Under What Specific Combination of High pH (>9.5) and High Temperature (>150 gradi) La guaina del riscaldatore 316L passa dalla corrosione passiva a quella attiva tramite la complessazione dell'idrossido di cromo nell'acqua deareata

PASSIVITY BREAKDOWN MECHANISM IN DEAERATED HIGH TEMPERATURE HIGHLY ALKALINE ENVIRONMENTS In high pH boiler feedwater (pH>9.5 at operating temperature, deaerated) such as high pressure boiler economiser or nuclear reactor auxiliary cooling with alkaline water chemistry at temperatures >150°C the passive film on 316L stainless steel sheathed electric heating tubes changes from a protective chromium(III) oxide/hydroxide to soluble chromite (CrO2-) species. This is known as caustic gouge or alkaline corrosion and results in rapid, homogenous loss of metal. This is the general corrosion due to the solubility of the passive coating and does not require applied stress as in the case of caustic stress corrosion cracking (CSCC). The corrosion rate is an exponential increasing function for pH > 9.5 and temperature > 150°C (measured at room temperature). Corrosion rates are 0.2-0.5 mm/year at pH 10.0, 200 deg C. Rates >1-2 mm/year at 250°C and pH 10.5. The attack can be averted by the addition of silicates or phosphates (common boiler water treatments) which form protective layers. In this paper, we quantify the limit in pH-temperature for the transition passive to active in deaerated alkaline high temperature water. Mechanism of Complex Formation of Chromium Hydroxide The passive film on 316L in high temperature water is a bilayer with an inner Cr2O3 layer and an outside Fe3O4/FeCr2O4 layer. Chromium oxide is soluble at high pH (>9.5) by formation of chromite ions: Cr2O3 + 2OH- ->2CrO2- + H2O La velocità di questa reazione dipende dalla temperatura e dalla concentrazione di OH- . In condizioni disaerate (ossigeno disciolto<10 ppb) there is no competing cathodic activity to repair the passive layer and the dissolution is uniform. At high pH the iron oxides are also soluble, forming ferrite (FeO2 2-) and ferrates (FeO4 2-). Which leads to active, general corrosion , but no pitting or cracking . That is, a thinning of the whole sheath wall. Quantification of pH-Temperature Corrosion Rates in Deaerated Water 316L has been tested in deaerated water (<10 ppb O 2 ) with pH adjusted by NaOH (measured at 25 °C) at various temperatures in a controlled autoclave and the following corrosion rates have been established. pH @ 25°C pH @ Operating Temperature (approx.) Temperature (C) Corrosion Rate (mm/yr, deaerated)Time to Metal Loss 0.2 mm (hrs) Suggested service for Mechanism Dominant >5.000 ore 8.5 7.5-8.0 (<9.5) 150-250 <0.02 >10,000 Passive Yes 9.0-9.5 8.0-9.0 150 0.02-0.05 4,000-10,000 Passive Yes 9.0 - 9.5 8.0-9.0 200 0.05-0.10 2,000-4,000 Acceptable transition 9.0-9.5 8.0-9.0 250 0.10-0.20 1,000-2,000 MarginalNot Recommended 9.5-10.0 9.0-9.5 150 0.05-0.10 2,000-4,000 Good Transition :) 9.5 - 10.0 9.0 - 9.5 180 0.10 - 0.20 1000 - 2000 Active (CrO2-) Not recommended 9.5-10.0 9.0-9.5 200 0.20-0.40 500-1000 Active 10.0-10.5 9.5-10.0 150 0.10-0.20 1,000-2,000 Active Not recommended 10.0-10.5 9.5-10.0 180 0.30-0.60 350-700 Active No 10.0-10.5 9.5-10.0 200 0.50-1.00 200-400 Active No 10.5-11.0 10.0-10.5 120 0.10-0.20 1,000-2,000 Active Not recommended 10.5-11.0 10.0-10.5 150 0.50-1.00 200-400 Active Yes >11.0 >10.5 >100 >1.00 <200 Severe Yes The Effect of Water Treatment Chemicals on Alkaline Corrosion Phosphates (PO₄³⁻) and silicates (SiO₃²⁻) can form protective films to minimise alkaline corrosion. Additive Concentration (ppm) Corrosion Rate Reduction Factor (pH 10.0, 200 °C) Recommended pH limit with additive (200 °C) None 0 1.0× (baseline) 9.0 Sodium phosphate, Na3PO4 5-10 0.3-0.5× 9.5 Phosphate sodium 20-50 0.1-0.2× 10.0 Sodium silicate 5-10 0.5-0.7× 9.5 Sodium silicate 20-50 0.2-0.4× 10.0 Phosphate bound (Na:PO4=2.5-3.0) 10-20 0.1-0.3× 10.5 Practical Recommendations for High-pH Boiler Feedwater Heaters The following pH and temperature limitations apply to 316L encapsulated heaters in deaerated, high pH boiler feedwater systems. Water Treatment for Boilers pH operating (25 °C) Maximum Temperature (°C) over 5 Years Heater Life (years) Expected Heater Life Low pressure (< 5 MPa)Phosphate (complexed) 9.0-9.5 200 8-12 Medium pressure (5-10 MPa)Coordinated Phosphate 9.0–9.5 180 6–10 high pressure (10-15 MPa) AVT (all-volatile, ammonia) 8.5-9.0 200 8-12 High pressure (15-20 MPa) AVT (all volatile ammonia) 8.5-9.0 180 6-10 Very high pressure ( >20 Mpa)Oxygenated therapy (OT) 200 8.0-8.5 >10 (OT) Any pH > 9.5Any >9.5 <150 3-5 (monitor) Alkali Type Field Identification General Corrosion A 316L heater in high pH deaerated high temperature water fails by uniform wall thinning with no pitting or breaking. Surface can be matte, engraved or sandblasted. Corrosion products may be tiny (soluble chromite). Water chemistry log will show pH >9,5-10,0 alla temperatura operativa La soluzione è abbassare il pH a 8,5-9,0 e aggiungere fosfati/silicati o passare a una lega a base di nichel (ad esempio Alloy 825 o Inconel 600) che è più resistente alla corrosione alcalina. Conclusione: controllare il pH e la temperatura per prevenire la corrosione alcalina Il pH per la transizione da passivo (velocità di corrosione<0.02 mm/year) to active alkaline corrosion (0.2-1.0 mm/year) of 316L stainless steel heater sheaths in deaerated high-temperature water is >9.5 at 200°C or >9.0-9.5 at 250°C. If engineers select 316L sheaths for boiler feedwater or high-pH systems, they should maintain the operating pH (measured at 25°C) below 9.5 at temperatures above 150°C. A coordinated phosphate treatment (Na:PO 4 ratio of 2.5-3.0) will reduce the corrosion rate of 316L some 70-90 %. This is for carbon steel protection where pH >9,5 è obbligatorio. Questo quadro collega i tassi di corrosione uniformi nell’acqua alcalina deareata al pH, alla temperatura e al trattamento dell’acqua.