Metal corrosion resistance

Atmospheric corrosion test
In order to test the atmospheric corrosion resistance of metal paint layers and conversion coatings in various natural atmospheric environments, it is common to place these materials or components (such as metals, alloy materials, or test samples that have covered various layers or coatings). It is placed in the atmospheric corrosion test site, allowing it to blow, sun and rain. After a period of time, carefully observe the corrosion of the surface to record, or calculate the corrosion rate of the increase and decrease of weight, and finally report the atmospheric corrosion results of these materials in a certain period of time for scientific research or production. reference. The results obtained by this approach are true and reliable, especially the longer the test time, the higher the reliability of the results, and the more valuable it is. However, since the corrosion rate of most metals in the atmosphere is relatively small, it is necessary to measure the results accurately, which requires a long test time, which is usually calculated in years, and the shorter ones are 3-5 years. 5-15 years. Therefore, as a judgment of production or as a scientific research technology, it is not feasible to test too long.

1. Classification of atmospheric conditions According to the environmental conditions of the area where the exposed site is located, the atmospheric conditions can be divided into four categories: industrial atmosphere, maritime atmosphere, rural atmosphere, and suburban atmosphere.
(1) Industrial atmosphere. The exposed field is in an industrial area where the factory is concentrated, and has atmospheric conditions that are seriously polluted by industrial media (such as S02, H2S, NH3, coal ash, etc.).
(2) Oceanic atmosphere. The exposed field is within 200m of the seashore and is exposed to atmospheric conditions contaminated by marine salt spray.
(3) Rural atmosphere. The exposed field is in a country far from the city, the air is clean, and there are basically no atmospheric conditions contaminated by industrial media or marine salt spray.
(4) Suburban atmosphere. Exposure fields are located in urban areas with atmospheric conditions that are slightly contaminated by industrial media.

2. Exposure method
According to the purpose and requirements of product testing, the main methods of atmospheric exposure testing are as follows:
(1) Open exposure. The open exposed sample is placed directly on the outdoor frame. The frame is made of a material that can withstand corrosion. The specimen faces the south on the frame and the height of the plant near the frame should not exceed 0.2 m.
(2) occlusion exposure. The exposed specimens are placed in a shelter with a height of not less than 3 m. The shelter is an umbrella-shaped roof, which strictly prevents rainwater from leaking and can directly or partially shield the sunlight from directly on the sample.
(3) Closed exposure. The closed exposed sample is placed in a louver box that is required to prevent atmospheric deposition, solar radiation, and strong winds from blowing, but should be kept in circulation with air from outside the tank. The top of the box is not allowed to leak, and it is properly tilted, with sloping and rainwater grooves. The louver box is made into a movable type to ensure the atmosphere exchange inside and outside the box, but the rain and snow will not enter the box. The size of the louver box depends on the size and quantity of the sample, and is placed on the open space of the test field. The spacing between the two louvers should be More than twice the height of the louver.

3. The selection and requirements of atmospheric exposure fields are divided into outdoor atmospheric exposure fields and indoor atmospheric exposure fields according to various atmospheric conditions. There are strict requirements in site selection, otherwise the assessment results will be affected.
A. Selection and requirements for outdoor atmospheric exposure fields (1) Exposure sites should be located in completely open areas, fully exposed to the atmosphere (air, sunlight, rain, dew, fog, frost, snow, etc.), surrounding buildings The trees and sample racks should be spaced apart to ensure that the shadows of surrounding buildings and trees are not projected onto the test sample at any time.
(2) Except for the industrial atmosphere exposure field, chimneys, vents, and buildings that generate a large amount of CO2, SO2 and other gas and coal ash are not allowed in the vicinity of other atmospheric exposure fields. Items that do not meet the requirements should be indicated on the test record card.
(3) The metal frame on which the sample is placed shall be coated with anti-rust paint (two layers of epoxy iron red primer and two layers of aluminum alkoxide paint). The shelf shall be 0.8 to 1.0 m from the ground, facing the south, and the frame and level. The direction is 45o. The magnetic insulator was placed on the sample.
(4) Set the exposure field in the coastal area. In order to prevent the typhoon from blowing down the sample holder, effective fixing devices should be adopted for the sample and the sample holder.
(5) When the sample is exposed to the atmosphere, various meteorological data of the exposed field shall be collected or measured for analysis of the test results. The contents include: regional extreme temperature, temperature frequency statistics; monthly, daily maximum, minimum, average temperature and relative humidity; rainfall and rain days, sunny days, days of exposure, fog days and sunshine.
(6) During atmospheric exposure, the corresponding harmful media in the atmosphere should be analyzed periodically according to the type of exposed atmosphere.
B. Selection and Requirements for Indoor Atmospheric Exposure Fields Indoor exposure fields are generally divided into two types: well ventilated and dark and humid.
(1) Conditions that are well ventilated. Temperature and humidity conditions are close to outdoor cloudy and sunny days. However, it is necessary to ensure that each sample in the room is free from the sun and rain. The structure of the exposed field should follow the form and principle of a blind box in which the meteorological instrument is placed, that is, the louvered windows on both sides of the wall, and its position should be adjacent to the outdoor exposed field.
(2) Dark, humid conditions. This indoor exposure field is mainly to test the protective performance of the coating in the basement or cave, so the exposure field should also be located in the basement or cave. Since the temperature and humidity in the basement or cave are different from those in the outdoor, it is necessary to have meteorological instruments that automatically record temperature and humidity, and statistically record the data.
There is no special requirement for the suspension method of the sample in the indoor exposure field. The sheet sample can be vertically suspended, and the samples with complicated shapes can be placed on the wooden sample holder in order.

