What cause corrosion-resistant hose get corroded
In automobiles, hoses are used to transport various liquids and gases, including fuel, lubricating oil, refrigerant and water. Hoses installed in automobiles must be subjected to various environmental factors under driving conditions for a long time. In order to produce and develop hose products that meet actual use requirements, it is inevitable that the correct evaluation and testing of the performance of hoses will become a very important task.
1. Performance requirements of various types of automobile hoses
Automobile hoses must have certain rigidity and flexibility, and certain resistance to high and low temperatures, pressure, weather, liquid transportation and mechanical vibration. Automobile hoses can be divided into fuel hoses, air conditioning hoses, brake hoses, cooling hoses, power steering hoses and air delivery hoses, etc. Hoses for different purposes have some different requirements. Table 1 shows the performance requirements of various types of hoses and some commonly used testing methods.
Hose type standard number main test items
Brake pipe
ISO3996 GB/T7127
Hydraulic test; necking test; volume expansion test; bursting pressure test; brake fluid compatibility test; flexural fatigue test; pull-off test; water absorption test; low temperature bending test; dynamic ozone test; high temperature pulse test; salt spray test
Cooling pipe
HG/T2491 (WSE-M96D34)
Adhesion strength; bursting pressure; outer diameter change; brittleness; temperature; ozone; aging; thermal aging (resistance to bursting pressure after aging after coolant filling; bending test; low temperature; flexibility; compression permanent deformation; pulse strength; electrochemical corrosion)
Air conditioning pipe
ISO8066 GB/T20025
Refrigerant leakage and penetration test; aging test; low temperature flexure test; vacuum test; static pressure length change test; burst pressure; R134a extraction test; R134a resistance test; ozone cleanliness pulse test; moisture ingress test; overall sealing pressure change
Fuel pipe
ISO4639 GB/T10542 HG/T3665 HG/T3666
Liquid resistance (C liquid oxygenated fuel oxidized fuel No. 3 oil); air tightness; burst pressure; adhesion strength; C liquid after extraction ozone test; low temperature flexure; cleanliness and extractables; fuel penetration; vacuum test; hose tensile permanent deformation and tearing; oxygenated fuel long-term cycle test; flame resistance; accelerated aging; copper sheet deposition
Power steering pipe
ISO11425
Pulse test; burst pressure; hydraulic length change test; low temperature flexure; bonding strength; ozone resistance; volume expansion; cleanliness; joint corrosion; liquid resistance; vibration fatigue
2. Inspection methods for hose material properties
Common hose material properties tests include tensile properties, hardness, tear strength, adhesion strength, liquid resistance, air aging, compression permanent deformation and tensile permanent deformation, ozone aging, low temperature performance, metal corrosion and permeability, etc. Usually, you should select the inner rubber according to its heat resistance and resistance to the ability to transmit liquid. The outer layer of rubber needs to be heat and ozone resistant and have good adhesion to the inner rubber. These tests are generally carried out with test pieces and are mainly used for product quality control. Its vulcanization conditions are different from the actual vulcanization conditions of the hose. Therefore, the evaluation of materials often stipulates that samples should be prepared from the hose.
In addition, when evaluating the durability of the material, samples should be cut from the hose after storage or use under certain conditions. Due to the particularity of the use conditions of the hose, these conventional physical property tests are also different from those of other rubber products. For example, usually carry out tensile permanent deformation measurement. As a vessel wall material, it often require permeability test. It requires Metal corrosion test due to contact with metal pipe joints. Rubber hoses are mainly to transport various liquids. Therefore, liquid resistance test is an important material test item. Liquid resistance test is often carried out with commercial liquids. Because it is close to the use conditions. However, due to large volatility, the comparability of test results is poor. Therefore, you’d better use standard or reference liquids for testing.
Commonly used standard or reference liquids are as follows:
1) Reference fuel: prepared with isooctane and toluene. The more toluene, the higher the aromatic hydrocarbon content, and the greater the swelling effect. The commonly used C liquid has a ratio of isooctane and toluene of 50/50, which can produce the effect of high aromatic gasoline for automobiles. Liquids B C D simulate various commercial gasolines with various expansion effects. F is standard diesel, which is composed of straight-chain alkanes and methylnaphthalene, and the expansion effect is lower than B. Some standards use 90/10 No. 3 oil/paraxylene as standard diesel. In addition, G H I K is a four-alcohol fuel (oxygenated fuel) composed of different proportions of isooctane, toluene, methanol, ethanol, diisobutylene and water. Germany’s FAM-1 and FAM-2 are similar reference fuels. The standard oxygenated fuel is a reference fuel with a peroxidation index of 90 after adding tert-butyl hydroperoxide.
