What Is the Minimum Recommended 316 Stainless Steel Sheath Wall Thickness for an Electric Immersion Heater Operating in 5% Sodium Hypochlorite Solution at 40 Degrees Celsius for Bleach Production Service?

Chemical process engineers building electric immersion heaters for sodium hypochlorite production, bleach manufacturing and water treatment disinfection systems experience substantial hurdles when using 316 stainless steel as the sheath material. Sodium hypochlorite is a powerful oxidiser and is a high-pH solution that aggressively destroys many metals. 316 stainless steel is more resistant than 304 due to the presence of molybdenum, but it is still susceptible to pitting, crevice corrosion and stress corrosion cracking in hypochlorite service, especially at extreme temperatures and concentrations. The wall thickness of the sheath controls the corrosion allowance and, more importantly, the time necessary for localised pitting to permeate the wall. This article presents minimum recommended wall thickness for 316 sheaths in dilute sodium hypochlorite service based on published corrosion data and field experience from the bleach production business.

Corrosion Mechanisms of 316 Stainless Steel in Sodium Hypochlorite Solution
Sodium hypochlorite (NaOCl) is a strong oxidising agent that preserves the passive film on 316 stainless steel in optimum conditions. But several conditions favour the localised attack. The hypochlorite ion is a chloride-containing oxidising agent, and hence simultaneously offers the aggressive chloride anion and the oxidative potential required to break down the passive film. Pitting of 316 may take place at a rate of 0.5 – 2.0 mm per year at concentrations > 1 % accessible chlorine and > 35°C. The pitting is very localised with pits reaching a depth of 1 mm in months with little effect on the areas surrounding the pitting. A single deep pit in the sheath of the immersion heater is sufficient to breach the wall leading to process fluid penetration, electrical failure, and contamination of the hypochlorite solution with metal ions that catalyse breakdown. General corrosion rates are low, usually <0.05 mm/year, such that wall thickness is destroyed mostly by pitting not by uniform thinning. This means that raising the wall thickness will only help to a limited extent, because a pit developing in a localised weak area will penetrate a 2.0 mm wall only slightly slower than a 1.0 mm wall. The real metric is not the average thickness, but the lack of surface defects and residual stresses, which are the locations of pit initiation.

Influence of Wall Thickness on Pitting Penetration Time
After the initiation of a pit the pitting penetration rate is roughly constant at a given hypochlorite content and temperature. Results of immersion experiments in 5% NaOCl at 40°C reveal pit initiation after 200–500 hours for polished samples of 316. Pits, once established, grow at rates of 0.03–0.08 mm day −1 (about 0.9–2.4 mm month −1 ). At this rate, a wall thickness of 1.0 mm would be pierced in 13–33 days from pit initiation. A 2.0 mm wall would last for 22-67 days from initiation. The service life is the sum of initiation time and propagation time. The entire life is about 200-500 hours+13-33 days for a 1.0 mm sheath, or about 20-50 days. Total life for a 2.0 mm sheath is 200–500 hours + 22–67 days or ~30–80 days. The thicker wall adds just 10-30 days to life which is a minor improvement for the additional material cost and thermal penalty. Due to these short service lives, 316 is rarely specified for continuous immersion in sodium hypochlorite greater than 1% concentration and 30°C. If extended service life is required then some other materials such as titanium or specific alloys have to be used.

Minimum Wall Thickness Recommendations for Hypochlorite Service 
The following table shows minimum suggested sheath wall thicknesses for electric immersion heaters in sodium hypochlorite service. Values are based on continuous operation, good design of the system to minimise the possibility of cracks and stagnation, and acceptance of a relatively limited service life measured in weeks or months rather than years.

Available Chlorine Concentration Solution Temperature Expected Pitting Rate (After Initiation) Minimum Recommended Wall Thickness of 316Minimum Thickness – Total Service Life ExpectedAlternate Recommendation
1 – 2% 20 – 30°C 0.5 – 1.0 mm/month 1.6 mm 2 – 4 monthsMarginal Titanium preferable
1 – 2% 30 – 40 o C 0.8 – 1.5 mm/month 2.0 mm 1.5 – 3 monthsTitanium is a good choice
2 – 5% 20 – 30°C 0.8 – 1.5 mm/month 2.0 mm 1.5 – 2.5 monthsTitanium necessary for dependable service
2 – 5% 30 – 40°C 1.2 – 2.0 mm/month 2.5 mm 1 – 2 months Titanium required 5 – 10% 20 – 30°C 1.5 – 2.5 mm/monthNot recommended < 1 month Use titanium or Alloy C-276
Any concentration Above 40°C 2.0 – 4.0 mm/mthWeeks Not recommendedTitanium or non-metalic heating
Periodical or batch serviceAny temperature Lower due to drying 1.2 – 1.6 mm Variable Acceptable for non critical applications
316 stainless steel is a poor choice for any sodium hypochlorite service above 2% concentration or 30C regardless of wall thickness. Its short service life and the possibility of rapid pitting failure make it unsuitable for crucial procedures. Titanium (Grade 2 or Grade 7) demonstrates excellent resistance to sodium hypochlorite with corrosion rates <0.01 mm/year at high concentrations and temperatures. The higher initial cost of titanium – generally 3-5 times that of 316 – is rapidly repaid through prolonged service life and less maintenance.

Design Modifications for Improved 316 Sheath Life in Hypochlorite Service
If 316 is required for dilute hypochlorite service because of cost or material availability, four design adjustments can increase the service life beyond the numbers provided in the table. 1. Maintain a hypochlorite pH greater than 11. Solutions of sodium hypochlorite at less than pH 11 have a faster rate of decomposition and are more hostile to stainless steel. Pitting rates are reduced 50-70% with automatic pH control with caustic addition. The second adjustment is to make the sheath surface as smooth as possible. Electropolished 316 < 0.2 micron surface finish The Ra is lower compared to the as-swaged material, which results in an increase in initiation time by a ratio of 2-4. The final change is to run at the lowest watt density achievable. Decreasing the watt density from 10 W/cm2 to 5 W/cm2 decreases the sheath surface temperature by 10-15 oC and the pitting rate by ~30-40% in the temperature sensitive regime of 30-50 oC. The fourth alteration is periodic cleaning of the sheath surface to eliminate sediments which generate sites for crevice. A weekly water washing or dilute acid wash can greatly extend service life. For most industrial bleach production applications these changes are not sufficient to make 316 a reliable material. For any heater to be immersed in sodium hypochlorite for more than intermittent use, titanium sheaths should be specified by the engineers. The cost of a single 316 heater failure, including lost output, solution contamination and labour for repair, is usually more than the added cost of titanium. Always include the actual concentration, temperature and anticipated daily hours of operation when specifying hypochlorite service. The supplier suggesting 316 for continuous hypochlorite service beyond 2% concentration without mentioning the limitations of this material is not offering competent engineering advice. In instances where titanium is too expensive, other heating methods can be considered, for example, external tank heating through a titanium heat exchanger or steam injection, which does not involve direct contact of metal with the hostile hypochlorite solution.