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Superior protection: an in depth look at the protective properties of silicate and epoxy zinc primers.

The application of zinc-rich primers on ferrous substrates is a very efficient method of anticorrosion protection. It is a common fact that in order to achieve a long-life coating system, a zinc primer needs to be applied as the first coat. Zinc primers offer threefold protection in that they seal the underlying metal from contact with its corrosive environment, creating a barrier effect (see figure 1 below); they provide galvanic protection (see figure 2 below); and they repair minor damages in the coating to form a barrier to further electrochemical action (see picture 1 on the next page).

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Zinc dust content. The anticorrosive and mechanical properties of zinc primers are highly dependent upon the level of zinc dust present. Compared to epoxy the silicates can be pigmented at extremely high levels of zinc dust giving zinc to zinc metal contact and, consequently, excellent cathodic protection properties like those obtained from galvanizing. The silicate will create a primary valency bond with the substrate as well as the zinc particles while the epoxy will create a secondary valency bond. The primary valency bonds are up to six times stronger than the secondary valency bonds. There is a trend in new international standards that the demand for zinc content is the same for zinc epoxy and zinc silicate given weight by weight. When standardizing the level of the zinc content in inorganic and organic coatings the values should be given volume by volume. It is clear from tests (see figure 3 on the next page) that Zn-silicate has much lower potential compared to Zn-epoxy even if the weight percentage of Zn-dust is higher. By comparing 86% weight Zn-dust epoxy and silicate primers by volume instead, the zinc epoxy will contain a volume of approximately 55% Zn-dust while the zinc silicate will contain a volume of 70% Zn-dust.

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Another topic that is highlighted more often is the composition of zinc dust. Some users of zinc primer specify metallic zinc content and other specify total zinc or zinc dust content. The general rules are:

* Total zinc is the sum of all the zinc compounds in the formulation including zinc dust, zinc oxide and zinc phosphate.

* The composition of the zinc dust is specified by ASTM D520 and ISO 3549.

* Metallic zinc can be calculated from the zinc dust content. The zinc dust should contain minimum 94% metallic zinc according to ISO 3549.

The standards also specify the content of impurities allowed in the zinc dust. A new trend is the demand for low lead zinc according to ASTM D520 type II (maximum 0.01%) or type III (maximum 0.002%).

INORGANIC OR ORGANIC ZINC PRIMERS

The zinc primers can be divided into different groups depending on the binder and solvent. The main groups used by the industry today are shown below in chart 1.

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The cathodic properties of a zinc primer. The type of corrosion protection and the galvanic potential can easily be illustrated by electrical impedance testing. The potential of bare steel in seawater is approximately -650mV SCE (Saturated Calomel Electrode) and zinc has a potential of approximately -1050 mV SCE. As long as the potential is below -800 mV the primer still gives cathodic protection. Above this level the primer will only give you a barrier effect. For unexposed zinc primers the potential is usually between -900 mV and -1050 mV depending on the type of binder, zinc content and film thickness (see figure 4 above). Three different formulations were tested on four test panels. Seventy-five microns of zinc ethyl silicate with 86% zinc dust in dry film gave by far the lowest electrical potential. The other panels are covered with zinc epoxy and in the beginning the potential is the same independent of zinc content and film thickness. During the exposure it is obvious that higher zinc content gives lower potential and is more important than higher film thickness. The reason is that there is better conductivity when the zinc content is higher. After a while the conductivity drops when the Zn is sacrificing and creating Zn-salts instead. These salts will fill the voids in the film and the primer will have greater potential until the protection occurs only by barrier effect. Higher film thickness will of course also contribute to better corrosion protection even if it is less important compared to the zinc content. The difference after outdoor exposure is clear (see picture 2 below). A low zinc content epoxy primer applied in different film thickness is exposed at seaside. The area with only 15 pm is full of rust all over the area while the area with 35pm has started to rust only in the scribe. The area with 80 pm film thickness is still protecting the scribe and looks perfect.

