Corrosion test in artificial atmosphere – The secret of the salt spray test

Friends who make galvanized sheet must be familiar with the term \"salt spray test\", and they can be deeply tortured and disgusted. Today, Xiao Wu will talk about the salt spray test, so that you will not \"talk about salt color change\" in the future!

1. Why Take a Salt Spray Test?
In the past, Cao Mengde said, \"Although the turtle lives for a long time, it still has its own time.\" That is, although the turtle has a long life, there will always be 24 hours to die.

Although the life of steel is longer than that of a turtle, it cannot get rid of the natural law of birth, old age, disease and death (not to mention steel, the universe will eventually die). The death of steel is called failure. Failure can take many forms, the three basic forms being breakage, corrosion and wear.

Salt spray test is closely related to corrosion.

To select a material suitable for the ondesign life, it is necessary to predict the life of the material. In action Once a certain material has been selected and used, it is almost impossible to change it halfway through, so the evaluation before the material is selected is even more important.

Steel, however, generally has a long service life. How to choose the right material from the point of view of corrosion?

Laboratory accelerated corrosion tests emerged, and the salt spray test is one of them. In the salt spray test, test pieces are placed in a fine mist of a common sodium chloride solution and sometimes other chemicals to compare the expected use of different materials.

2. Classification Of Salt Spray Test

For the requirements of salt spray test, our country has formulated many standard methods. Among them, GB/T 10125-2012 Artificial Atmospheric Corrosion Test Salt Spray Test is a comprehensive standard that covers all three sospray tests, namely neutral salt spray (NSS), acetic acid spray (ASS) and copper-accelerated acetic acid bath mist (CAS).

The specific working methods of the three salt spray tests are not described here, but it is necessary to state their applicable objects, which are mentioned in the text of GB/T 10125-2012:

(1) The neutral salt spray test is suitable for:
—metal and its alloy;
—metal coating (anode or cathode);
—conversion coating;
—anodised coating;
- Organic coatings on metal substrates.
(2) The acetic acid salt spray test is suitable for copper + nickel + chromium or nickel + chromium decorative coatings, and also for anodized aluminum films.

(3) Copper Accelerated Acetic Salt Spray Test is suitable for copper + nickel + chromium or nickel + chromium decorative coating, also suitable for aluminum anodic oxide film.
In addition, the promulgation of GB/T 10125-2012 is also necessary to remindand:

3. Advantages and Disadvantages of Three Salt Spray Test Methods
(1) Neutral Salt Spray Test
is the application for a wide variety of objects, such as the galvanized sheet that our steel merchants usually deal with, is it better to use the Neutral Salt Spray Test to evaluate.
The disadvantage of the neutral salt spray test method is that the test period is long (due to the weak corrosion of the medium) and the reliability is poor (especially in nickel, cadmium, chrome plating and chrome plating). The results obtained under real conditions are relatively poor).

(2) Acetate Salt Spray Test
ASD test is suitable for urban environments

The painted parts of cars are mainly copper-nickel-chrome coating and nickel-chrome coating, and are also suitable for aluminum anodic oxidation film. The ASS test can well distinguish the quality difference of Ni+Cr plating system en can satisfactorily reproduce the corrosion characteristics after the outdoor exposure corrosion test.
The disadvantage of the ASD test is that the improvement of the accelerator pedalation coefficient is not satisfactory (such as the test period of the Ni+Cr system is usually 8~114 hours), and the reproducibility is still in some cases not ideal (as the result of the micro-discontinuous chrome plating system is somewhat unusual).

(3) Copper Accelerated Acetate Salt Spray Test
A large number of studies have shown that the results of the CASS test and the air exposure test of decorative coatings are in good agreement. It is especially suitable for the quality inspection of nickel plating-chrome plating. Due to the short test time, it has a positive significance for the development of new corrosion-resistant materials. The data shows that the CASS score of 16 hours is equivalent to the result of a year of driving in Detroit in the Unitedthe States, and the 18 hours equals the result of exposure to the industrial atmosphere in the United States during one year.

It should be noted that during the CASS test, the test parameters should be strictly controlled, such as the pH value of the solution, the amount of spray, the cleanliness of the sample surface, the position of the sample, the condensation rate of the solution in the box, etc. This method is mainly applicable to the corrosion resistance test of cathode coatings (copper, nickel, chromium coatings, etc.). ) on steel substrates or zinc alloy castings and on aluminum or aluminum alloys, but not for comparison of the relative corrosion resistance of Cu-Ni-Cr and Ni-Cr coatings, nor for stainless steel and brass matrix.

4. Dry goods! ! Salt spray corrosion test time of galvanized layers with different thicknesses
Friends who make galvanized sheets often have to say \"white rust\" and \"rode rust\", so what do they mean?
White rust refers to the corrosion of the zinc layer. Because the corrosion products are mainly ZnO and white, it is called white rust.
Red rust refers to the corrosion of the steel substrate, because the corrosion product is mainly Fe2O3, which is red, so it is called red rust.

(The following salt spray test data is a neutral salt spray test)

Understanding this, the following conclusion lies obvious: the time of the appearance of white rust has nothing to do with the thickness of the galvanized layer!As long as the zinc layer on the surface begins to corrode, it is white rust, no matter how thick the zinc layer is.The appearance time of white rust is closely related to the passivation degree of the surface.

Studies have shown that non-passivated hot-dip galvanized products usually show 5% white rust in 2~8 hours, after which a large area of ​​white rust appears.AaOn the other hand, after 24 hours, a small piece of white rust began to appear on the surface of the passivated hot-dip galvanized product, and the surface was only partially corroded when 5% of the red rust appeared.

The appearance time of rust is closely related to the thickness of the galvanized layer, because it is only when the galvanized layer continues to corrode that the steel starts to corrode.

The relationship between the appearance time of red rust and the thickness of the galvanized layer (without passivation) is as follows:

The weight of the single-sided galvanized layer is not less than 70g/m2, which can ensure the time of red rust is not less than 96 hours (required by most users).

It should be noted that the samples of oil-coated products should be cleaned before the salt spray test to prevent the oil from affecting the accuracy of the test. In addition, the anti-rust oil itself is also a cost-effective anticorrosive surface treatment. Anti-rust oil can provide a physical barrier to block the corrosion of the zinc layer by corrosive media to a certain extent. Therefore, even for passivated products, a layer of anti-corrosion oil can provide better corrosion resistance during long-distance transportation.

Prev: The testing principle of the paper burst testing machine