Anti-corrosion coatings in low-temperature evaporation chambers:

The locations where micro-cracks are most likely to form first under alternating acid-alkali conditions,

thereby triggering corrosion of the substrate

1. The interface zone where the liquid level fluctuates (the most critical area, where damage occurs first)

This is the area subjected to the most intense alternating acid-alkali impact:

The liquid level fluctuates wildly, alternating between immersion and exposure to vapour, causing the coating to repeatedly absorb and lose water, as well as undergo thermal expansion and contraction.

Coupled with constant wetting by secondary steam condensate and salt crystallisation causing expansion and cracking, hairline micro-cracks first appear in the coating. Corrosion penetrates directly from these cracks into the steel substrate, gradually leading to flaking, blistering and peeling.

2. Tank weld seams, weld toes and repair areas

Due to the inherently non-uniform metallography of the weld seams and high residual stresses, the coating in these areas is prone to micro-pores and micro-cracks.

Under alternating acid-alkali conditions, corrosive media seep in through the pre-existing micro-cracks in the weld seam coating and propagate inwards along the intergranular boundaries of the weld, leading to preferential corrosion of the weld seam and the coating peeling off in sheets.

3. Nozzle openings, manhole flange corners and right-angle bends

At all right angles, sharp corners and the edges of openings, the coating is prone to thinning, sagging and stress concentration during application.

Fluctuations in temperature and negative pressure induce deformation stresses, whilst repeated penetration by acids and alkalis causes chipping and cracking to begin at micro-cracks in the edges, gradually spreading inwards.

4. Base of internal supports, stiffening ribs and cylinder body junctions

The bases of stiffening ribs and supports are dead zones for liquid accumulation and points of stress concentration.

Acid and alkali waste liquids tend to stagnate and concentrate here, whilst structural deformation pulls at the coating,

initially causing fine radial cracks; once the medium penetrates, this leads to hidden corrosion beneath the scale.

5. Top of the vapour zone and areas surrounding the demister

Secondary steam carrying acid and alkali mist condenses over time, leaving the coating continuously exposed to alternating layers of moist acidic and alkaline environments;

Significant temperature fluctuations under negative pressure cause thermal fatigue in the coating, resulting in fine, mesh-like micro-cracks,

which are initially imperceptible to the naked eye but gradually allow corrosion to penetrate the substrate through these fissures.

6. Material scouring zones and areas where the circulating feed stream strikes the wall directly

High-velocity feed scouring combined with alternating acid and alkali components;

The coating surface first develops fine abrasion grooves, which gradually progress into cracks;

The medium penetrates along these grooves, forming localised pitting corrosion.

7. Dead zones with residual liquid where wet and dry conditions alternate during pressure retention and drainage

After shutdown, residual liquid is drained and air-dried, but the area is re-immersed during pressure retention;

in these dead zones, the cycle of wet and dry conditions is most severe;

repeated crystallisation and expansion of acids, alkalis and salts force open micro-cracks, which gradually spread from localised areas to cause widespread failure.