Several major causes of environmental aging of materials The atmospheric corrosion effect is mainly determined by the common, synergistic or interactive effects of various environmental factors. These environmental factors can be divided into decisive factors and accelerating factors. The environmental factors are different for different materials. For example, the decisive factors for metallic materials are the factors that can form visible water films and invisible water films (rainfall, condensation, snow melting, frost, fog reduction, relative humidity, etc.), that is, the factors that determine whether electrochemical corrosion can occur or not. Sexual factors are mainly atmospheric pollution (corrosive components) and temperature and other factors that affect the corrosion rate. In general, the environmental factors in the atmospheric environment mainly include the following eight types.
First, relative humidity
Relative humidity is one of the most important factors that affect atmospheric corrosion of metals. In the metal atmospheric corrosion process, atmospheric moisture condenses on the metal surface to form a water film and oxygen enters the metal surface through the water film. This is the basic condition for atmospheric corrosion. The formation of the water film is related to the relative humidity in the atmosphere. The different surface states of different substances or different substances have different adsorption capacities for water molecules. The formation of a water film on the surface of the object has a close relationship with the characteristics of the object itself. When the relative humidity in the air reaches a critical value, water forms a water film on the metal surface, thereby promoting the development of the electrochemical process, and the corrosion rate rapidly increases. The relative humidity at this time is called critical critical humidity of the metal. The critical relative humidity for atmospheric corrosion of common metals is: steel 65%, zinc 70%, copper 60%, nickel 70%, and aluminum 76%. The critical relative humidity of a metal varies depending on the state of the metal surface. The rougher the metal surface, the lower the critical relative humidity. Therefore, the metal surface is stained with salts or dust that tend to absorb moisture, and its critical value will also be reduced. In addition, the relative humidity in the air also affects the thickness of the water film on the metal surface and the frequency of alternating wet and dry.
As mentioned before, the relative humidity in the atmosphere has an important influence on the moisture absorption, decomposition, and deterioration of the charge; in addition, it also has a certain influence on the speed of plastic aging. In general, the relative humidity is high and it tends to cause accelerated aging of the material.
Second, the temperature
Atmospheric temperature and changes are important factors affecting atmospheric corrosion. Because it affects the condensation of water vapor on the metal surface, the solubility of various corrosive gases and salts in the water film, the resistance of the water film, and the corrosion reaction speed of the cathode and anode in the electrolytic cell. The influence of temperature should be considered together with the relative humidity of the atmosphere. When the relative humidity is lower than the critical relative humidity of metal, the effect of temperature on atmospheric corrosion is small. No matter how high the temperature is, the metal corrosion is slight in a dry environment. However, when the relative humidity reaches the metal critical relative humidity, the effect of temperature is very obvious. According to the general chemical reaction, the temperature increases by 2 times for every 10°C increase in temperature.
The temperature in the atmosphere is not high, but due to the participation and coordination of light and oxygen at the same time in the atmosphere, the thermal factors accelerate the aging of polymer materials. The higher the temperature, the faster the acceleration effect. The bigger. In addition, the temperature of the atmosphere will change with the change of regions and seasons, and there will also be a temperature difference between day and night. This alternating role of hot and cold will also have some impact on the aging of certain polymer materials.
Third, sunshine hours
Sunshine time has a close relationship with atmospheric corrosion (aging) of polymer materials and coatings. The longer the sunshine duration, the faster the polymer material ages. Although the amount of ultraviolet light in sunlight is small, but the light energy is very large, it is a destructive to many polymer materials, and it is the most important factor causing the aging of polymer materials; infrared also has an important impact on the aging of polymer materials. Because the material absorbs infrared light and transforms it into heat energy, heat can accelerate the aging of the material; visible light also affects the aging of the material, because under certain conditions, visible light can also cause certain polymer degradation and damage the polymer material containing pigments. effect.
For metal materials, the longer the sunshine duration, the faster the water film on the metal surface disappears and the time for surface wetting decreases, which in turn reduces the total amount of corrosion.
IV. Oxygen and Ozone
Oxygen can react with many substances. The corrosion of steel is the result of the oxidation reaction between iron and oxygen. Aging of a polymer material is actually an oxidation reaction that takes place under the influence of heat or light, or an oxidation reaction of both.
