Waterproofing Solution for the Bottom Edge Plate of Steel Storage Tanks

CTPU Waterproofing Solution for Tank Bottom Edge Plates

I. Preface

Currently, the conventional practice for waterproofing tank bottom plates in China involves sealing joints with asphalt or applying asphalt-sand coatings. Some also use rubber-modified asphalt or epoxy glass cloth for waterproofing. However, the former suffers from poor aging resistance and insufficient bonding strength, while the latter exhibits poor elasticity, leading to cracking and delamination after use—both resulting in less-than-ideal performance. Additionally, there has been an attempt to employ ternary ethylene propylene diene monomer (EPDM) rubber sheets for corrosion protection; yet, their surface tends to crack easily, thereby losing their waterproof and anticorrosive effectiveness. In response to these issues, our company has conducted on-site investigations and research, and has developed an effective and comprehensive waterproofing and anticorrosion solution. Specifically, based on our CTPU elastic polyurethane waterproofing coating, we have adjusted the formulation by adding a certain amount of filler to create granular compounds and paste-like materials. We then employ a construction technique that involves laminating polyester waterproof mesh fabric onto the coating to enhance its strength, thereby effectively blocking the ingress of corrosive agents such as rainwater and dew into the edge plates and tank bottom plates, thus achieving the goal of protecting the tank bottom plates.

II. Causes of Corrosion on the Bottom Edge Plates of Oil Tanks

To analyze the causes of corrosion at the outer edge of the tank bottom plate, we can identify two primary factors: First, the junction between the tank base and the bottom plate experiences radial expansion and contraction due to environmental changes, particularly fluctuations in temperature; second, variations in the load caused by the oil storage and transfer volumes lead to deformation of the tank. When the tank is filled with liquid, the hydrostatic pressure generates significant hoop stresses, causing horizontal displacement of the tank along its radius. Since the edge plates are firmly welded to the bottom plate and cannot expand outward, they undergo deformation at the edge, resulting in edge stresses. These edge stresses, combined with the resistance exerted by the base on the edge plates, ultimately lead to plastic deformation of the outer ring portion of the bottom plate. When the tank is empty, the tank body returns to its original shape, but the edge plates remain permanently deformed and curl upward due to the plastic deformation that has occurred.

The aforementioned factors create a gap between the outer edge of the tank bottom plate and the base. The size of this gap continuously expands and contracts in response to the tank’s movements, providing an entry point for external corrosive agents such as rainwater and dew. This leads to crevice corrosion: these corrosive agents infiltrate day after day, and because the crevices are extremely narrow, water has difficulty evaporating and tends to accumulate over long periods between the bottom plate and the base, resulting in severe electrochemical corrosion. Eventually, this corrosion causes rusting and perforation beneath the bottom plate. Since this type of corrosion occurs between the tank bottom and the base—areas that are typically hidden from view—it often goes unnoticed and is therefore the most dangerous form of corrosion. Protecting the edge plates at the bottom of the oil tank involves blocking the above-mentioned invasion pathways, effectively preventing environmental factors from penetrating around the bottom of the tank. The goal is to safeguard the tank bottom plate (especially the edge plates) by isolating it from water, atmospheric elements, and other corrosive agents.

III. Protective Measures for the Bottom Edge Plates of Oil Tanks

Based on the deformation of the above-mentioned oil tank and the corrosion mechanisms at the outer edge of its bottom, the waterproofing material used for the bottom plate should possess waterproof properties, weather resistance, strong adhesion to both the edge plates and the base, and high elasticity. By "waterproof properties," we mean that the waterproof layer must be a complete, continuous, and integral structure capable of effectively preventing the intrusion of water, air, and other elements. "Weather resistance" refers to the material's ability to withstand atmospheric corrosion and ultraviolet radiation without developing cracks that could compromise its waterproof performance; it should also remain stable at high temperatures without flowing and at low temperatures without becoming brittle. High elasticity ensures that the material can accommodate deformations in the bottom plate caused by movements or changes in load on the oil tank. Based on these requirements for anticorrosive materials for edge plates, we have developed a specialized anticorrosive material for oil tank edge plates—CTPU. This material is a polymer coating composed of two components: Component A, a color paste consisting of chain extenders, fillers, pigments, antioxidants, UV absorbers, and other additives; and Component B, a pre-polymer containing isocyanate groups (-NCO). Its key features are: 1. Excellent waterproof performance; 2. Good adhesion; 3. Superior mechanical properties; 4. High elongation and tensile strength; 5. Long service life.

IV. CTPU Anti-Corrosion Construction Plan

The anti-corrosion process flow for the outer edge plates of oil storage tanks is shown in the figure below:

1. Clean the surface of the cement foundation of the storage tank.

2. Apply CTPU waterproof coating as an anti-corrosion primer, then paste PE waterproof tape.

3. Prepare a layer of CTPU waterproof coating granules and shape it into a water-guiding slope.

4. Scraped-applied CTPU waterproof coating and corrosion-resistant elastic sealant

5. After the corrosion-resistant elastic mortar layer is formed, apply the elastic waterproof mesh fabric.

6. Apply the second coat of CTPU waterproof coating and corrosion-resistant elastic mortar layer.

7. Roll-applied CTPU waterproof coating—corrosion-resistant and weather-resistant topcoat.