1. Why Q235NH cannot protect itself on the inner wall
Q235NH only forms a protective stable patina in atmospheric environment (open air, oxygen, alternating wet and dry).
The inside of a flower box is:
Long-term damp / wet
Directly contact with soil, water, fertilizer, plant roots
Poor ventilation, almost no air circulation
Contains chloride, sulfate, organic acids from soil & fertilizer
These conditions destroy the patina formation mechanism of weathering steel.
Q235NH will rust quickly, locally corrode deeply, and even perforate within 1–3 years.
2. What happens if no inner coating?
Inner wall rusts rapidly, large areas of loose rust
Serious pitting corrosion, easy to penetrate
Rust water seeps out, pollutes the exterior and ground
Service life is greatly shortened
3. Recommended inner wall anti-corrosion solutions
Heavy-duty anti-corrosion paint (epoxy primer + polyurethane topcoat)
Bituminous anti-corrosion coating
PE/HDPE plastic liner
Composite anti-corrosion layer for long-term soil contact
All must cover full inner surface, including bottom, corners and welds.
4. Only the outer surface doesn't need coating
Outside: exposed to air → forms stable patina, no paint needed.
Inside: buried humid environment → must be coated.
