. Keep submerged at least 1 to 2 years
Bottom in vapour heater (inverted)Not recommended Meniscus shifts to top; comparable harm <1 year
Weekly rotate heater 180° Immersion anyDamage between meniscus and tip is 2-3 times extension
Engineering Beyond Submergence Control

Titanium grade has a substantial effect on the rate of pit propagation but not on the site of maximum attack. Grade 7 (palladium-stabilized) enhances pit propagation 50-70% at meniscus and bottom tip sites. Wall thickness gives a corrosion allowance; a 2.5 mm wall with 1.0 mm per year pitting at the worst spot will survive 2.5 years. Sludge level should be monitored regularly; a level controller to keep the immersion full at all times eliminates the meniscus. Periodic sludge blowdown (removal of settled solids from the bottom on a regular basis) minimizes under-deposit corrosion at the tip. If partial submergence cannot be avoided, a PTFE sleeve is installed in the expected meniscus zone to isolate the titanium from the concentrated sludge film.

Making an Educated Specification

Design a tank to hold a vertical titanium heat rod in phosphoric acid sludge at 150°C at 100% submergence at all times. Install level control system with warning at 90% submergence and heater cut-out at 80% to prevent exposure of meniscus. For existing heaters that are required to function with partial immersion, specify Grade 7 titanium and a PTFE shrink sleeve of 100 mm length at the predicted sludge line. Ultrasonic thickness mapping of the rod at the meniscus zone and bottom tip should be performed every three months during operation. If the pit depth is greater than 0.5 mm at either site, rotate the heater 180° (if symmetrical) or replace it. With the knowledge of how submergence depth influences the position of maximum pit depth, the engineer can apply specific inspection and mitigation procedures to the vertical titanium heaters in phosphoric acid sludge.