The industry is currently looking for alternative (wet) paint systems that achieve the same results in terms of corrosion protection and paint adhesion as chromate/Chrome-VI-based paint. MM31, a new development in chemical technology from AD Chemicals, could be the answer. The product is easy to apply in just 1 step in the mechanical pre-treatment and significantly improves corrosion resistance and paint adhesion.
Why pre-treatment on steel
Organic coatings are applied to steel to prevent or at least delay damage caused by corrosion. This is often based on a combination of the following mechanisms:
- protection against (aggressive) ions
- barrier to moisture and oxygen
- slow anodic dissolution reaction with pigments (chromate, phosphate, etc.)
- cathodic corrosion protection (zinc dust).
To obtain good durable adhesion under corrosive conditions, the steel must be properly prepared before applying a coating. The purpose of the pre-treatment is to get a consistent surface quality in terms of:
- cleanliness: dirt, oil, grease, paint residues, rust, mill scale
- roughness: can contribute to the adhesion and appearance of the paint layer
- chemistry: metallic, oxidised or with a chemical conversion layer
Common pre-treatment methods
The most common pre-treatment methods for steel listed in the table below.
| Method Pre-treatment |
Purpose |
Disadvantages |
| Water-based degreasing |
Remove: oil/grease/dirt |
Not practical outside of paint shop, rinsing necessary, waste |
| Stain |
Remove: mill scale, rust |
Not practical outside of paint shop, rinsing necessary, waste |
| Solvent-based degreasing |
Remove: oil/grease/dirt |
• Safety, Environment
• Dirt not completely gone
• Fire hazard |
| Mechanical (including blasting/sanding) |
• Removal: mill scale, rust
• Create uniform roughness |
• Remove oil/fat first
• Remove dust afterwards necessary |
| Chemical conversion layers |
More passive layer than metal: often with additional adhesion and corrosion properties |
|
Each pre-treatment method has its advantages and disadvantages. In general it can be said that today the following 5 points are the main challenge for the preservation of the steel industry:
- Improve working conditions;
- No Cr6 + in (conversion) coatings
- No exposure to solvents due to OPS.
- Reduce environmentally harmful emissions;
- Replacing solvent cleaners (thinner, xylene, heptane)
- 0% VOC emission
- Extend the time between mechanical treatment and coating;
- Prevention of flash rust in temporary conditioned storage
- Waste reduction
- g. replacing pre-treatment bath with a no-rinse treatment
- Cleanliness of the surface after blasting;
- Fat and oil still blasted into the surface. Dust present as a result of the blasting.
New pre-treatment method: combination of blasting and chemistry
Generally, it can be said that in the wet paint industry, a mechanical pre-treatment by way of blasting or sanding is the most commonly used method. What if the aforementioned pre-treatment methods can be combined?
This is the power offered by a new development in surface treatment from AD Chemicals. Known under the brand name MM31, its product combines blasting and chemical pre-treatment. If a chemical surface treatment is carried out, the procedure includes degreasing, staining and applying a conversion coating. The conversion coating provides corrosion protection and paint adhesion. The degreasing and staining step is similar to the degreasing and blasting method. After blasting, there is no additional added value to the metal, as is the case with a chemical pre-treatment by applying a conversion coating. This means that this mechanical procedure basically only ensures good paint adhesion and does not impart additional added corrosion-resistant properties to the substrate. However, by applying a conversion coating after blasting, the two worlds come together, improving the quality of the coated product in terms of adhesion, as well as corrosion resistance.
Corrosion resistance and paint adhesion improve significantly in just 1 step!
Risk of corrosion after blasting
Steel structures that are coated after sand or grit blasting run a risk of corrosion despite high-quality systems and careful preparation of the steel surface. Even in unexpected places, like flat parts. This can have various causes. Sometimes layer thicknesses can be too thin, but the cause, more often than not, is in incorrect surface treatment after blasting. The abrasive is dirty or even contains grease. A solvent-based degreaser is often used before or after the blasting process. This ensures that the grease layer is smeared rather than removed, with all the inevitable consequences. This undesirable result can be demonstrated with a water break test. The water break test is a simple, fast and non-destructive test for the presence of hydrophobic films. The test is applied to freshly cleaned metals, such as steel and other metals that are hydrophilic in a completely clean state and in this case often shows contamination of the substrate. The test is described in ASTM F21 and F22 or MIL-DTL-53072.
