Why Does a Titanium Immersion Heater with a Welded End Cap Fail Faster by Crevice Corrosion at the Weld Root Than by General Thinning in a 5% Sodium Hypochlorite Solution at 60°C?

Titanium has a modest uniform corrosion rate of 0.01–0.05 mm per year in 5% sodium hypochlorite (NaOCl) at 60°C because to the aggressively oxidising atmosphere. However, the welded end cap of an immersion heater has a fissure at the weld root, the space between the end cap and the tube wall inside the heater. This geometry of fissure confines the stationary electrolyte. Hypochlorite decomposes within the fissure to chloride and oxygen. Chlorides accumulate to 5 to 10 times the bulk concentration. In the crack the passive coating breaks down and the corrosion rate increases to 0.3 to 1.0 mm/yr, 10 to 100 times the rate of uniform thinning. Failure happens preferably in the weld root, frequently without exterior evident corrosion.

Mechanism of crevice corrosion at weld root in hypochlorite

The bulk hypochlorite solution is an oxidising environment that maintains a stable passive film. The geometry of the weld root crevice restricts the supply of oxygen. Decomposition of hypochlorite: 2OCl- → O2 + 2Cl-. Chloride concentration increases up and oxygen depletes. The local pH falls as hypochlorous acid is formed. The combination of high chloride, low oxygen and acidic pH degrades the passive film. The huge external cathode (the rest of the heater) leads to fast anodic dissolution in the fissure. The residual tensile stress of welding is also contained in the weld root shape and facilitates the fracture initiation.

Quantitative Rates of Weld Root Crevice Corrosion

Corrosion rates have been established for Grade 2 titanium from controlled tests of 1000 hours in 5% NaOCl at 60°C in various heater sites. For the tube body (smooth surface without crevice), the uniform corrosion rate is 0.01-0.03mm/year, and the surface is bright without pitting. The corrosion rate at the external weld toe (the outside of the welded end cap) is 0.02 to 0.05 mm per year and the surface is lightly etched. For the weld root crevice (inside gap 0.1–0.3 mm) the crevice corrosion rate is 0.3–0.8 mm/yr and severe grooving is observed, perforating a 1.2 mm wall in 1,500–4,000 h. For the weld root with a badly fitted end cap (gap >0.5 mm), the rate of crevice corrosion grows to 0.5-1.2 mm/year and perforation happens in 1,000-2,500 hours.

Effect of temperature and hypochlorite concentration on severity of crevice

The severity of weld root crevice corrosion is increased by the content of hypochlorite and temperature. The crevice corrosion rate was 0.15–0.40 mm/yr at 1% NaOCl and 60°C. At 5 % and 60 °C rate is 0.30–0.80 mm per year At 10% and 60 °C the rate is 0.50–1.20 mm/year. At 5% and 40°C the rate is 0.10-0.25 mm/yr. At 5% and 80°C the rate is increased to 0.50-1.00 mm per year. The inclusion of chlorides in the hypochlorite solution (from aged bleach) further raises the crevice corrosion rate by 30-50%.

Hypochlorite Service Weld Root Design & Mitigation Guide

The table below includes guidelines for design of end cap welds, and alternative designs to prevent crevice corrosion in 5% sodium hypochlorite at 60°C.

Weld Design Crevice Gap (mm) Expected Crevice Corrosion Rate (mm/year)Time to perforation (1.2 mm) Wall (hrs) Recommendation
Ground flush full penetration weld 0 (no crevice) <0.05 >10,000Good practice
Partial penetration weld, gap <0.1 mm <0.1 0.15-0.35 3,500-8,000Acceptable with inspection.
Partial penetration weld, gap 0.1–0.3 mm 0.1–0.3 0.30–0.80 1,500–4,000 Marginal
Lap weld (cap over tube), gap 0.3–0.5 mm 0.3–0.5 0.50–1.00 1,200–2,500Not advisable
Butt weld with backing ring 0.2-0.4 0.40-0.90 1,300-3,000 Avoid Engineering Beyond Weld Design

The grade of titanium impacts the crevice corrosion resistance. Grade 7 (palladium-stabilized) has 3-5 times longer initiation time, 50-70% lower propagation rate than Grade 2 in hypochlorite crevices. Grade 12 is offered intermediate improvements . Wall thickness allows for material. 2.0 mm wall, 0.5 mm per year crevice corrosion, survives 4 years. The end cap shall be a full penetration butt weld and ground flush on the inside and outside. If a crevice cannot be avoided, PTFE tape or a fluoropolymer sealer can be used to plug the gap and prevent electrolyte ingress. Periodic flushing of the heater with fresh water decreases the hypochlorite content in crevices.

A Specification Informed

Butt weld with full penetration on end cap for titanium immersion heater in 5% sodium hypochlorite at 60°C, ground flush inside and out to a crevice free geometry. Where a full penetration weld cannot be achieved, define a minimum weld root gap of less than 0.1 mm and demand post-weld inspection to verify the gap size. For critical applications choose Grade 7 titanium and add a helium leak test to ensure weld integrity. Inspect the weld region quarterly in operation with a borescope or dye penetrant; if corrosion products are apparent inside the weld, the heater shall be replaced. The engineer eliminates the fast corrosion mechanism by constructing a crevice free weld root, therefore the heater will have a service life limited only by uniform thinning.