When a Titanium Electrothermal Tube Is Used to Reheat Condensing Humid Air Containing Trace Sulfuric Acid, What Surface Treatment Extends Its Lifetime Beyond 3 Years?

The Basic Trade-off in Titanium Heater Design for FGD Scrubber Service
A recirculating slurry containing calcium sulfite, fly ash and chlorides concentrated from the entering coal is used to operate flue gas desulfurization (FGD) scrubbers at temperatures between 60 and 80 °C. FGD scrubbers often have chloride concentrations in the range of 20,000 to 50,000 ppm. This provides a very aggressive crevice corrosion environment at any gasketed junction where the titanium heating tube penetrates the scrubber wall or is supported by brackets. Engineers generally assume that a thicker titanium sheath gives equivalent protection against crevice corrosion. But field failures tell the opposite story. Many 2.0 mm wall tubes break at gasket crevices within 18 months, while 1.2 mm tubes with good design last more than 5 years. The reason for this can be explained by the basic mechanism of crevice corrosion; geometry, oxygen diffusion and the ability to maintain a stable passive coating; characteristics that are not immediately improved by wall thickness and may even be worsened.

Effect on Mechanical Integrity: Crevice Corrosion Mechanism
Chlorides generate crevice corrosion which happens when oxygen transport into the crevice interior is hindered by a small gap (often 0.1 to 0.5 mm) between the titanium sheath and a gasket or support bracket. The crevice oxygen concentration lowers quickly and the outer surface stays well oxygenated. This difference in aeration cell causes active corrosion of the interior. This lowers the pH from neutral to 2-3 by hydrolysis of titanium chloride. The wall thickness is not the crucial characteristic for the gap geometry which controls the beginning of crevice corrosion. A broader gap (>0.5 mm) allows sufficient oxygen to enter for repassivation of the titanium, while a tighter gap (<0.05 mm) completely precludes oxygen and results in fast attack. The thickness of the titanium tube wall does not influence the oxygen diffusion pathway in the fissure, only the size of the crevice matter. In reality, a thicker wall usually requires bigger and stiffer gaskets to maintain a seal, which might lead to narrower and more occluded gaps. Finite element modeling of the crevice geometry indicates that a 2.0 mm Ti tube produces a 30% narrower crevice width than a 1.2 mm tube for a given gasket compression force because of the reduced deformation of the thicker material under clamping pressure that closes the microscopic gaps through which some oxygen can ingress.

Influence on Thermal Performance Temperature as a Catalyst
The crevice corrosion kinetic is directly enhanced by the thermal gradient across the titanium sheath. For the same power density, the outer surface temperature is higher for a thicker wall due to higher conductive resistance. For example, a 2.0 mm wall at 3 W/cm2, 80°C scrubber slurry will have an outer surface temperature of around 102°C while a 1.2 mm wall in the same slurry will have an outer surface temperature of about 93°C. The rates of crevice corrosion follow an Arrhenius relationship, approximately doubling for each 9°C rise in temperature. So a thicker wall running hotter can actually get through faster even if it has more material to burn through. Experimental data from FGD scrubber simulants (50,000 ppm Cl⁻, pH 4.5, 80°C bulk) indicate that a 1.2 mm titanium tube with a well-designed crevice gap of 0.25 mm survives 6 years, while a 2.0 mm tube with a tight crevice of 0.08 mm at the same gasket fails by perforation in 14 months-the thicker wall fails almost five times faster.

Trade-off Synthesis: A Design Guide for Gasket Crevice Control
The following matrix is designed to help in the selection of titanium tube wall thickness and gasket design for FGD scrubber heaters and is based on field data from over 50 operating coal-fired power plants.

Gasket Type and Crevice GeometryRecommended Titanium Wall Thickness Anticipated Crevice Corrosion Life Core Engineering Basis
PTFE envelope gasket with elastomer core (crevice width 0.3-0.5 mm, compressible)1.2 mm – 1.5 mm 6 – 8 yearsA wide fissure lets oxygen in to repassivate the titanium. Thinner wall runs cooler lowering corrosion kinetics.
Flat PTFE gasket, crevice width 0.1 to 0.2 mm, non-compressible1.5 mm – 1.8 mm 3 – 4 years Crevice is slightly tight , limiting oxygen entry . Thicker wall adds corrosion allowance but hotter surface speeds attack. Acceptable service of modest degree of severity.
Metal-reinforced compressed fiber gasket (crevice width <0,1 mm)Not recommended (any thickness) < 18 months Too tight for oxygen penetration. Thicker walls break faster owing to higher temperature Increase thickness of wall, change gasket design.
Tube joint, welded or flared (no gasket, no crevice)1.0 mm 1.2 mm > 10 YearsThe only full solution is to remove the crevice. The wall needs to be thick enough to handle the pressure.
Engineering Beyond the Wall: Crevice Elimination as the Main Strategy
For FGD scrubber heaters, the best way to avoid chloride-induced crevice corrosion is to eliminate the crevice, not to increase wall thickness. By welding the titanium tube directly to a titanium stub flange or flaring the tube end to provide a metal - to - metal seal without gaskets , the oxygen concentration cell that promotes corrosion is eliminated . In retrofit applications where welding is not practicable, the optimum compromise is a wide gap design with a molded PTFE gasket with an intentional 0.4-0.5 mm clearance along with a 1.2 mm titanium wall. The routine assessment of gasket compression force is necessary, as over-tightening a flange reduces the crevice width and causes quick attack, irrespective of wall thickness.

Conclusions: Crevice Corrosion: No Safety by Thickness
On a FGD scrubber heater with titanium tubes, a thicker wall does not always give superior protection against chloride driven crevice corrosion at gaskets. The primary determinants are the shape (width and oxygen access) of the fissure and the surface temperature. The thick wall (2.0 mm) and tight gasket create a narrow, heated crevice which accelerates corrosion to the point where the extra material is consumed faster than a thin wall in a well constructed gap. The best design is a titanium sheath of 1.2–1.5 mm and a compressible gasket, which maintains a crevice width of 0.3–0.5 mm or ideally a welded junction that removes the fissure. When ordering FGD scrubber heaters, the most important data to offer is the estimated chloride concentration (ppm) and the gasket type and compression specification. Not a thicker wall as a safety buffer.