Determination of Corrosion Resistance of Long-lasting Anticorrosive Coatings

It is an important protective performance of the coating film to resist the action of external media and prevent the corrosion of the coated substrate. Corrosion includes rust caused by external natural media and corrosion caused by industrial media. The corrosion caused by natural media is ubiquitous, and the corrosion conditions caused by industrial media are more severe. Moreover, the conditions that cause corrosion are ever-changing, and the environments of various coated objects are different, which makes it difficult to evaluate the corrosion resistance of the coating film. Corrosion resistance is one of the most complex topics when designing and producing coating products.

The most practical method to evaluate the corrosion resistance of the coating film is the physical test, which is to apply the coating on the object to be coated, use it under actual conditions, and observe the corrosion situation for a long time to judge whether the coating is suitable for corrosion resistance. Use requirements, which take a long time and are difficult to achieve. A step back is the hanging panel simulation test, which is to make a sample of the paint and test it under the same environmental conditions as possible, using the sample instead of the real thing, such as the actual sea test of the bottom coating in seawater, the hanging panel test of the floating raft in the harbor, etc. , The test cycle of this method is still too long. For example, the Japanese JIS specification stipulates that the test period for antirust performance is as long as 2 years. Now more and more simulated accelerated test methods in the laboratory are used. These methods include salt spray test, damp heat test, water vapor permeability test, etc. It can be carried out indoors to shorten the test period, but only relatively limited results can be obtained. Some people point out that the methods commonly used at present are not good and accurate methods. They are only comparable to the same type of coating products, and cannot be used for The evaluation of different systems of coatings, in addition, the results of the test are often different from the actual situation. Therefore, although it is feasible to use these test methods as quality control, it is necessary to use several tests at the same time to evaluate the quality of corrosion resistance based on their comprehensive results. Now the world is constantly studying more accurate simulation test methods.

(1) Salt spray test

Salt spray test is a method commonly used to test the corrosion resistance of coating films.

The salt spray in the atmosphere is a dispersion system composed of tiny liquid picks of suspended chlorides, which are formed by the transport of tiny water picks from sea spray and waves hitting the shore through airflow. Generally, the atmosphere in coastal or offshore areas is full of salt spray. Since chlorides in salt spray, such as sodium chloride and magnesium chloride, have the ability to absorb moisture at very low relative humidity, and chlorides are highly corrosive, salt spray is very harmful to metal materials and metal materials in coastal or offshore areas. Its protective layer has a strong corrosion effect.

At present, the brine formulas used in the salt spray test standards of various countries can be roughly divided into two categories: one is pure sodium chloride brine; the other is artificial seawater. Their pH values ​​are all controlled in the range of 6.5~7.2, generally known as neutral salt spray test. Pure sodium chloride brine, there are 3%, 5% and 20%. Because the relative humidity in the Test Chamber will drop if the concentration is too high, the salt crystallization on the surface of the sample will reduce the corrosion strength, and the nozzle is often easy to block, so it is generally used between 3% and 5%.

The artificial seawater formula is as follows:

Its purpose is to make the composition of the solution closer to the natural seawater, so as to simulate the corrosion conditions of the real ocean atmosphere. From the test results, the corrosion rate is not as fast as that of pure sodium chloride brine.

In order to improve the effect of the salt spray test, the acetic acid salt spray test has been developed, that is, the pH value of pure sodium chloride brine is adjusted to acidity (pH value between 3.1 and 3.3) with acetic acid. A further development is the acetic acid salt spray test modified by copper chloride, that is, in addition to adjusting the acidity with acetic acid, an appropriate amount of cucl2·2H20 is added. The purpose of these two methods is to try to overcome the reliability and reproducibility problems of previous salt spray tests, and greatly accelerate the corrosion rate. See AsTMG 43-75(80).

There is a close relationship between temperature and corrosion rate. When the t temperature is high, the corrosion is accelerated, but the reliability and reproducibility decrease. At present, most national standards stipulate two kinds of 35°C and 40°C.

The spray cycle can be continuous spray or intermittent spray according to different requirements (for example, within an hour, spray for 15 minutes, stop spraying for 45 minutes), but the destruction speed of continuous spray is the fastest.

At present, my country's national standard GB/T1771-91 "Determination of the resistance of paints and varnishes to neutral salt spray" is equivalent to the international standard Is07253-1984. The concentration of salt water in the test is 50g/L±10g/L, and the pH value is 6.5~7.2, temperature 35℃±2℃, frontier spraying. The test conditions are exactly the same as the US ASTMB117-73 (1979) standard.

At present, most of the salt spray test equipment is nozzle type, even if the air with a certain pressure passes through the nozzle in the Test Chamber to spray the salt water into a mist and settle on the test sample plate. The nozzle can be made of glass, plastic or other alloy steel. The following items should be paid attention to when using the nozzle type salt spray chamber test.

(1) During the test, it is necessary to check frequently whether the nozzle is blocked, so as to ensure the normal operation of the spray.

(2) The compressed air used needs to be degreased through the air filter and heated and saturated by the air saturator.

