The Corrosive Environment and Classification.
Types of Anticorrosive Coatings and Applications.
Examples of ISO Test Methods and Performance for Anticorrosive Coatings.
1. The Corrosive Environment
Anticorrosive industrial coatings (solvent-borne, water-borne, or powder) are exposed to various environments ranging from constant immersion in water and burial in soil, to atmospheric pollution in industrial areas and ultraviolet radiation. The specific requirements for anticorrosive coating systems are highly dependent on the environmental conditions that the coating may experience during the entire time of service. Anticorrosive coatings applied to protect metal against corrosion may be exposed to chemicals and rain, whereas coatings buried in soil may be exposed to bacteria and humidity.
2. Classification of Environments
The versatility and different corrosivities of the environments that anticorrosive coatings may encounter during service needed a classification of the different types of environments. ISO 12944 “Coatings and varnishes–Corrosion protection of steel structures by protective coating systems” divides the environment into three types of exposure: atmospheric, splash zone, and immersion.
2.1. Atmospheric Exposure:
Coatings applied in atmospheric environments are subjected to varying local conditions, such as ultraviolet radiation, heat, and moisture, salt and gas concentrations. The atmospheric environment can be classified according to six corrosivity categories ranging from very low (C1) to very high corrosivity (C5-I and C5-M), as illustrated in Table 1. Such categorizations can be subject to debate because it is often difficult to specify corrosivity categories for one particular location (for example, roads may be heavily salted during the winter in rural areas where the location can, in principle, be categorized as both C3 and C5). However, they provide a frame of reference for typical conditions encountered by coatings during service.
2.2. Splash Zone:
Structures situated near the waterline of large bodies of water, such as parts of offshore plants and foundations of wind turbines, are located in a so-called “splash zone.” Splash zones are extremely aggressive environments because they combine an oxygen-rich atmosphere with continuous splashing of electrolytes from surface water (fresh water), waste-water, and especially salt water from the sea. The degradation of coatings situated in the splash zone is likely to be accelerated further by exposure to ultraviolet radiation and the mechanical stress induced by continuous exposure to alternating periods of moisture and dryness.
When structures are immersed in water or buried in soil, the aggressiveness of the environment is very specific because the combination of temperature, salinity, pH, and the content of dissolved gasses, especially oxygen, determines the overall corrosivity of the environment. The following corrosivity categories are suggested: Im1 for immersion in fresh water, Im2 for immersion in sea water, and Im3 for burial in soil. The aggressiveness of soil on buried structures and coatings is mainly determined by the type of soil, humidity, bacteria, salt, and oxygen content, as well as acidity (pH). In comparison, the aggressiveness of a fresh water environment is mainly determined by the type and content of dissolved salts and oxygen. As opposed to fresh water, sea water has a high content of dissolved salts, especially sodium chloride, that are very aggressive towards metals and anticorrosive coatings. Structures immersed in water or buried in soil may also be affected by sand, gravel or stones; they are also subjected to biofouling.
2.4. Table 1 – Corrosivity Categories and Environmental Impact Factors:
||Indoor in dry rooms (relative humidity <60%)
||Indoor in non-heated and ventilated rooms
||Indoor with high humidity and pollution (production areas). Rural environments far from industrial area
||Urban or industrial areas
||Very heavy industry
||Industrial areas with high relative humidity
||Very heavy marine
||Coastal and offshore areas
||Immersion in fresh water
||Immersion in sea water
||Medium-very heavy, depends on aggressiveness of soil
||Buried in soil
3. Types of Anticorrosive Coatings and Applications
3.1. Barrier Coatings:
Barrier coatings are used as primer, intermediate, or topcoat, and they are often applied on immersed structures. Barrier coatings are typified by an inert pigmentation, typically titanium dioxide, micaceous iron oxide, or other pigments and/or fillers; lamellar aluminum is also often applied. The relatively low pigment volume concentration results in dense and cohesive coatings with low permeability. The degree of protection offered by a barrier coating system is highly dependent on the thickness of the coating system as well as the type and nature of the binder system. The delamination of both defect-free and artificially damaged barrier coatings has been reported to be significantly reduced when the thickness of the coating is increased because coatings behave as semi-permeable membranes. In general, the anticorrosive performance of barrier systems increases when the same film thickness is built up from multiple successive thinner coats rather than one single coat; labor costs and potential revenue loss due to downtime push towards fewer and thicker coats.
The original assumption was that barrier coatings inhibit corrosion by acting as a barrier to water and oxygen from the environment. However, studies indicate that the mechanism of barrier protection relies on the ionic impermeability of the coatings (example: rebar coatings).