2) Standard mineral oil: a high-viscosity petroleum-based oil. The higher its aniline point, the lower its swelling ability. The No. 1 standard oil has the highest aniline point and is a low-volume incremental oil that can simulate high-viscosity lubricating oil. The No. 2 oil has a medium aniline point and is a medium-volume incremental oil. The No. 3 oil has the lowest aniline point and is a high-volume incremental oil that can simulate hydraulic oil. The newly added IRM902 and IRM903 are close to No. 2 and No. 3 oils, respectively.
3) Working fluid: The actual liquid used often contains many chemical additives, such as engine oil, which is a base oil with 5~25% additives added. Therefore, three representative working fluids are proposed, namely 101 simulated diester type lubricating oil. 102 is a No. 1 standard oil with 5% hydrocarbon preservative, which is to simulate a certain high hydraulic oil. 103 simulates the phosphate ester hydraulic oil used in aircraft. A 50/50 ethylene glycol/water mixture can produce the effect of engine coolant. Glass washing liquid prepared with different proportions of water, ethanol and humidifier can also be a working fluid.
Commonly used hose inspection methods
Many countries’ general rubber test standards have specified a set of methods for testing the reliability of various types of rubber hoses. In addition, there are also many special test methods in some product standards. These methods are:
1. Hose size measurement
Inner diameter outer diameter; reinforcement layer; outer diameter wall thickness; concentricity; inner and outer layer rubber thickness; assembly; inner diameter. The new national standard and ISO have added length and measurement point marks. And it stipulates the measurement method of hose length without pipe joints and with various pipe joints.
2. Hydraulic test-verification pressure test.
Check whether the hose and assembly have leakage, deformation and damage under the verification pressure for 30s-60s.
Pressure deformation test: Keep at the specified pressure (working pressure verification pressure or other pressure lower than verification pressure) for 1 minute. Measure the length and outer diameter change of the hose, as well as the torsion angle and bending.
Burst pressure test: Determine the pressure when the hose bursts at the specified pressure increase speed.
Leakage test: Keep at a static pressure of 70% of the minimum burst pressure for 5 minutes, repeat once, and check for leakage or damage. Since the test often uses water, the viscosity is different from that of the actual liquid. And the burst pressure and leakage pressure at room temperature may be slightly lower. Usually the working pressure designed for the hose is the maximum pressure available for use. The pressure of the non-destructive test to verify the firmness of the hose is called the verification pressure, which is usually 1.5-2 times the working pressure. The pressure when the hose bursts is called the burst pressure, which is 3-10 times the working pressure.
3. Low temperature bending test
Low temperature rigidity: After the hose is clamped on a torsion wheel with a diameter 12 times the inner diameter of the hose, it is parked at low temperature for 6 hours and twisted 180° within 12s. The ratio of the torque measured to the torque measured at standard temperature.
Low temperature bending: The hose is clamped on a torsion wheel with a diameter 12 times the inner diameter of the hose. After being parked at low temperature for 24 hours, it is twisted 180° within 10s. Check whether the inner and outer rubbers are brittle and damaged. The simplest test to measure the low temperature brittleness of the hose is to bend the sample 90° at low temperature. Or freeze a section of the hose and compress it by 1/2 to see if it is brittle. Another method is to use a certain weight of a heavy hammer to fall freely and impact the sample to see if the sample is brittle.
4. Bending test
After bending the hose to a certain degree, measure the ratio of the minimum outer diameter of the bent part to the outer diameter before bending, the passing capacity of the steel ball and the bending force when pressurized in the tube.
5. Flattening test
Evacuate the hose within 1 minute, keep it for 10 minutes, and then roll a steel ball with a diameter of 0.9 times the inner diameter of the hose to check the degree of collapse of the hose. Some standards use the measurement of the change rate of the outer diameter of the hose to indicate the degree of deformation of the hose.
6. Interlayer adhesion strength test
Automotive hoses are mostly braided hoses with a diameter of less than 50mm. The test usually uses 10mm or 25mm wide strip specimens. There are also 25mm wide rings, peeled at 90°, and the stretching speed is 25mm/s.
7. Liquid wall penetration test
Under normal pressure, connect the hose to a container filled with a certain liquid and seal the container mouth. Place the test device horizontally, and then regularly weigh the mass change of the entire test device caused by the liquid penetrating outward through the hose, so as to find the liquid penetration rate.
Apply 50kPa of air pressure to the test liquid, release the pressure after a certain period of time, and measure the reduced volume of the liquid. The measurement principle of the refrigerant penetration rate of the air conditioning pipe is basically the same, except that the refrigerant filling method is more complicated, and a reference sample test without liquid filling should be carried out at the same time.
8. Volume expansion test
The hose should not produce obvious volume changes under the pressure of the transmitted liquid. The method of measuring volume expansion is to connect the hose to a hydraulic source and the other end to a measuring tube for measuring the volume of liquid after the hose expands. Increase the pressure in the hose to the test pressure to expand the hose. Then turn off the hydraulic source and open the valve connected to the measuring tube. At this time, the liquid in the volume expansion part rises into the measuring tube, and the expanded volume can be measured.