Conversion coating properties for blasting applications
MM31 is a Chromium-3 containing product that offers a remedy for the aforementioned causes of corrosion. Notably, it removes grease, it ensures better adhesion of the coating system and protects against flash rust and creep corrosion.
In addition to adequate degreasing, a so-called conversion layer is created by using MM31. This conversion layer is a non-organic, water-based coating which is formed on the metal by way of the constituents, the metal itself contributing to the formation of the layer. From a chemical technical point of view, the following happens: Chromium-3 acts as a strong carbide former and bonds with the iron parts. After blasting, a protective crystalline layer with a surface of ‘hook’ forms, which increases the adhesion of the coating. Moreover, these crystals ‘refine’ the dips in the steel surface that arise during blasting, creating a relief with fewer high peaks, resulting in a more even surface.
The purpose of a conversion layer is twofold. The layer improves anti-corrosion properties and provides good adhesion for organic coatings. The thickness of a conversion layer is indicated in milligrams of conversion layer or conversion elements per m², because the layer is usually too thin for the measuring range of the common layer thickness gauges. It is therefore a wafer-thin layer (<0.1 μm) that is many times thinner than a regular coating layer, which, for example, has a layer thickness of 60-1000 μm. An important point is that the conversion layer should not be confused with a regular coating layer such as a primer.
The test results speak for themselves:
The product is applied directly after blasting, or following any other type of mechanical pre-treatment, in-line with a misting installation or manually by means of a cloth or a low-pressure nebuliser (plant sprayer) at room temperature. MM31 can be used for treating completely new constructions as well as for maintenance work on location.
MM31 is water-thin and when applied it also easily treats difficult-to-reach corners and edges of a workpiece. The surface turns blue or dark grey when properly operated, similar to the blue glow that is also characteristic of surface treatments such as iron phosphating, and thus immediately detects the imperfections. After drying, any paint system can be applied.
The quality that can be achieved meets the Qualisteelcoat or GSB Steel requirements. The product is free of ADR hazard labels.
Protection against flash rust
Normally, blasted objects should be coated immediately within a few hours after blasting, or the first forms of corrosion will occur. MM31 offers protection against flash rust, so uncoated objects can be stored longer before being coated. In covered storage for up to 2-4 weeks and in many cases up to 48 hours! As a result, a higher efficiency and output can be achieved when it comes to the processes carried out by coating companies. This offers great advantages in situations such as internal transport and delays in applying the coating, and it makes it possible to buffer parts to be coated prior to the coating process.
Solvent replacement
The replacement of solvent cleaners such as thinner, xylene, heptane is an important challenge for the preservation of the steel industry in connection with the risk of fire hazards, working conditions for employees and the 0% VOC emission target. MM31 offers an answer to this and can replace these solvents in the degreasing step in the pre-treatment of metal. The outcome? A better quality, a safer working environment and cost savings can be achieved.
Reinforcement paint systems without chromium (VI)
Another important aspect is that, at present, in many sectors such as agriculture, machine manufacturing, infrastructure, tank construction and the transport sector, alternatives to Chromium-6-containing primers are sought after for good corrosion protection on products. The harmful effects of such systems for human beings and the environment are now widely known. MM31 makes it possible to improve the final quality on a variety of chromium (VI)-free coating systems. The product can therefore make a valuable contribution to achieving high-quality coating systems of a comparable quality to Chromium-6. This fulfils a long-expected market need. AD Chemicals is therefore happy to enter into talks with potential paint suppliers and coating companies about creating more synergy between the pre-treatment and the paint system.
Comparison test?
Are you curious about what this technology can mean for your process and how it relates to your current situation? Contact us via chemicals@adinternationalbv.com for more information.
