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Three effective methods to predict the corrosion resistance of coatings: salt spray corrosion test, external natural corrosion and early corrosion prediction

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There are several ways to determine long-term durability, i.e. the performance of the coating in long-term atmospheric external tests.

External exposure
First, one can actually conduct long-term testing by exposing the test panels to aggressive areas or by coating chemical tanks, bridges, or other facilities. External testing is considered to be the most reliable method to determine long-term performance, although this method has obvious disadvantages, especially the time required to make proper judgments.
Acceleration method
The second method is to accelerate the degradation, which is usually completed by salt spray test chamber, humidity chamber and ultraviolet concentrator. Here, degradation (such as rust, scratch undercutting or blistering) will occur in a shorter period of time, so degradation can be observed in about 1000 hours instead of 5 years. An important question is whether this degradation reflects the degradation generated in the external environment.
Early detection of degradation
Another approach is to detect degradation early, rather than waiting for traditional degradation methods, such as rust and blistering. A major example is the use of electrochemical methods to detect degradation early. There are other types of tests, including water permeability or other characteristics that detect early degradation.

Early detection of degradation can also be achieved by quantitative visual assessment of rust or blistering. This technique examines the surface in detail, rather than comparing it with ASTM standards.

In addition, these three basic methods, external exposure, accelerated laboratory salt spray corrosion test and early degradation detection, can also be combined.

In outdoor exposure, the degradation time range is tens of thousands of hours (Fig. 1). (Ten thousand hours a little over a year.) Degradation usually starts between 30000 and 50000 hours (3 to 5 years). When using accelerated tests, such as humidity test and salt spray test, the time is usually reduced by an order of magnitude (about 1000-2000 hours).

By using accelerated testing, you can reduce the time frame by about an order of magnitude. Of course, the issue of the effectiveness of the assessment must still be addressed. Another way to reduce the radiation intensity is to use the early degradation of external radiation. Don't look at the parameters such as rust rate and blistering. These parameters may not be obvious within 3 to 5 years. Maybe some parameters (such as electrochemical reaction) will show up in 1000 to 3000 hours of outdoor exposure (about a few months rather than a few years).

The properties of coatings can be evaluated and compared in several ways. Suppose someone has many coating systems and wants to know how coating A compares to coating B.

Two common methods are time to failure and rating after a given time (e.g. 36 months).

Time between failures is considered the best method. Figure 2 shows the rust grades of alkyd coatings exposed to marine environments. Use five repetitions. The data shows that the performance difference between duplicates is very large, which is typical. (See Fig. 3 for scribed undercut of 5 repeated epoxy coating systems.) The worst one is the one with triangle; In the whole 54 month experiment, the best one is perfect (rust grade is 10), and the others are in between. The dotted line with triangle is the average of five repeats.

These data help to illustrate the difference between average rating and time between failures. As shown by the horizontal dotted line, when the fault is defined as 7 rust grades, this is very typical, and the mean time to failure is shown as about 48 months. As can be seen, the worst single panel failed at about 28 months. This panel represents 1 of 5 samples, or 20% of samples. If 20% of the surface fails within 28 months, it will be more meaningful than knowing that the average grade after 36 months is 8, or the average grade after 48 months is 6.

Linear correlation of alkyd coatings on blast cleaned steel surfaces

For the selected coating group, it is meaningful to check the correlation between the salt spray test and the external exposure.

The first group is a series of 20 lead-free and chromium free oily and alkyd coating systems applied to SSPC-SP 10. Coated panels were exposed to salt spray (ASTM B 117) for 8000 hours and at LaQue Center marine site (250 meters long) in Kure Beach, North Carolina for 12 years.

Evaluate the rust and blistering of the coating in the salt fog, and the rust and scratch undercut at the offshore site. The parameter of interest here is the time when the coating fails due to rust (i.e., the time when the coating system degrades to Grade 7 rust according to SSPC-Vis2 or ASTM D 610).

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