Study on anti-flash rust in two-part water-based epoxy coating

 Introduction
With the promotion of sustainable development, environmental regulations are becoming more and more stringent, water-based epoxy coatings because of its low pollution, strong environmental protection, excellent anti-corrosion performance, has become an important direction of coating development. In water-based epoxy coating, epoxy resin as a film-forming substance, water as a dispersion medium, its solvent content is low, small environmental pollution, easy to store and construction. However, due to the large evaporation potential heat of water in water-based coatings, the rate of volatility is slow, in the construction process with the metal substrate contact with a longer time, often will produce flash rust phenomenon, the coating film corrosion resistance has a great impact .
, water-based epoxy coating systems often add anti-flash rust additives. At present, the main components of traditional anti-flash rust additives to sodium nitrite, sodium tantalum, chromate, etc. , such products are cheap, small dosage, the effect is obvious, but the disadvantage is not environmentally friendly, toxic. Therefore, Europe and the United States each additive manufacturers take the lead in the study of organic polymer amine compounding products, some domestic manufacturers have also begun to follow up, this category of products is based on traditional products to develop alternatives, or its performance further improved and broadened.
this study mainly aims at the properties of coating anti-flash rust and salt spray resistance, and discusses the resins, anti-rust pigments, different types of anti-flash rust additives and construction processes in water-based epoxy systems.
The effect of different water-based two-component epoxy system on the performance of metal surface coating
substation resin as the main film-forming substance, its emulsion particle size, molecular structure, emulsification process and the degree of reaction cross-curing between curing agents have greatly affected the performance of the coating film, the difference between different epoxy system is obvious, so the selection of suitable epoxy system to meet the performance requirements of the coating is the most basic guarantee. In this study, several home-made and commercially available water-based epoxy systems were selected as the main body (blank varnish model, dry film thickness of 40 to 60 m), and the effect of different types of epoxy system on coating film performance was discussed.
1 Effect of different water-based epoxy systems on coating performance
As can be known from Table 1 data, Araldite-PZ-3891 is equipped with Aradur-3951 epoxy resin system, and its resin emulsion is emulsified It needs to be emulsified with a certain amount of emulsifying agent, and because the resin itself is drying slowly, so the rust resistance is not good; Epikure-8538-Y-68 curing agent matching Epicote-6520 -WH-53 epoxy emulsion system and CTW-6062 epoxy system overall corrosion resistance is relatively poor;
CTW-6064 supporting CTW-6061 epoxy system and Beckopox-EP-387w/52WA with Becopo-EH-2150w/70EP epoxy system anti-flash rust and salt spray and water resistance performance compared to several other resin systems are better, drying speed is also faster. However, the price of Beckopox-EP-387w/52WA supporting epoxy system is high, so this study chose CTW-6064 supporting CTW-6061 epoxy system as the main body, and studied its anti-flash rust and corrosion resistance.
The effect of active rust-proof pigments on anti-flash rust performance in water-based two-component epoxy coatings
in order to increase the passivation and rust-proofing ability of the coating film on the surface of the metal substate, a moderate amount of active rust-proof pigment must be added to the formula. Active rust-proof pigments need to have a certain degree of solubility in water-based epoxy coatings. Too low solubility can not effectively passivate the metal substate; This study explores the effects of different kinds of active anti-rust pigments on the anti-flash rust performance, salt-resistant water and neutral salt spray-resistant properties of water-based two-part epoxy coating paint film.Table
2 Effect of active anti-rust pigments on the performance of water-based epoxy coatings
Pictures from Table 2 show that the corrosion result of zinc phosphate is no significant corrosion of the plate surface, but the widening of the line >2.5mm, indicating that only in the early stages of corrosion, the metal surface will produce cathodes and anodes, so that dissolved iron salts and ferrous salts are hydrolyzed. Zinc phosphate and triethic iron ions have a strong contraction ability, this phosphate root ion and iron anode reaction, the formation of iron phosphate and insoluble conjunctivities, is the first corrosion and then protection type, has a certain amount of flash rust inhibition ability, but for the coating film of the early corrosion resistance effect is not good;
Aluminium tripolyphosphate anti-rust pigment has a certain acidity, it will lead to water-based epoxy coating during storage of pH constantly reduced, easy to lead to the occurrence of instantaneous corrosion, and thus affect the corrosion resistance of the coating film;
test and comparison, it is found that the water-based epoxy system with zinc phosphate aluminum and zinc phosphate combination when used in the anti-corrosion performance is the best. Mainly because zinc phosphate aluminum in the water-based system can release more zinc ions and phosphate root ions, and zinc tantalum with synergetic effect, can play a good passivation effect on the metal substrate, not only solve the problem of water-based coatings on the metal substrate flash rust, film can slowly release corrosion ions, improve the long-lasting anti-corrosion effect.
Rapid screening and identification of anti-flash rust additives
Because water-based epoxy antiseptic coatings are a very complex composition, the components of the various raw materials will more or less have a certain impact on flash rust. In the course of the experiment, if the flash rust inhibitor in accordance with the recommended amount of added to the water-based epoxy coating to carry out one or several experimental tests, found that the effect is not good and then abandoned is not very unscientific, not only time-wasting, but also cause waste of materials.
