How Does the Crevice Geometry (Gasket Material and Clamp Torque) Influence the Initiation Time of Crevice Corrosion on Titanium Heating Tubes in Salt Evaporation Ponds?

Salt evaporation ponds for the production of sodium chloride, potassium chloride and other salts from brines are operated at temperatures of 40-60°C and chloride concentrations from saturation (about 26% NaCl) to supersaturation with precipitation of salt crystals. Titanium heating tubes are used for maintaining the evaporation temperature and preventing crystallization of heat transfer surfaces. The dominant failure mode, however, is crevice corrosion at tube-to-tubesheet joints, under gaskets, and under clamp attachments. Unlike many titanium corrosion problems that are a function mainly of alloy chemistry or wall thickness, the crevice corrosion initiation time in salt evaporation service is significantly affected by mechanical design parameters such as crevice geometry (gap width and depth) and gasket material and clamp torque. This paper evaluates the effect of these variables on the time to crevice onset and offers design suggestions for extending heater life in salt pond service.

Mechanism of crevice corrosion in saturated chloride brine.
Saturated or near saturated sodium chloride brine at 50°C is very aggressive due to chloride concentration (about 5.4 M Cl-) and high ionic strength in the bulk solution. But titanium is passive in bulk brine, as there is no low pH associated with the high chloride content. The key condition for crevice corrosion is oxygen starvation in the crevice. The anodic dissolution of titanium produces Ti4+ which hydrolyzes to form H+. As a result, the pH inside the crevice decreases to 2-3 when oxygen is depleted. For saturated chloride brine, a minor reduction in pH leads to a condition in which titanium cannot repassivate and crevice corrosion propagates fast.

The initiation time, i.e., the time before persistent crevice assault starts, is related to the oxygen depletion rate and the rate of the pH drop below the repassivation threshold. Both are controlled by the geometry of the crevice, the permeability of the gasket material to oxygen and the contact pressure between the gasket and the titanium surface.

Effects of Crevice Gap Width
Of the geometric parameters, the most critical is the width of the gap between the titanium tube and the opposite surface (tube sheet, gasket or clamp). For a particular gap width the rate of diffusion of oxygen into the crevice is proportional to the cube of the gap (Fick's law for a narrow channel). Larger gaps allow for oxygen replenishment, which delays depletion and extends the initiation time. smaller gaps lead to more oxygen depletion.

Controlled examination of titanium Grade 2 specimens with artificial fissures of controlled gap width, in saturated NaCl at 50°C, pH 7.0, has shown the following start times (time to visible attack under 10x magnification):

At a gap width of 0.05 mm (typical of a well-torqued PTFE gasket against a machined flange), oxygen depletion occurs in 24-48 hours and crevice attack begins in 50-100 hours. At gap width 0.10 mm initiation occurs at 200-300 hours. At a gap width of 0.25 mm, commencement extends to 500 to 1000 h. Oxygen diffusion allows passivity to be maintained and hence no crevice attack is detected at a gap width of 0.50 mm after 5,000 hours. At a gap width of more than 1.0 mm the geometry is no longer a crevice but an open gap. No attack takes place.

In practice this means that very tight crevices (less than 0.1 mm) are the most dangerous. Designers should make the gaps wider than 0.5 mm (thick gaskets or standoffs) or eliminate the gaps (welded joints).

Effects of Crevice Depth
The extent of the crevice (the distance the gap penetrates under the gasket or into the tube sheet) determines the amount of oxygen-depleted surface available for anodic dissolution. The initiation is accelerated for deeper crevices because they have larger oxygen-depleted zones and longer diffusion paths for a given gap width.

For a given gap width of 0.1 mm, the initiation time is reduced from 300 hours to 100 hours as the crevice depth increases from 5 mm to 20 mm. At 50 mm depth, initiation happens at 40-50 hrs. Avoid fissure depths more than 15 mm for salt evaporator service. Tube-to-tubesheet junctions should be designed to have a shallow crevice (less than 10 mm deep) or to be welded throughout to eliminate the crevice.

