In Flue Gas Desulfurization Scrubbers at 70°C, How Does Chloride Concentration Above 20,000 ppm Dictate the Minimum Titanium Tube Wall for Pinhole Prevention?

Coal-fired power plants and industrial boilers are equipped with flue gas desulfurization (FGD) scrubbers that operate in some of the most corrosive conditions of environmental equipment. The scrubber fluid is a complicated mixture of calcium sulfite/sulfate, fly ash and dissolved chlorides which concentrate as the scrubbing loop is recirculated. Titanium Grade 2 has been the standard material of construction for reheater tubes and spray headers in FGD systems due to its resistance to sulfite attack. However, chloride concentrations are often greater than 20,000 ppm (2%) in FGD scrubbers and may even reach 50,000-80,000 ppm in zero-liquid-discharge systems. At these chloride concentrations titanium is sensitive to pitting and crevice corrosion and the only protection against through-wall perforation is the tube wall itself, both thickness and metallurgical quality. This article presents the minimum titanium tube wall thickness required to preclude pinhole perforation as a function of chloride concentration, temperature and crevice in FGD scrubber service.

Pitting of Metals in High Chloride FGD Liquor
The pitting corrosion of titanium in high-Cl and low-pH FGD settings follows a well-documented initiation-propagation pattern. The breakdown of the passive TiO 2 film occurs at discrete places where chloride ions replace oxygen in the film lattice. Once a pit is started, the acidic chloride solution in the pit (pH can drop to 1-2) continues active dissolution, while the surrounding passive surface is protected. The pit expands hemispherically and the time to perforation is approximately proportional to the square of the wall thickness for a particular pit growth rate.

The critical pitting temperature (CPT) of Grade 2 titanium in chloride solutions decreases with increasing chloride concentration. At a pH of 4 (typical of FGD scrubbers), 20,000 ppm Cl- gives a CPT of about 85°C, above the average scrubber working temperature of 70°C and provides a safety margin. At 40,000 ppm Cl- the CPT decreases to 70°C, i.e. the scrubber works exactly at the threshold. The CPT drops to 55°C at 60,000 ppm Cl- and active pitting can occur at normal service temperatures. The rates of pit propagation rise logarithmically with increasing chloride concentration: at 20,000 ppm Cl- and 70°C a propagating pit deepens at 0.2-0.3 mm per year; at 40,000 ppm Cl- the rate is 0.5-0.8 mm per year; at 60,000 ppm Cl- rates of 1.0-1.5 mm per year are recorded.

Wall Thickness: A Pinhole Prevention Approach
The minimum wall thickness required to give 5 years service life without pinhole perforation of FGD scrubber heaters and tube bundles is dependent upon the predicted pit propagation rate and the statistical distribution of pit start sites. A conservative pit growth model (pit depth=k*t^0.5 where k is a function of chloride concentration) is used and the tube life is assumed to be determined by the deepest pit. The needed wall thickness is computed as:

The pit propagation constant k at 20,000 ppm Cl- is 0.28 mm/year^0.5. For 5 year life, max pit depth = 0.28 × √5 = 0.63 mm. The safety margin for the 1.2 mm wall tube is about 0.6 mm, which is sufficient for clean tubes with no preexisting problems.

40,000 ppm Cl- k = 0.48 mm/year^0.5 Maximum pit depth in 5 years = 0.48 * sqrt(5) = 1.07 mm. 1.65mm wall tube provide margin, 1.2mm tube would perforate.

k = 0.70 mm/year 0.5 at 60,000 ppm Cl-. Maximum depth of pit in 5 years = 0.7 × √5 = 1.57 mm. A 2.0 mm wall tube is necessary for reliable 5 year service. 1.65 mm will only give 3-4 years.

Titanium Tubes for FGD Scrubber Application Matrix
FGD System Type Typical Chloride ConcentrationRecommended Titanium Grade Minimum Wall Thickness Service Life Expected
Coal fired power plant, wet limestone (bleed stream regulated)15,000-25,000 ppm Grade 2 1.2-1.5 mm 5-8 years
Coal fired power plant, high chloride coal 25,000-40,000 ppm Grade 2 or Grade 7 1.65 mm 4-6 years
Zero-liquid-discharge FGD (closed loop) 40,000-60,000 ppm Grade 7 (Ti-0.15Pd) 2.0 mm 5-7 years
Industrial boiler FGD (variable coal) 10,000-20,000 ppm Grade 2 .8-.12 mm 8-10 years
Waste-to-energy FGD (high chloride from waste) 50,000-80,000 ppm Grade 12 or Grade 7 2.5 mm 4-6 years
Tube Quality And Crevice Problems
Pre-existing crevices in the tube do not prevent pinhole perforation, even if the wall thickness is increased. In FGD scrubber service, crevices are present at tube-to-tubesheet rolled joints, under gaskets and under deposits of gypsum or fly ash. Crevice corrosion increases the rate of pit initiation and propagation by a factor of 3-10. Welded tube-to-tubesheet junctions (no fissures) are preferred to rolled or flanged connections at any chloride concentration above 20,000 ppm. Tube surfaces should be devoid of scale, embedded iron particles (from forming tools), and surface scratches deeper than 10 percent of wall thickness. The best pitting resistance is offered by electropolished or pickled finishes.

For FGD scrubbers working at 70°C with chloride concentrations above 20,000 ppm, a minimum titanium tube wall thickness of 1.65 mm is required for Grade 2 in moderately concentrated systems (20,000-40,000 ppm), rising to 2.0 mm for Grade 7 systems beyond 40,000 ppm. Engineers should specify pickled or electropolished tube surfaces, welded rather than rolled tube-to-tubesheet joints and monitor chloride concentration weekly to detect upsets before pitting perforation occurs. For chloride levels over 60,000 ppm, other materials such as grade 12 titanium or zirconium should be considered.