3.2. Sacrificial coatings:
Sacrificial coatings rely on the principle of galvanic corrosion for the protection of metals against corrosion. The substrate is protected by a metal or alloy that is electrochemically more active than the material to be protected. Coatings formulated with metallic zinc powder have been extensively employed for corrosion protection of steel structures for several decades. Unlike barrier coatings, sacrificial coatings are only applied as primers because they are only effective if the coating is in direct contact with the substrate due to the requirement of electrical contact between the substrate and the sacrificial metal. Sacrificial coatings should only be applied with great care on structures submerged in water due to the subsequent permeation of water, which may cause the sacrificial metal to corrode very quickly. In zinc-rich primers, zinc is used to produce an anodically active coating. Zinc acts as an anode and sacrifices itself to protect the metal, which becomes a cathode. The resistance towards corrosion is dependent on the transfer of galvanic current by the zinc primer, but as long as the conductivity in the system is preserved, and as long as there is sufficient zinc to act as anode, the metal will be galvanically protected. Once the galvanic protection stops, any subsequent protection is attributable to the barrier effect of the coating only.
3.3. Inhibitive coatings:
Inhibitive coatings arc primarily applied as primers because they are solely effective if inhibitive pigments can react with the metal. These coatings are mainly applied to substrates subject to environments with a risk of atmospheric corrosion, in particular industrial environments, and are generally not recommended for immersion in water or burial in soil. The anticorrosive mechanism of inhibitive coatings relies on passivation of the substrate and build-up of a protective layer consisting of insoluble metallic complexes, which impede transport of aggressive species by acting as a barrier. The inhibitive pigments are inorganic salts, which are slightly water soluble. When the coating is permeated by moisture, the constituents of the pigments are partly dissolved and carried to the substrate surface. At the surface of the substrate, the dissolved ions react with the substrate and form a reaction product that passivates the surface of substrate. This means that inhibitive pigments must be high enough to ensure sufficient leaching from the coating. However, if the solubility of the inhibiting pigments is too high, blistering can occur. The efficiency of inhibitive pigments is very dependent on the barrier properties of a coating. The effect of the inhibitive pigments will be more apparent in coatings with a certain degree of permeability (e.g. when the pigmentation is near the critical pigment-volume concentration) because the solubility of the pigments and the mass transfer within the coating will be important in this case.
3.4. Table-2 Summary of Application:
The table on the next page lists advantages and disadvantages for the three different types of anticorrosion coatings mentioned under items 3.1., 3.2., and 3.3. Although this table was written primarily with liquid industrial coatings in mind, the information therein is generally applicable for powder coatings. For those not familiar with the expression lamellar pigments, those are flat, platy pigment (or fillers), such as mica or aluminium flakes.
4. Accelerated testing of anticorrosive coatings
Modern high-performance anticorrosive coating systems are quite durable; they often show little signs of deterioration for several years after exposure to natural outdoor conditions. As a consequence, accelerated test methods have become an important tool in the development of anticorrosive coatings. The purpose of accelerated testing is twofold. Coating suppliers use accelerated test methods to screen and develop novel high-performance coatings, whereas potential customers may use the accelerated test data to compare the performance of different anticorrosive coatings (e.g., prequalification tests). The accelerated laboratory test methods seek to intensify the effects from the environments so the coating breakdown occurs more rapidly than in naturally occurring environments. It is essential that the utilized accelerated test methods reflect the type of environment encountered by the coating during service. However, many of these accelerated exposure tests will not, within their exposure time, visually show the negative effects on intact coated surfaces. Therefore, the behaviour of coatings around artificially made damages is given significant consideration in the design of novel high-performance anticorrosive coatings. An important aspect in relation to the accelerated testing of coating performance is the correlation with natural outdoor exposure. Traditional accelerated testing, such as simple salt spray chambers where aerosols of seawater are continuously sprayed on coatings containing an artificial damage, have often shown a fair-to-poor correlation with natural exposure; however, salt spray testing still remains a standard for powder coating applications in North America, as are water immersion and water condensation (condensing humidity) testing. Despite the continuous review of procedures for accelerated test methods, many problems continue to persist. The continuous high temperature (35Â°C, or 95Â°F) and salt concentration (5 wt %) of the salt fog do not fit any common conditions for usage. An important aspect in relation to application of elevated temperatures is to ensure that the temperature during accelerated weathering does not exceed the glass transition temperature (TG) of the coating, because this will result in failures unrelated to atmospheric and chemical exposure.
4.1. Table 3 – Test Methods and Exposure for Performance of Protective Coatings:
A schematic overview of some test methods for determination of the anticorrosive performance of coatings is given in the following table on page 6 for corrsivity categories starting from C2, and durability ranges from low to high.
5. Special Note:
The information disclosed in this paper was taken from the following “The Free Library by Farlex website, entitled Anticorrosive Coatings: A Review by Sorensen, P.A.; Kiil, S.; Dam-Johansen, K.; Weinell, C.E.: http://www.thefreelibrary.com/Anticorrosive+coatings%3a+a+review-a0200166359 Although most of the information was copied verbatim from the website, it was edited for content and shortened, and several errors in the text or tables were corrected.
Bruno Fawer, B.Sc.,
MBA Associate Consultant
Powder Coating Consultants
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