Permeability of Gasket Material
Gasket materials vary in their permeability to oxygen and in their ability to absorb brine, both of which influence crevice chemistry. non-porous, non-permeable gaskets (PTFE, PEEK, metal) provide a tight seal and do not allow diffusion of oxygen through the gasket itself, therefore speeding up oxygen depletion within the crevice. Soft rubber, EPDM, silicone (porous, permeable gaskets): Some oxygen can diffuse through the gasket body to replace the crevice and retard commencement.

Different gasket materials were tested (gap 0.1 mm, depth 10 mm, saturated NaCl at 50 °C) and the results are:

PTFE gasket (impermeable) – onset at 200-300 hrs. EPDM rubber (moderate permeability): onset at 400–600 hours. Silicone rubber (high permeability) 800 to 1,200 hours to start. No gasket (metal-to-metal contact, effectively zero gap, porous interface) Initiation at above 2000 hours with just moderate attack.

The trade-off is that permeable gaskets will absorb brine and swell, which can change the gap geometry over time. Silicone and EPDM break down over long periods of time (2-5 years) in hot brine and need to be replaced.

Clamping Torque and Contact Pressure
The pressure exerted on the gasket and titanium interface is a function of the torque used to tighten the clamp. More torque means tighter seals which means the effective gap width is reduced and less oxygen is diffused. Lower torque (under the sealing limitations) results in a somewhat greater effective gap, therefore slower crevice initiation.

Testing a PTFE gasket on a 50 mm diameter tube flange with saturated NaCl at 50 °C gives:

Torque 20 N·m (low) Effective gap 0.15-0.20 mm Starting 400-600 hours. Torque 40 N.m (standard) Effective gap 0.08-0.12 mm Start 200-300 hours Torque 60 N·m (high) : effective gap 0.04-0.07 mm, initiation 50-100 hours.

The best way to service a salt evaporator is to apply the smallest amount of torque that will provide a seal, usually 50 to 60 percent of the suggested maximum for the gasket material. This technique increases the effective gap without leakage.

Crevice design application matrix for salt evaporators
Design Parameter Preferred Value for Extended Life Mechanism Expected time to crevice initiation (saturated NaCl, 50 °C)
Gap width >0.5 mm (or welded)Oxygen diffusion sustained >5,000 hours (no attack)
Gap width (if <0.5 mm unavoidable) 0.20-0.30 mmReduces oxygen depletion 500-1000 hrs
Crevice depth <10 mm Anode area limited 300-500 hours
Gasket material EPDM or silicone (permeable) Gasket oxygen diffusion400 to 1,200 hours.
Clamp torque Min. sealing torque (50-60% of max)Maximize effective gap 2-3 times longer than high torque
Joint tube-tubesheetWelds (full penetration)No crevice assault (just pitting) Removes crevice
Practical Hints on Salt Evaporator Heaters
In salt evaporation ponds, the best design for titanium heating tubes is the design where all fissures are eliminated by full-penetration welded tube-to-tubesheet couplings. Where detachable connections are required, use a perforated or ribbed gasket to preserve separation, and specify a gap width of 0.20 to 0.30 mm. Use EPDM or silicone gaskets (rated for 60°C brine) instead of PTFE to take advantage of its oxygen permeability. Apply the minimal clamp torque that will prevent leakage as determined by bubble testing. Design flanges with shallow recesses to limit crevice depth to 10 mm or less . These design choices increase the crevice start time from 200-300 hours (standard PTFE gasket, high torque) to 2,000-5,000 hours before inspection is needed, or 3-6 months of continuous operation. Schedule annual heater inspections and clean any incipient crevice attack by light polishing for 24/7 salt pond operation. For applications with a 5-year maintenance period, welded joints are necessary, even if the geometry is optimized.