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Electrical impedance testing is a well known test for zinc primers. Most of the zinc primers will still have cathodic protection after 100 days and the primers with high zinc content applied in high film thickness will last for more than 300 days. Compared to real life testing of zinc primers this is not very long but still it is almost a year. By making a scratch of a standardized size in the film the time for testing can be reduced dramatically (see figure 4 above). After four days the potential increased by 123 mV. In the standardized test with panels without scratch it takes more than 100 days to achieve the same increase in potential. By implementation of this test it is possible to test and screen a lot of formulations after relatively short time with exposure.

It is also possible to test other types of primers and multiple layers by electrical impedance. It is not only the benefit of the pure zinc primer that can be evaluated but also the barrier effect of the total system. As long as the potential is higher or lower compared to steel the corrosion has not started.

The curing mechanism. Even if the cured product based on ethyl silicate and alkali silicate is the same the initial product is very different. The ethyl silicate is an organic binder before curing while the alkali silicate is an inorganic binder. The ethyl silicate needs humidity to cure while the alkali silicate needs to get rid of the water before it cures. The speed of curing ethyl silicates compared to alkali silicates is faster and easier. The curing process of an alkali silicate is complex and depends on the type of metal in the alkali silicate. The alkali silicates used are sodium (Na), potassium (K), and lithium, (Li), or a mixture of some of them.

During the curing process the binder will react with metal, drop in pH or heat in the beginning with further reaction with the C[O.sub.2] in the air. These reactions will make acids that react with Zn-ions to form a zinc silicate polymer. Further developments are the introduction of colloidal silica and styrene acryl in combination with the alkali silicate. This is done to lower the pH of the paint which allows for better application and top coating properties. Unfortunately this creates other limitations. Ethyl silicates must be pre-hydrolyzed to achieve proper curing. The pre-hydrolyzed binder will react with water and Zn-ions to create zinc silicate polymer and ethanol. The ethanol will evaporate fast and contribute to the high VOC level. The finished matrix will only contain inorganic material.

The waterborne silicates can be made with zero-VOC and is much more environment friendly compared to even waterborne epoxy that needs co-solvents for film formation.

Zinc epoxies are most used among organic zinc primers. The solventborne epoxies are normally 2-components. The waterborne epoxies used to be 3-components because a combination of zinc and water will react and create hydrogen gas. New technology has resulted in 2-component waterborne epoxies. Epoxy is normally cured with polyamides and amine adducts. Polyamides are used because of their reduced tendency to react with the zinc powder.

Mechanical resistance The adhesion and cohesion strength of inorganic and organic zinc primers are quite different. As mentioned earlier the silicates will react by forming primary valancy bonding with the steel and give excellent adhesion if the pretreatment is good enough. Because of this reaction the silicate should only be applied on Sa 2 1/2, or concrete which has a lot of metal ions accessible. On all other types of surfaces like St 2 or old paint the silicate will have poor adhesion. The alkali silicate are very sensitive to impurities such as oil and fat on the surface, since they do not contain any solvents for secondary cleaning of residual oils and contaminations. Because of the chemical reactions in the silicates the cohesion is also acceptable despite the extremely high pigment volume concentration (PVC). The adhesion of silicate primers are very good because of the primary valency bonding between the silicate and substrate. Zinc epoxy primers have properties similar to other epoxy primers, with good adhesion and mechanical strength because of the strong physical attraction in secondary valency bonds. It is possible to have good adhesion to most surfaces depending on the formulation. The adhesion and cohesion strength are strongly dependent on the ratio between PVC and CPVC, or critical pigment volume concentration (see figure 5 below).