4. Sample requirements and exposure methods
For the sample for atmospheric exposure test, the sheet sample shall be made of a steel plate (or other metal plate) having a size of 50 mm × 100 mm × (1 to 2) mm. The specifications of the finished sample are not limited, but the main surface shall be exposed when exposed. Place it up and face the south. Each sample should have a number that is not easy to disappear. It can be stamped or affixed with a plate.
The method of exposure of the sample shall meet the following requirements:
(1) The sample can be exposed all year round, but if the same batch of samples are tested in different areas of the exposure field, the test time should be the same.
(2) Before the test, the special record card is used to record the sample number, coating structure, thickness, appearance color, etc., and prepare the test outline (including test purpose, requirements, inspection cycle, etc.). Each sample should be left with 1-3 pieces and stored in a desiccator for comparative observation during the test.
(3) During the first 3 months of exposure, the number of inspections should be frequent, such as 2 to 3 times per month. Pay attention to the time when the corrosion point begins to appear and record on the card. After 3 months, the number of inspections can be reduced appropriately, but it should also be done once a month. It can be checked every 3 months after one year.
(4) For samples exposed in winter, when snow is encountered, the snow on the surface of the sample shall be removed in time.
(5) When taking the sample, do not directly touch the test surface of the sample by hand (the sheet sample can contact the edge, and the irregular sample can only contact the non-primary surface). The two samples must not overlap each other. Rubbing to avoid mechanical damage affecting the test results.

5. Qualitative assessment of test results
Qualitative assessment of test results can be observed with the naked eye or magnifying glass (3 to 5 times), and some simple tools and equipment can be used.
Corrosion of various coatings or conversion coatings in the atmosphere has its unique performance (such as color, shape, etc.). From the corrosion products and their development, the corrosion of coatings, conversion coatings and base metals can be judged. Degree to determine its protective performance.
The qualitative inspection and recording of coatings mainly have the following five aspects.
(1) The color and shape of the corrosion products of the coating and the base metal are shown in Table 4-3.

(2) Gloss. The gloss of the coating after exposure to the atmosphere refers to the extent to which the coating is oxidized when it is etched in a film.
The inspection and evaluation should be compared with the same original sample stored, according to the degree of disappearance of the coating gloss:
1) Good - no change or change is not obvious.
2) Dimly - the gloss is slightly changed, but it remains lustrous.
3) Dim - dark and dull.
(3) Corrosion rate. Corrosion rate refers to the ratio of the penetration of the coating to the corrosion point of the base metal. It is the main content to evaluate the protective performance of the cathode coating. It can be observed and calculated in the following manner.
A transparent plaque (5mm × 5mm) plexiglass plate is used to cover the surface of the sample to be tested (film layer), and the total number of squares of the surface of the coating (film layer) is counted, denoted by N, Count the number of squares with corrosion points, denoted by n, then the corrosion rate is:
Corrosion rate = × 100% (4 - 3)
(4) Cracking. Calculate the percentage of cracking area, which is the same as the percentage of corrosion rate area.
(5) Determine the corrosion level from the qualitative assessment results. The corrosion grade of the coating is determined by qualitative evaluation results and is generally divided into five grades.
1) Grade 1: There is no change in the surface of the coating (film layer), or only the gloss is dark.
2) Grade 2: The coating (film layer) has corrosion spots or membranous oxides, or the gloss is dim, but there is no corrosion of the base metal.
3) Level 3: The base metal corrosion point appears, but less than 10% of the total area.
4) Class 4: The area of ​​corrosion of the base metal is less than 30% of the total area, or the cracking area of ​​the coating is the same.
5) Class 5: The corrosion area of ​​the base metal exceeds 30% of the surface area, or the degree of cracking of the coating reaches the same level.
Coatings up to level 5 have been severely corroded and can be terminated.

6. Quantitative evaluation of test results Quantitative evaluation of atmospheric exposure test, mainly using weighing method, that is, measuring the weight loss of coating corrosion per unit time, and determining the corrosion rate.
Quantitatively evaluated samples shall be weighed according to the requirements before the test. After a period of exposure test, the samples shall be taken and impregnated with the solution specified in Table 4-4 to remove the corrosion products, then dry and weighed according to the two times. The weight weighed is calculated according to Equation 4-4.

Where G - the surface area of ​​the coating loses weight per unit of time, ;
Mo - the mass of the sample before the test, g;
M t - the mass of the sample after removal of the corrosion product, g;
S - the surface area of ​​the sample, ;
t - test time, a.