Thus, a quick screening and identification method for anti-flash rust additives is introduced: the flash rust inhibitor is added to the ionized water according to its recommended amount, stirred well, the drop is added to the substrate that has been normally treated, and the droplet 24 h is kept dry, if this method is tested without rust inhibitors can be the next verification experiment. This method not only eliminates the interference of other parts of water-based coatings to the inhibition of flash rust, but also is simple, inexpensive and efficient. The result is shown in Figure 1.
Figure 1 Flash Rust Inhibitor Quick Identification Method
From the figure, it can be found that the screening of flash rust inhibitors and blank samples, can effectively inhibit the occurrence of corrosion on the surface of the metal substrate, therefore, can be followed by the performance of the flash rust inhibition test and performance evaluation.
effect of anti-flash rust additives on the anti-flash rust performance of two-group water-based epoxy coatings
Flash rust is a unique phenomenon in water-based coatings and occurs instantaneously on the surface of metal substrates. Rust caused when the paint film is not dry seriously affects the corrosion resistance and adhesion of the coating film. Through the analysis of the principle of applying water-based epoxy coating on the metal surface to produce flash rust, it is necessary to passivate the metal substrate surface before the water-based coating is applied, so a certain amount of anti-flash rust additives need to be added to the water-based coating formulation. This study analyzes the effects of different anti-flash rust additives on film coating performance.
table 3 anti-flash rust additives on the performance of the paint film
the metal substrate is surface-treated, such as sand blasting, etc., before coating protection. Because there will be a time difference between metal substrate pre-treatment and coating construction, some substrate surfaces have produced slight rust before coating, the general rust inhibitors can not play a good passivation protection effect.
As can be known from Table 3, sodium nitrite (NaNO2) solution as a traditional acid anti-flash rust additive, with strong reductive properties, the metal substrate has a good passivation and protective effect, but this kind of reduced material due to strong ion resistance, will lead to long-term water resistance of the paint film decreased, and will have a certain impact on the storage stability of the coating, at the same time its salt spray resistance is relatively poor. The modified organic chelate FR-355 and aminoxate Halox-515 do not contain sodium nitrite, which has good flash rust inhibition ability on the general metal substrate, but the anti-flash rust inhibition effect on the surface of the metal substrate that has been partially rusted is poor;
Raybo-60 and FA-179 are chelating series, both anti-flash rust additives can solve the problem of water-based coating on the metal surface to produce flash rust, and after drying most of the form of insoluble water compound adsorption to the metal surface, so not only can effectively inhibit the production of sparkling rust, but also improve the paint film salt spray resistance and rust resistance. After testing and comparison, the experiment selected FA-179 as the anti-flash rust additive in the two-part water-based epoxy system, and studied the performance effect of the addition of FA-179 on the two-group water-based epoxy coating.
Table 4 FA-179 Flash Rust Inhibitor Dosage on the performance of the coating film
The picture shows that when the addition of the anti-flash rust additive FA-179 gradually increased, the anti-flash rust resistance, salt spray resistance and water resistance of the coating have a significant improvement effect, but when its dosage >1.2%, the performance of the coating decreased. Therefore, the optimal addition for FA-179 is determined to be 1.2%.
Effect of paint film thickness on anti-flash rust
This experiment examines the effect of water-based two-part epoxy coating on anti-flash rust performance of different wet film thickness and paint film's table drying time during construction.
Table 5 Effect of different wet film thickness on the performance of the coating flash rust
As can be known from Table 5, with the increase of wet film thickness, the table drying time of the two-component watery epoxy coating is continuously extended, and the resistance to flash rust decreases gradually. Through analysis, it can be found that the increase of wet film thickness slows down the rate of moisture evaporation, indirectly prolongs the contact time between moisture and metal substrates, and increases the risk of flash rust in the coating film. Therefore, in the construction process of two-group water-based epoxy coating, the thickness of a film should be strictly controlled to reduce the chance of flash rust. For some anti-corrosion performance requirements are strict, the need for high film thickness of the field, can reduce the thickness of a single film, divided into multiple coatings to achieve performance requirements.
In conclusion
by selecting the water-based epoxy emulsion CTW-6064 with modified fatty amine curing agent CTW-6061 epoxy system as the main film-forming substance, with zinc phosphate aluminum and zinc phosphate compounding as environmentally friendly rust-proof pigments, can effectively passivate the metal substrate protection, and improve the corrosion resistance of the coating film. Selecting the flash rust inhibitor FA-179 of the organic zinc chelate type and adding 1.2% of the total formulation can improve the anti-flash rust performance of the two-part watery epoxy coating system. In the coating process in order to reduce the risk of flash rust, as far as possible to control the thickness of a single film, for the thickness of the paint film requirements, should be divided into 2 or more coating to achieve the desired film thickness.
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authors , Zo Huiming, Liu Ming, Zhang Hanqing, etc.
(CNOOC Changzhou Coating and Chemical Research Institute Co., Ltd.)