Alkali pipes often leak in winter; I seek advice from those with experience
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Our company produces and uses liquid caustic soda, in concentrations of 30% and 50%. We use steam for insulation in winter, and leaks in the caustic soda pipes occur very frequently. I would like to know whether there is a relationship between the corrosion resistance of liquid caustic soda and temperature; I hope experts can provide some guidance
. Search for this paper; it should be able to help clarify your doubts
Analysis of corrosion causes: Ordinary carbon steel forms a surface film composed mainly of Fe3O4 or Fe2O3 in alkaline solutions. Meanwhile, the precipitation of carbides and nitrides at the grain boundaries renders the surface film there unstable, making it prone to dissolution. Under the action of external stress, grain boundary cracks are formed, causing selective dissolution of FeO2- in the newly exposed iron, thereby resulting in stress corrosion. Carbon steel is prone to alkaline embrittlement in all concentration ranges above 5% NaOH solution, with concentrations around 30% being the most hazardous. The lowest temperature at which alkaline embrittlement occurs is 50°C, and it tends to happen most frequently in high-temperature areas near the boiling point. See Figure 1. During the use of the pipeline, in summer or when the pipeline is not heated, an alkali solution with a concentration of 30–40% does not cause alkali embrittlement ； In winter, when the pipes are heated and the temperature exceeds 50°C while the alkali concentration remains at 30–40%, alkali embrittlement occurs, as actual alkali-containing pipes usually reach temperatures above 50°C when heated. Furthermore, an alkaline solution will dissolve iron only under highly concentrated conditions, through the following reactions: 3Fe + 7NaOH → Na3FeO3·2Na2FeO2 + 7H; Na3FeO3·2Na2FeO2 + 4H2O → 7NaOH + Fe3O4 + H; 7H + H → 4H2; 3Fe + 4H2O → Fe3O4 + 4H2. Therefore, liquid alkali corrosion is related to temperature.
Is using steam for insulation making the temperature a bit too high? Too much heat can cause alkaline embrittlement. Electric heating or hot water would be better, I think. My workplace uses electric heating, with splash-proof protective rings installed on the flanges.
It is best to use hot water for heating, and the weld seam must undergo heat treatment; after such treatment, the hardness of the weld seam and its heat-affected zone will be similar to that of the base material, which is better.
An increase in temperature exacerbates corrosion; furthermore, the insulation layer loses its effectiveness when it becomes damp, which also increases corrosion on the outside of the pipes. Our company offers online repair and sealing techniques, as well as pipeline reinforcement services