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Where will the break come in a system with a zinc primer as a first coat? It is of course desirable to have a film that is impossible to break, but so far this has not happened. By using zinc primers with good adhesion properties there is minimum risk for under film corrosion. The paint system should have higher cohesion and adhesion strength compared to the cohesion strength of the zinc primer. This will make the cohesion in the zinc primer the weakest link so that if the paint system takes on mechanical damages down to the steel it will have as much zinc as possible accessible to protect the scribe (see figure 6 below).

[FIGURE 6 OMITTED]

The high PVC/CPVC ratio of the silicates also gives some limitations. First of all they have a big risk for mud cracking if they are applied too thick. The ethyl silicates have normally the highest PVC/CPVC ratio of the silicates and mud cracking will appear at lower film thickness compared to the alkali silicates. One of the reasons for the higher PVC/CPVC ratio of the ethyl silicates is the use of pigments and extenders in the paint formulation. Zinc pigments need very little binder to be wetted compared to normal extenders and the alkali silicates have normally higher zinc content than the ethyl silicates. But the most important issue with regard to mud cracking is the speed of curing. Ethyl silicates cure much faster than alkali silicates. Mud cracking appears when the silicate matrix cannot take the internal stresses that develop during the curing of the primer. At too high thicknesses, the upper layer of the inorganic zinc will cure faster than the layer underneath, causing excessive stresses which the inorganic silicate matrix cannot withstand.

Another benefit of using inorganic zinc silicates are that when applied it does not shrink. Together with the surface tension of the coating this property makes these types of primers very good in covering edges, corners and bolts. The organic zinc primers are not as good because they will always have some shrinkage.

Chemical and heat resistance. When it comes to chemical resistance the silicate matrix is very inert and, with the exception of strong acid and alkalis, does not degrade in most industrial and marine environments. The film is inert to crude and refined oils, greases, solvents, chlorine, C[O.sub.2] and industrial gases. In general it is resistant to most chemicals within the pH range 6-10 depending on the temperature. The corrosion resistance of zinc to these chemicals is usually the primary consideration for its use, but in some cases, the effect of zinc corrosion on a consumer product or chemical is of greater concern than the actual corrosion rate of zinc. One example is the formation of zinc salts that is not acceptable for cargoes such as jet fuel. Zinc epoxy is normally not used for chemical protection because the epoxy used is not optimized for this purpose and the product will at the same time have limitations because of the zinc.

Inorganic zinc silicate has high heat-resistance and is excellent against radiation including nuclear radiation. The heat resistance is limited by the zinc to maximum 400[degrees]C since the melting point of zinc is 419.5[degrees]C. Above this temperature it is not possible for zinc to react with the oxygen in the air or with the silicate, rendering the zinc inert and leaving it as a simple barrier coating. The zinc epoxy primers are limited by the type of epoxy and will normally resist temperatures up to 120[degrees]C. Zinc primers are also limited when in contact with hot water. The galvanic effect will start to decrease at 60[degrees]C and with further increase in temperature the potential difference will be zero or may actually be reversed at 70[degrees]C. This will only happen when the water is soft, or distilled. When the water temperature is above 120[degrees]C there are normally not any problems regarding the reversed potential.

Top coating of zinc primers. All inorganic zinc silicates are formulated with PVC above CPVC. As mentioned earlier the film will be porous and allow liquid and air to come into the film. This will create problems with popping in the next coat and in the end create pinholes. This can be avoided by using a mist coat technique or by using a tie-coat. The zinc epoxy will normally not have this problem but since many of the formulations have PVC/CPVC ratio close to one is possible.

The alkali silicates are binders with a high pH. This high pH will create adhesion failures of the next coat if precautions are not taken. Usually products based on lithium silicate and with more than 90% zinc dust by weight can be top coated quite fast. By using slower curing alkali silicates and/or a lower content of zinc the time to over coating will increase. The paint will be similar to fresh concrete and the pH will be reduced during the curing process. By applying water to the surface it is possible to reduce the pH. Ethyl silicates will not have the same problem and can easily be formulated down to 30% zinc dust by weight and still have good top coating properties.