(3)喷雾压力应严格控制,使在规定值上下很窄的范围内波动,以免影响试验的重现性。

(4)把需要进行相互比较的同一批样板应尽量在同一次试验里进行。样板涂漆表面与盐雾沉降方向成30°角放置。每次检査后,应交换样板的放置位置,以消除设备内喷雾量及温度的不均匀所引起的误差。

标准中规定被测试涂膜样板需要在封边后测试,观察涂膜状况有无变色、起泡、生锈和脱落现象,按其轻重程度,起泡大小和面积、锈点大小来分级。有的标准中还规定在测试前对涂膜斜十字切割露底,放入盐雾箱中测试,其结果以在切痕周边锈蚀蔓延的距高和附着力损失的距高来评定。

美国AsTMD2933-74(81)提出一个循环试验法,即样板在盐雾箱中放置4小时后,不冲洗或干燥立即放入温度37.8℃,相对湿度100%的湿热试验箱中18小时,然后不干燥直接放入温度为_23.3℃±2℃冷冻箱中2小时,此为1循环,重复试验至产品规定的要求,一般为5~35循环。据称35次循环试验的结果比在美国佛罗里达内陆曝日西2年的条件还要:苛刻,但不及在佛罗里达海滩曝日西18个那样严重。

(二)湿热试验

湿热试验也是检测涂联耐腐蚀性的一种方法,一般与盐雾试验同时进行。

饱和水蒸气对漆膜的破坏作用主要基于以下两点。

(1)水对漆膜有渗透作用,透过漆膜之一层或多层,在漆膜与漆膜之间积聚,产生了最初的起泡;随后再向深入一步发展,最后达到漆膜与底板之间产生最后的起泡,水分与金属底板接角生,产生电化学腐蚀作用。

(2)漆膜本身可以吸收一部分水分,使漆膜发生膨胀,降低了漆膜和底板的附着力,从而产生起泡现象。

一般在相对湿度较低的情况下,漆膜附着力的变化是不明显的,但随着相对湿度增加到90%,甚至于更高,附着力的丧失就会变得很快,除了个别漆膜外,多数漆膜的附着力均不能恢复。

在相同的相对湿度下,温度越高,则绝对湿度越大,周围空间水蒸气压力增加,水气向漆膜内扩散就越显著,加快了受潮速度。同时温度越高,高分子链的热运动越厉害,分子间的作用力减弱,加速形成分子间的空隙,有利于水分的进入。

Under the same absolute humidity, the lower the temperature, the higher the relative humidity, and the greater the tendency of moisture to penetrate into the paint film. On the other hand, when the relative humidity is high, the tendency of moisture condensation increases, and the moisture condensed on the surface of the paint increases, so the speed of the paint getting wet increases.

According to some of the above reasons, there are many kinds of damp heat test cycles suggested at present, including short cycle of high temperature and high humidity, test cycle of alternating temperature and humidity, and test cycle of constant humidity and constant temperature.

From the actual test situation, although the temperature is too high, although the cycle is short and the damage is fast, in some cases, the change is too fast to distinguish the good from the bad of the model, and even distort the truth of the test. In the test cycle of alternating temperature and humidity, due to the low temperature and high humidity stage, water vapor will condense on the surface of the paint film, which is conducive to the penetration of water into the interior of the paint film, thus accelerating the damage to the paint film, but from the current test trend See, there is a tendency to hopefully not have too much condensation in the damp heat test. Because of too much condensation, a layer of water film is formed on the paint film, which can easily cause too much soluble substances in the paint film to be dissolved out, which is inconsistent with the actual humidity and heat conditions.

my country's national standard GB/T1740-79 (89) "Determination of Humidity and Heat Resistance of Paint Films" stipulates the constant temperature and humidity test cycle method.

At present, the humidity and heat resistance test is generally carried out in a temperature and humidity control box. Since the main influencing factors in the damp heat test are temperature and humidity, special attention should be paid to the control of these two factors in each test, so as not to affect the test results. In addition, during the test, a certain distance should be maintained between the vertically suspended samples, subject to non-overlapping and collision (2~4cm); the samples should also be exchanged during each periodic inspection to minimize equipment damage Experimental error caused by uneven internal temperature and humidity. Attention should also be paid to the use of distilled water or high ion exchange resin to purify water for test water.

For the evaluation of the model, it is mainly to observe whether the coating film has blisters, rust and peeling off, and grade it according to the degree of damage.

(3) Water vapor permeability test

As mentioned earlier, water vapor can penetrate through the coating film to reach the substrate, thus causing corrosion. Therefore, the water vapor permeability test of the coating film can judge the corrosion resistance of the coating film. This test is carried out with Yougao coating film, so the inspection procedures are more complicated. It is measured with a permeability test cup, clamping the Yougao coating film in the test cup, adding different constant relative humidity on both sides of the film, and calculating the water vapor passing through the test film with a specified surface area within a certain period of time Mass, expressed in g/(m2·d) for water vapor transmission rate. A lower value means less water vapor permeation. It can be used as an indication of the corrosion resistance of various coatings. This method is generally used in the research of coating products.

(4) Filiform corrosion test on steel surface

Filiform corrosion (filamentous corrosion) on the surface of steel can make the surface coating appear loose and linear bulges, commonly known as filiform corrosion, which is usually radiated by one or several corrosion growth points, test and evaluate paint and The filiform corrosion of the steel surface caused by scratches in the varnish coating in the presence of trace amounts of salt and specified relative humidity is also a test method for corrosion resistance. Our country equivalently adopts Is04623-1984, and formulated GB/T13452.4-92 "Filiform Corrosion Test on Painted and Varnished Steel Surface". For scratches of 20mm and 50mm in length, the metal surface can be clearly seen on the scratches. Immerse the sample in 1g/L chlorinated fine solution for 30-60 seconds (soaking method), or put it into the neutral salt spray in accordance with GB/T1771-91 for the time specified in the agreement (salt spray method), and then Put it into a Test Chamber at 40°C ± 2°C and a relative humidity of 80% ± 5%, conduct inspection according to the specified time, and evaluate according to the degree and quantity of filiform corrosion that the scratches expand.