The existing protective coating on the double skinned kerosene storage tank had broken down in places and after 10 years in service was looking tired.
During our survey we observed that the existing coating was poorly adhered by means of a cross hatch adhesion test.
It also appeared that at manufacture the tank had not been abrasive blasted prior to painting as there was no visible surface profile and mill scale still appeared to be present.
Mill scale is a flaky surface that forms on steel or iron products during their production, particularly during hot rolling or continuous casting processes. It is composed mainly of iron oxides, such as magnetite (Fe₃O₄), hematite (Fe₂O₃), and wüstite (FeO). The mill scale forms as a thin, brittle layer on the metal’s surface due to the reaction of the iron with oxygen at high temperatures.
Characteristics of Mill Scale:
Appearance: Mill scale is dark gray to black in color, often flaky or powdery in texture.
Composition: It is primarily composed of iron oxides, which makes it a valuable material in some recycling processes.
Adherence: Mill scale typically adheres to the surface of the metal but can flake off during handling, transportation, or subsequent processing steps.
Thickness: The thickness of the mill scale layer can vary depending on the process and conditions, usually ranging from a few micrometers to a few millimeters.
Common Occurrences:
Hot Rolling: When steel is hot-rolled, it is passed through rolls at high temperatures. During this process, the surface of the steel reacts with the oxygen in the air, forming mill scale.
Continuous Casting: In continuous casting, where molten steel is solidified into slabs, billets, or blooms, mill scale forms on the surface as the material cools and reacts with the surrounding atmosphere.
Uses and Challenges:
Recycling: Mill scale is often recycled in steel production, where it is used as a raw material in the production of iron or in cement manufacturing as a source of iron.
Surface Preparation: Before further processing, such as painting or coating, mill scale must be removed, as it can cause poor adhesion and lead to corrosion if left on the surface.
Environmental Impact: If not properly managed, mill scale can contribute to environmental pollution, particularly in waterways, due to its iron content.
Given the poor adhesion of the existing coating and suspected presence of mill scale we advised that to provided a long lasting protective coating solution it would be necessary to remove all of the existing coatings and the mill scale beneath.
Given the tanks location within a busy manufacturing facilities yard area we proposed to do this by method of water entrained abrasive blasting. It is not possible to remove mill scale by other means of surface preparation such as water jetting.
Initially surrounding structures were protected to prevent damage from rebounding blast abrasive.
All existing coatings and mill scale were then removed using our Hodge Clemco Aquagrit slurry blast unit, for dust free surface preparation
The additional benefit of water entrained abrasive blasting is that soluble salts such as chlorides and sulphates if present are removed during the preparation process.
We had identified areas of weld spatter that did not conform to ISO8501-3 at time of survey. These areas were dressed with flap discs in line with best protective coating practice.
As part of our standard quality assurance the prepared surface was tested for the presence of soluble salts.
If these are present failure to remove these will likely lead to osmotic blistering in the new protective coatings as moisture will be drawn through the cured film via osmosis, leading to premature coating failure and reduced life expectancy.
One of the down sides of wet blasting methods is the generation of flash rust. Any loose corrosion deposits were first removed by brushing and then all surfaces to be coated solvent wiped to ensure that only firmly adherent corrosion remained.
Prior to coating application commencing and during the climatic conditions were tested and recorded to ensure compliance with the manufacturers recommendations.
The flash rust present was acceptable as were applying Corroless EPF rust stabilising epoxy primer. This contains a pigment that stabilises corrosion present. It also contains self leafing glass flake which laminate within the cured film to create a more tortuous route for moisture to reach the substrate, reducing permeability.
Coating permeability refers to the ability of a protective coating, such as paint, varnish, or sealant, to allow substances like gases, liquids, or vapors to pass through it. Permeability is a crucial factor in determining the effectiveness of a coating in protecting underlying materials (such as metals, concrete, or wood) from environmental factors like moisture, oxygen, chemicals, and corrosive agents.
Key Aspects of Coating Permeability:
Permeability to Water Vapor:
Water Vapor Transmission Rate (WVTR): This measures how much water vapor can pass through a coating over a certain period. High WVTR indicates that the coating is more permeable to moisture, which might be undesirable in protective coatings where moisture resistance is essential.
Moisture Barrier: A coating with low water vapor permeability acts as a good moisture barrier, helping to prevent corrosion, mold growth, or degradation of the substrate.
Oxygen Permeability:
Oxygen permeability is another critical aspect, especially for coatings applied on metal surfaces. If oxygen can penetrate the coating, it may lead to oxidation (rusting) of the underlying metal.
Corrosion Resistance: A coating with low oxygen permeability helps prevent corrosion by limiting the amount of oxygen that can reach the metal surface.
Chemical Permeability:
Some coatings are designed to resist specific chemicals or solvents. If a coating is permeable to certain chemicals, it might allow those chemicals to reach and damage the substrate, leading to potential structural or aesthetic failures.
Chemical Resistance: The ability of a coating to resist penetration by chemicals depends on its composition, thickness, and the nature of the chemicals it encounters.
Porosity and Microstructure:
The microstructure of a coating, including its porosity (the presence of tiny pores or voids), significantly affects its permeability. Coatings with high porosity are generally more permeable because the pores provide pathways for substances to pass through.
Thickness and Uniformity: A thicker and more uniform coating generally has lower permeability because it presents more material for substances to pass through, reducing the likelihood of penetration.
Testing and Measurement:
Coating permeability can be tested using various standardized methods that measure the transmission rates of water vapor, oxygen, or other gases through the coating. The results are often used to assess the coating’s suitability for specific applications.
Practical Implications:
Protective Coatings: For coatings intended to protect metals from corrosion, low permeability to moisture and oxygen is crucial. These coatings must form a continuous, non-porous barrier to effectively prevent contact between the metal and the environment.
Breathable Coatings: In some applications, such as in building materials or breathable fabrics, a degree of permeability is desirable. These coatings allow moisture to escape from beneath the coating, preventing issues like blistering or mold growth.
Coating Selection: Understanding the permeability characteristics of a coating helps in selecting the right product for specific environmental conditions and service requirements.
In summary, coating permeability is a critical property that influences the protective performance of coatings. By controlling permeability, manufacturers can tailor coatings to provide optimal protection against moisture, oxygen, chemicals, and other environmental factors, ensuring the longevity and integrity of the coated material.
All welds and edges received a heavy stripe coat of Corroless EPF applied by brush. Stripe coating is essential to ensure full film build in areas where protective coatings thin through gravity and surface tension.