When it comes to ethyl silicates and organic zinc epoxy the pH on the surface is not any topic but the products need to be cured before they are top coated. Insufficient curing of the primer before top coating is the main cause of failure of coating systems based on ethyl silicates. If the primer is top coated before it is cured the film will not develop the necessary physical and corrosion protection properties. This can however easily be avoided by determining the degree of cure before top coating through tests for hardness and/or solvent sensibility of the primer. A solventborne zinc epoxy primer is more tolerant to be top coated but to develop the best mechanical properties and avoid solvent entrapment it is better to have it fully cured. The waterborne zinc epoxy can be top coated very early and almost as fast as the solvent-borne.

Cutting and weldability. The high performance shopprimers used today are based on zinc ethyl silicate. These shopprimers are very good for the welding and cutting process because they do not contain any organic materials when cured. The silicates are unbeatable on back burning, pore formation and welding fumes as soon as it is cured. Uncured ethyl silicate has poor welding and cutting properties because it still contains some organic material. Any organic material will contribute to gas formation and then increased content of pores. The fastest shopprimers to weld in also has a very low content of zinc. A high content of zinc will cause problems with zinc fever for the welders and pores in the welding seem. Zinc epoxy was used in the shopprimer market for some years but its consumption has decreased due to high zinc content and the organic vehicle that creates pores in the welding seems.

DISCUSSION

Both inorganic and organic zinc primers offer many benefits but some properties are better for one type of primer. Because of these properties the areas of use are different for the two types of primers. Applications for inorganic zinc silicate primers include barge decks because of their abrasion resistance; high strength bolted steel because of high friction (slip) coefficient; pipelines, power plants and power transmission lines; bridges, ships and tankers including interior and exterior surfaces and storage tanks; oil rigs and offshore drilling platforms; rocket launch gantries; water tanks; construction materials operating over 100[degrees]C such as chimneys, kilns, incinerators, furnaces, ductwork and piping; coal washeries; wharf facilities; mineral processing plants; as shopprimer; and inside the Statue of Liberty.

Applications for organic zinc epoxy primers include bad pretreatment steel; maintenance projects; touching up zinc silicate shopprimed steel; high performance coatings with good flexibility; and steel that is impossible or hard to blast to Sa 2 1/2 because of cost or accessibility.

The type of primer normally specified for new building has been zinc silicate followed by zinc ethyl silicate. This is because within the parameters of zinc silicates the solvent-borne ethyl silicates have been found to be more tolerant than the water-borne alkali metal zinc silicates. The zinc silicates offer the best corrosion protection--especially alone--adhesion to Sa 2 1/2, chemical resistance, heat resistance, abrasion resistance, welding and cutting properties. The corrosion protection properties of inorganic zinc primers and organic zinc primers are compared in figure 3. Even if the zinc epoxy primer with higher zinc content is compared to the zinc ethyl silicate the electrical potential of the zinc epoxy is higher.

While zinc silicate is a typical new building coating the zinc epoxy is more of a maintenance primer. The epoxy is easier to apply in higher film thickness without cracking and can be applied with conventional airless spray while alkali silicates normally need special equipment. Top coating is also easier compared to a silicate which may create popping and also adhesion failure because of high alkalinity on alkali silicate paint. Shelf life of a zinc epoxy is far better than ethyl silicate. Zinc epoxy is also more surface tolerant.

The development of zinc silicate shopprimers started a revolution for welders and cutters in the steel business. After years of slow welding with numerous pores in the welding seem and slow cutting with large damages to the shopprimer, it is now possible perform this work with minimal problems. A total inorganic film with a low content of zinc will give very good properties. The next generation of shopprimers will include inorganic and organic water-borne products. Today these kinds of products still have some challenges that need to be solved.