The effect of ozone on the polymer, like that of oxygen, is mainly oxidation. For example, for unsaturated rubbers, most ozone combines with double bonds to generate ozonide. This ozonide is very unstable and rearranges as an offensive oxide that breaks the molecular chains. Tests have shown that ozone acts on the rubber deformed by stress and breaks the molecular chain, and cracks perpendicular to the direction of stress are called “ozone crackingâ€. When acting on non-deformed rubber, only the oxide film is formed on the surface instead of the turtle. crack. The chemical activity of ozone is much higher than that of oxygen, and it is also more destructive than oxygen.
V. Rainfall
Rainfall has two main effects on the atmospheric corrosion of metals: on the one hand, rainfall increases the relative humidity in the atmosphere, causing the metal surface to become wet, which accelerates the metal atmospheric corrosion process; on the other hand, the metal surface can be washed off due to rainfall. Contaminants and dust reduce the corrosion of the liquid film and slow down the corrosion process, which is extremely evident in the marine atmosphere. In addition, since the rain in the atmosphere dissolves pollutants in the air, such as S02, Cl-, etc., it can promote the corrosion process. The pH value of rainwater in acid rain areas in China is small (the lowest value is only 3.5) and the corrosion is serious.
For polymer materials, rainfall can wash away the dust on the surface of the material to make it more exposed to large sunlight, which is conducive to photoaging. Rainwater, especially the water film formed by condensation, can penetrate into the interior of the material, allowing certain water-soluble substances, plasticizers and substances containing hydrophilic groups in the polymer system to be dissolved, extracted or absorbed by water. This will gradually change the material composition and proportion of materials and accelerate the aging of materials. Water is the root cause of the blistering of the paint coating.
6. Wind direction and wind speed
In a polluted environment, the influence of wind direction on the spread of pollutants is directly related to the corrosion rate. The wind direction varies with the season and should be taken into account when discriminating the effect of corrosion factors. The wind speed has a certain influence on the alternating dry and wet frequencies of the surface liquid film. Excessive wind speed in the wind and sand environment can play a role in the abrasion of the metal surface.
Seven, dust
The effect of solid dust particles on corrosion generally occurs in three situations: The dust particles are soluble and corrosive. When dissolved in a liquid film, they become corrosive and increase the corrosion rate. The dust particles themselves are not corrosive and do not dissolve. Can absorb corrosive substances, when dissolved in the water film, promote the corrosion process; itself no corrosive and adsorptive, but fall on the metal surface may cause gaps between the sand and the metal surface, easy to moisture condensation, and even local corrosion.
Eight, biological factors
In addition to physical and chemical factors, the external factors of aging of polymer materials include biological factors. The biological factors are mainly: the influence and destruction of microorganisms and insects. Microorganisms (molds and bacteria) can grow on the surfaces of certain polymer materials under suitable conditions of temperature and humidity, mainly because some plasticizers and oils and fats in these polymer systems provide nourishment for molds. Molds can be parasitized and propagated. Insects (such as termites, cockroaches) will feed on polymeric materials.
These eight factors we have in the field of environmental testing equipment derived from high and low temperature damp heat test chamber, ultraviolet aging test chamber, xenon lamp aging test chamber, ozone aging test chamber, rain test chamber, dust chamber, mold test chamber and other equipment.
Percussion drilling is employed when auger or wash boring is not possible in very stiff soil or rock. It also can be used in most soil types.
Here the advancement of a hole is achieved by alternatively lifting and dropping a heavy cutting or hammering bit that is attached to a rope or cable that is lowered into an open hole or inside a temporary casing (casings are hollow cylindrical pipes used for borehole stability and to prevent the loss of drilling fluid through the boreholes).
Usually a tripod is used to support the cable. The stroke of bit varies according to the ground condition. The major disadvantage of this method is that it is not possible to get good-quality undisturbed samples. In very hard rock (and especially fractured hard rock), down-the-hole (DTH) drilling can be employed. In this case the hammer, applying repeated percussive pressure, is located just behind the drill bit inside the hole, unlike the open percussion drilling, where the hammer is on top of the drilling string.
The drilling string provides the necessary force and rotation to the hammer and bit, as well as compressed air or fluids to the hammer and for the flushing of cuttings.
This arrangement also allows for much deeper percussion drilling. However, the DTH drills are typically more expensive.
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