At What Specific Threshold of Sulfuric Acid Concentration Does a 1.2 Millimeter 316 Stainless Steel Sheath Require Post-Weld Annealing to Prevent Intergranular Corrosion in Electric Immersion Heaters?

Welding of the attachment of mounting flanges and fittings to the 316 stainless steel sheath must be done for manufacturing engineers and quality assurance personnel in charge of electric immersion heaters for use in sulphuric acid service. Welding generates heat that can produce sensitisation, the formation of chromium carbides at the grain boundaries in the heat-affected zone. 316 sensitised loses its corrosion resistance and becomes susceptible to intergranular corrosion which preferentially attacks the grain boundaries producing fast material loss and possible heater failure. The wall thickness of the sheath controls the pace of cooling after welding and the necessity for post-weld annealing. In this work we determined the threshold of sulphuric acid concentration requiring post-weld annealing to avoid intergranular corrosion attack in a 1.2 mm 316 sheath.

Sensitisation and Intergranular Corrosion Mechanism in 316 
When 316 stainless steel is heated in the temperature range of 450°C to 850°C (sensitisation range), chromium carbides tend to precipitate at grain boundaries. This precipitation results in the regions near the grain boundaries to be depleted of chromium and thus deficient in the 12% chromium required for passive film production. In a corrosive environment, these zones of chromium depletion are preferentially corroded, resulting in grain boundary attack and ultimately in the removal of material. For a thin walled sheath of 1.2 mm thickness the cooling rate after welding is fast as the thermal mass is minimal and so the heat dissipates quickly. A thin wall will cool from welding temperature to below 450°C in seconds and spend little time in the sensitisation range. For thick-walled sheaths (more than 3 mm) the cooling rate is slower, which increases the danger of sensitisation. However, for a 1.2 mm wall, the cooling rate is generally quick enough to avoid sensitisation under conventional gas tungsten arc welding circumstances. It's not only wall thickness that's the key, it is the carbon content of the 316 material. Low carbon 316L (max 0.03% C) is particularly resistant to sensitisation independent of wall thickness. Standard 316 (max 0.08% carbon) can be sensitised if cooled too slowly.

Critical Concentration of Sulphuric Acid for Detection of Sensitisation
The intergranular corrosion of sensitised 316 is especially severe at certain concentrations of sulphuric acid. The Strauss test (ASTM A262 Practice E) is a test for sensitisation using boiling 16% sulphuric acid with copper sulphate. The most susceptible concentration of sulphuric acid to intergranular assault in service is between 10 and 40% at temperatures over 50°C. Below 10 % the acid is not active enough to target grain boundaries preferentially. Above 40% acid it becomes oxidising and repassivates the surface obscuring sensitisation. 1.2 mm 316 sheath in sulphuric acid. For 60-80°C the concentration threshold for the risk of intergranular corrosion is around 15-25%. A sensitised sheath may live for years without obvious attack below 15 per cent. At >25% acid, the oxidising tendency of the acid may prevent intergranular attack but pitting becomes a worry. A sensitised sheath in the critical 15-25% range can lose material at a rate of 0.5-2.0 mm/year along grain boundaries, piercing a 1.2 mm wall in a matter of months. So, for applications with sulphuric acid concentrations between 15% and 25% at temperatures over 50°C, post-weld annealing is strongly advised, unless the material is 316L.

Annealing Requirement Matrix for Welded 316 Sheaths 
The table below gives recommendations for post-weld annealing of 1.2 mm 316 sheaths according to the concentration of sulphuric acid, the operating temperature and the specific grade of 316 utilised. The welding assembly is heated to a temperature of 1,050°C for 15 minutes and rapidly cooled (water quench or forced gas).

Sulphuric Acid Concentration Operating Temperature 316 Grade Post Weld Annealing Needed? Alternative Recommendation
Less than 10 per cent of316 or 316L temperature No Standard welding permitted
10 – 15% Below 60°C 316L No Preferred 316L
10 – 15% <60°C 316No, however 316L recommended Good without annealing
10 – 15% 60 – 90°C 316L No Caution against attack
10 – 15% 60 – 90°C 316 Yes Anneal or use 316L 15 – 25%Below 50oC 316L No acceptable 316L 15 – 25%Table 11.2.2.1.2. Composition and applications of 316 and 316L SS at elevated temperaturesAny temperature Any 316 N/A Sensitisation is not the predominant failure mode; Pitting is dominant
Over 40%Any temperature Any 316 Not applicable Oxidizing acid; intergranular attack not likely
For the critical concentration range of 15-25% at temperatures over 50°C, if standard 316 is utilised, post-weld annealing of a 1.2 mm 316 sheath is needed. But annealing gives an added margin of safety even with 316L. If annealing is not performed, the zone of a welded flange attachment may develop intergranular corrosion penetrating the thin wall in 3 to 12 months, producing detachment of the flange or leakage of the sheath.

Practical Considerations on Annealing Thin Walled Sheaths
Practical difficulties of post-weld annealing of 1.2mm tube of 316 are discussed. If the assembly is not properly protected, the high temperature of 1050°C may allow for oxidation of the internal resistance wire. Annealing should be carried out in a controlled environment furnace using either argon or dissociated ammonia to avoid oxidation. Alternatively, the weld zone can be locally annealed by induction heating without heating the entire heater. The MgO insulation in the heater swages expands differently from the steel and can shatter the sheath, therefore full furnace annealing of heaters that have previously been swaged with MgO insulation is not possible. Therefore annealing must be done after welding but prior to swaging or localised to the weld area only. Engineers should stipulate that any welding performed on 316 sheaths that will see service in 15 to 25 percent sulphuric acid should be followed by localised induction annealing of the heat-affected zone. The annealed zone shall extend at least 10 mm on either side of the weld. Proper annealing can be checked using a ferrite meter or by submitting a sample weld for ASTM A262 Practice E testing. A supplier that cannot perform post-weld annealing or offer verification testing shall not be employed for sulphuric acid service in the critical concentration range . The cost of one intergranular corrosion failure, which is usually unexpected and without warning, is significantly greater than the cost of good annealing processes. For new installations, choosing 316L, rather than standard 316, eliminates the potential of sensitisation for 1.2 mm walls under all but the most extreme conditions and is strongly recommended for any sulphuric acid service above 10% concentration.