The Hidden Risk in the Most Common Spill Pallet Material
Galvanised steel is the workhorse material for spill pallets, drip trays, and secondary containment sumps across UK industry. It is robust, inexpensive, and suitable for a wide range of petroleum products, oils, and many solvents. But store an alkali on a galvanised steel pallet and you are setting up a progressive structural failure that may not be visible until the containment unit collapses under load. This is not a theoretical risk — it is a well-understood electrochemical reaction, and understanding it is essential for anyone specifying secondary containment equipment.
What Is Galvanising?
Hot-dip galvanising coats steel with a layer of metallic zinc, typically 45–85 micrometres thick for structural products. The zinc serves two protective functions:
- Barrier protection: The zinc coating physically separates the steel substrate from the corrosive environment
- Sacrificial (cathodic) protection: Because zinc is anodic relative to steel in the galvanic series, it preferentially corrodes if the barrier is breached, protecting the underlying steel
This mechanism works well in mildly acidic and neutral pH environments (pH 4–12.5), where zinc forms a stable zinc carbonate patina. Outside this range, the protection fails — and it fails rapidly.
The Chemistry of Alkali Attack on Zinc
Above pH 12.5, zinc undergoes amphoteric dissolution. Unlike most metals which corrode in acid but are stable in alkali, zinc is amphoteric — it dissolves in both strong acids AND strong alkalis. The reaction in alkali conditions is:
Zn + 2NaOH + H₂O → Na₂[Zn(OH)₄] + H₂
This produces sodium zincate (soluble) and hydrogen gas. The reaction is exothermic and self-accelerating — as the zinc dissolves, the pH at the metal surface increases, accelerating further dissolution. Hydrogen evolution creates porosity in the coating and blistering, which exposes fresh zinc and fresh steel to the alkali.
The net result: a galvanised steel spill pallet that contained even a small spill of sodium hydroxide (NaOH) solution at concentrations above approximately 5% w/w will show visible zinc layer damage within hours. Within days, the steel substrate will be exposed and corroding.
Which Alkalis Are Most Dangerous to Galvanised Steel?
| Substance | Typical pH | Attack Rate on Zinc | Notes |
|---|---|---|---|
| Sodium hydroxide (caustic soda) >5% | 13–14 | Rapid (hours) | Common cleaning chemical, battery electrolyte |
| Potassium hydroxide (KOH) >5% | 13–14 | Rapid (hours) | Used in biodiesel production, cleaning |
| Ammonium hydroxide (ammonia solution) | 11–12 | Moderate (days) | Refrigeration leaks; cleaning products |
| Sodium silicate (waterglass) | 11–13 | Slow to moderate | Industrial adhesive/sealant |
| Sodium carbonate (washing soda) >10% | 11–12 | Slow | Descaling, textile processing |
| Bleach (sodium hypochlorite) | 11–13 | Rapid (hours) | Also chloride attack on steel substrate |
Legal Implications Under COSHH
Regulation 7 of COSHH requires that control measures are effective and maintained. An alkali-degraded galvanised steel sump that has lost structural integrity is not an effective control measure — it will fail to contain a spill and may itself introduce contamination (zinc compounds washed into drains constitute a controlled water pollution risk under the Environmental Permitting (England and Wales) Regulations 2016, Regulation 38).
Furthermore, the hydrogen gas evolved during alkali attack on zinc presents an explosion hazard in enclosed storage spaces — relevant to DSEAR zone classification, particularly where the storage area is not adequately ventilated.
Correct Materials for Alkali Secondary Containment
The correct material choice depends on the alkali concentration and temperature:
- High-density polyethylene (HDPE): Suitable for most inorganic alkalis up to 50% concentration at ambient temperature. Chemical resistance should be verified against manufacturer data for the specific substance.
- Polypropylene (PP): Better temperature resistance than HDPE; suitable for warm alkali solutions.
- Fibreglass (GRP): Excellent alkali resistance with appropriate resin selection (vinyl ester or epoxy); used for large bunds.
- Stainless steel (316L): Resistant to dilute alkalis; avoid for chloride-containing alkalis (bleach) due to pitting corrosion risk.
- Avoid: Galvanised steel, aluminium (amphoteric like zinc), or uncoated mild steel for any pH above 12.
Inspection Protocol for Sites with Existing Galvanised Steel Bunds
If your site has galvanised steel drip trays or secondary containment and stores any alkaline substances, implement the following inspection protocol immediately:
- Review the Safety Data Sheets (SDS) for all stored substances — note pH values and alkali content
- Inspect galvanised surfaces for white powdery deposits (zinc hydroxide), blistering, or rust staining (steel substrate exposure)
- Any tray showing substrate corrosion must be replaced immediately — it provides no containment guarantee
- Specify polymer or stainless replacement units with documented chemical resistance confirmation from supplier
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