Coatings corrosion tests used

Hundreds of test methods are used to accelerate the aging of coatings. Several of them are widely used, such as salt spray and ultraviolet (UV) weathering. A review of all the corrosion tests used for paints, or even the major cyclic tests, is beyond the scope of this chapter. It is also unnecessary because this work has been presented elsewhere the reviews of Goldie [1], Appleman [2], and Skerry and colleagues [3] are particularly helpful. 

Attack on metals can be a function of fuel components as well as of water and oxygen. Organic acids react with cadmium plating and 2inc coatings. Traces of H2S and free sulfur react with silver used in older piston pumps and with copper used in bearings and brass fittings. Specification limits by copper and silver strip corrosion tests are requited for fuels to forestall these reactions. 

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

Practice for making and using U-bend stress corrosion test specimens Recommended practice for laboratory immersion corrosion testing of metals Method for vibratory cavitation erosion test Practice for recording data from atmospheric corrosion tests of metallic-coated steel specimens... 

One costly form of degradation is corrosion of metallic objects and structures. Organic coatings are widely used to protect these objects from corrosion. No completely acceptable and predictive methods exist for the evaluation of corrosion protection. Since corrosion is an electrochemical phenomenon, electrochemical testing may provide the evaluation tools which are wanting. 

Accelerated Corrosion Tests. There are as many as a dozen methods (salt fog, Kesternich, etc.) that are currently being used to investigate corrosion resistance of coating systems and a need to develop a better and more dependable method to predict in-use service. A severe drawback of all these tests is that their results often compare unsatisfactorily with practical experience. One reason for the discrepancies is assumed to be the variability of natural exposure conditions. Accordingly, cyclic testing procedures have been developed with which exposure conditions, especially temperature and humidity. 

This chapter deals with the protection of metal surfaces against corrosion by means of coatings that contain anticorrosive pigments. The degree of corrosion protection depends not only on the pigment, but also on the binder, and these must complement each other chemically. The uses of anticorrosive pigments in paint binders are summarized in Table 39. For standards, see Table 1 ( Corrosion testing )-... 

For the cyclic corrosion test, a layer of acrylosilane polymer coating (10-25 fim thick) was dip-coated onto the plasma-deposited substrates. The coated samples were then subjected to 25 scab cycles. The test results are plotted in Fig. 7. Corrosion performance (as described by the length of scribe creep) was correlated to the wattage used for plasma film deposition. As discussed in the previous section, the chemical structure and properties correlated with the deposition conditions, especially the power level applied. Therefore, atomic compositions for plasma polymers deposited at different power levels were also plotted in Fig. 7.A... 

After completing corrosion testing exposure, the panels were rinsed with distilled water and visual observations were made. The panels were then subjected to Turco 5469 paint stripper solution to strip off the E-coat or spray primers (including the controls) from the scribed surface, so that the effect of corrosion beneath the coatings and away from the scribes could be viewed. These panels were then used to estimate the average corrosion creep widths, in order to compare the corrosion performance of the different sample systems [5]. 

Although the panel with the plasma deposited film followed by priming with E-coat is visually better, the use of the corrosion width provides a method for quantifying the improvement in the corrosion performance. Also the factor of about 2 difference in corrosion width between the two chromate conversion-coated panels is difficult to obtain from the qualitative difference observed from the scanned images. It can be seen from this comparison of three panels that the use of the measured corrosion width makes the differentiation of corrosion performance much easier. This method of evaluating corrosion test results is used to determine if the combination of the two bests could indeed yield the better corrosion protection of aluminum alloys. 

E-coat stripping. Both plasma-treated panels show excellent corrosion protection performance as compared to the control panels. All [2A] panels with different plasma treatments and plasma polymer coatings, which were corrosion tested in both SO2 and Prohesion salt spray tests, were similarly scanned, and the corrosion width was evaluated by using a scanned image and computer calculation of the corroded area. Figure 31.19 compares the corrosion width obtained by the two methods. 

The purpose of cleaning steel is to remove dirt and leave the article in a state in which it can be dehvered for use without further finishing. The surface must therefore be covered with a tenacious corrosion-resistant coating as it emerges from the cleaning bath. Many emulsion cleaners remove lubricants and other unwanted dirt whUe depositing an anticorrosive coating on the metal. The primary test for efficacy in this situation is a corrosive test of the cleaned article. 

Corrosion tests in artificial atmospheres [1, 2, 3] are used as comparative tests for the evaluation of corrosivity of metals and metal alloys and corrosion protection capability of various corrosion protection means by metal plating, varnishing and paint coating as well as anodic and conversion coating. Therefore, it is essential to know precisely the corrosivity of the test cabinet environment. 

As may be seen from Table I, concentrations of SO2 in the atmosphere are subjected to considerable variations. Moreover, SO2 concentrations used in corrosion tests are larger by several orders of magnitude. Therefore, it becomes highly questionable whether data obtained at high SO2 concentration levels may be also applied to practical exposure conditions. SO2 permeabilities of a series of polymer films like polyethylene, polycarbonate. polyamide 2 gg well as polyacrylate and cellulose triacetate have been shown to depend on SO2 pressure, especially at higher SO2 concentration levels. In the case of organic coatings such data are not available. It is therefore recommended to study SO2 permeability at low SO2 concentrations comparable to practical exposure conditions. 

Several specimen were tested for each of the coating types listed in Table 7.3. In this sub-section, however, the term SPECIMEN No. X is also used for COATING TYPE No, X and should be so interpreted according to the context. The material used for the coated steel specimens were unalloyed steel St 37, Crevice and pitting corrosion testst and galvanic corrosion tests with copper as a contact matcrialj were performed on all specimens supplied,... 

The corrosion protection performance of the sol-gel films was estimated by accelerated test using immersion in a NaCl solution. Fig. 2 demonstrates the coated surface after the immersion tests. The reference sample prepared without titania nanoparticles is strongly attacked after 10 days in 0.05 M NaCl. In contrast, the sample with titania nanoparticles seems almost untouched even after one month in concentrated NaCl solution. 

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