CONCLUSION

There is no doubt that zinc-rich primers offer a very efficient method of anticorrosion protection. Zinc offers threefold protection since it seals the underlying metal from contact with its corrosive environment, provides galvanic protection and "repairs" minor damages in a coating forming a barrier to further electrochemical action. In Picture 1 is the formation of zinc salts and how the zinc protects the scribe on a test panel during exposure obvious.

Compared to epoxy the silicates can be pigmented at extremely high levels of zinc dust giving zinc to zinc metal contact and, consequently, excellent cathodic protection properties like those obtained with galvanizing. There is a trend in new international standards that the demand for zinc content is the same for zinc epoxy and zinc silicate given weight by weight. If it is a wish to have the same zinc content level for inorganic and organic coatings the values should be given volume by volume.

The type of corrosion protection and the galvanic potential can easily be illustrated by electrical impedance testing. The potential of bare steel in seawater is approximately -650mV SCE (Saturated Calomel Electrode) and zinc has a potential of approximately--1050 mV SCE. As long as the potential is below -800 mV the primer still gives cathodic protection. Above this level the primer will only give you a barrier effect. For unexposed zinc primers the potential are usually between--900 mV and -1000 mV depending on type of binder, zinc content and film thickness.

Earlier the type of zinc primer normally specified has been zinc silicate and most typical zinc ethyl silicate. This is because within the parameters of zinc silicates the solventborne ethyl silicates have been found to be generally more tolerant than the water-borne alkali metal zinc silicates. The zinc silicates give the best corrosion protection, adhesion to Sa 2 1/2, chemical resistance, heat resistance, abrasion resistance, and welding and cutting properties. The waterborne silicates can be made with zero-VOC and is more environment friendly compared even to waterborne epoxy.

The zinc epoxy is easier to apply in higher film thickness without mud cracking and can be applied with conventional airless spray while alkali silicates normally need special equipment. Top coating is also easier compared to silicate, which may give popping and adhesion failure because of high alkalinity on alkali silicate paint. The curing of the zinc epoxy is faster and is not depending on high humidity and the shelf life of a zinc epoxy is far longer than ethyl silicate. Zinc epoxy is also more surface tolerant and is a typical maintenance coating.

Zinc epoxy was used in the shopprimer market for some years but the consumption has decreased a lot. The reason is high zinc content and poor weldability. The silicates are unbeatable on back burning, pore formation and welding fumes.

Inorganic zinc should be used where high performance is necessary. In less severe environments, organic zincs may be used to provide good protection with better physical properties.

REFERENCES

(1.) Charles G. Munger, "Zinc silicate Coatings"--40 years experience, Journal of Protective Coatings & Linings, p.34-44, March 1985.

(2.) Clive H. Hare, "Protective Coatings"--Fundamentals of chemistry and composition, SSPC 94-17

(3.) Clive H. Hare, Matthew Steele and Steven P. Collins, "Zinc loadings, cathodic protection, and post-cathodic protective mechanisms in organic zinc-rich metal primers" JPCL September 2001 p.54-72

(4.) Mike Mitchell and Mark Summers, "How to select zinc silicate primers," PCE July 2001 p.12-15.

(5.) B. del Amo and C.A Giudice, "Influence of some variables of zinc-rich paints based on ethyl silicate and epoxy binders," American Paint & Coatings Journal September 23, 1991 p. 36-43.

(6.) D. Pereira, J. D. Scantlebury, M. G. S. Ferreira and M. E. Almeida, "The application of electrochemical measurements to the study and behavior of zinc-rich coatings," Corrosion Science, vol. 30, No. 11, pp. 1135-1147, 1990.

(7.) Ray Mudd, "Organic zinc-rich Coatings" Journal of Protective Coatings & Linings October 1995, p. 51-63.

Havard Undrum works for Jotun AS, Jotun Coatings Marine and Protective Coatings Laboratory, Sandefjord, Norway. He can be reached via e-mail at: havard.undrum@jotun.no.
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