9. Cleanliness and extraction test
For fuel hoses, C liquid is often injected into the hose, and it is emptied after being parked for 24 hours. The inner wall is cleaned with C liquid. Collect the injected and flushed C liquid, filter out the insoluble impurities, and dry and weigh them to obtain the weight of the insoluble impurities. The cleanliness is expressed by the number of impurities per unit surface area of the hose or the maximum size of the impurities. The filtered solution is evaporated, dried, and weighed to obtain the weight of the soluble matter. Methanol is then used to extract the waxy substance from the filtrate evaporated and dried. The obtained methanol extract is evaporated and dried, and the weight of the waxy substance is obtained.
10. Salt spray test Place the hose assembly in the salt spray formed by 35°C 5% sodium chloride aqueous solution for 24 hours, and then check whether the metal of the pipe joint is corroded. (Insertion force and pull-off force, flame resistance, conductivity, wear resistance, pollution, etc.)
Durability test of hose
The durability test of hose can reflect the actual performance and service life of the hose. The main methods are:
1. Hot air aging
Put the hose in an oven and measure the changes in the various properties of the hose after aging after a certain period of time. Or wrap the hose around a cylinder, take it out and straighten it after aging in the oven for a period of time to see if cracks occur.
2. Aging test of sealed medium
The medium flowing through the inside of the hose will cause chemical changes in the rubber and its additives, reinforcing fabrics and adhesive layers. Therefore, it is not sufficient to use only hot air aging or fatigue test as durability evaluation. The most ideal is to test the aging performance by long-term hydraulic pulse and circulating medium according to the actual use conditions. However, for a large number of hose tests, the performance of the hose is generally tested by sealing the medium in the hose for aging. Such as changes in physical properties, pressure resistance, low temperature performance and vacuum performance. The relevant standards for fuel hoses also stipulate that the method of measuring the flattening, bending, ozone resistance, bursting pressure, bonding strength and low temperature flexibility of the hose after circulating oxygen-containing fuel in the hose for 1000 hours at 60°C is specified.
3. Aging test under climate cycle conditions
Cyclic aging is a multi-parameter accelerated aging test method that simulates actual use conditions. Existing equipment can be used to repeat the cycle in sequence for each single aging test, or a multi-factor, artificially simulated accelerated aging test chamber can be used. These factors include oxygen, heat, ozone, ultraviolet light, moisture and other liquids, as well as dynamic stretching. Existing standards stipulate that the temperature and humidity conditions should be changed according to a certain procedure to accelerate aging, and then the appearance (such as frost and cracks) and various performance changes of the hose should be measured.
4. Fatigue life test When the automotive hose is used, it must withstand mechanical vibration, the long-term effects of the external environment and the internal medium. The fatigue life tests that simulate these conditions can be divided into the following three categories:
Rotation fatigue test
The two ends of a group of hoses are respectively installed on non-moving and movable horizontal rods parallel to each other. The two ends of the movable rod are connected to a turntable. The turntable drives one end of the hose to rotate in a circle in the vertical direction at a speed of 800r/min, seals the joint of the movable end, and connects the non-moving end to the hydraulic source. A certain pressure is applied, and the time of leakage and damage is recorded.
Non-flexure hydraulic pulse test
Bend the sample 90°or 180°, apply medium pressure, pulse frequency, liquid temperature and ambient temperature according to the use of the hose, until the specified number of pulse cycles or until leakage and damage occur, and record whether there is damage or the number of failures. —Hydraulic pulse test with flexural vibration is accompanied by periodic flexural while applying pulse pressure, including flexural hydraulic pulse test (half-Ω test) for rubber-plastic hose assemblies and flexural hydraulic pulse test for steel-wire reinforced rubber-plastic hose assemblies. The difference between the two is that the installation direction of the active pipe end is different.
Hydraulic pulse test is usually carried out at 150% of the working pressure. When using this method to evaluate the durability of hoses and assemblies, various complex temperature and pressure change programs can also be set as needed to carry out the test.
5. Ozone aging test of hoses
The test under static tensile conditions is generally to cut the hose into a long strip sample and stretch it 20% or bend it into a ring. Or fix the hose on a cylinder that can stretch the hose by 120%. The hose can also be directly bent into a semicircular ring and wound around a mandrel with a diameter of 120% of the inner diameter of the hose. Under a certain ozone concentration, after a certain period of time, observe the cracking of the hose.
The surface cracking test conducted in the atmosphere is a low-concentration ozone test. And the method is similar to this. In the test of brake hose, a hose ozone resistance test under dynamic conditions was designed. One end of the hose was fixed and the other end moved relative to the fixed end at a frequency of 0.3Hz. After a certain period of time, the cracking of the hose was checked.
The ozone concentration used in the test is 25~200pphm. The ozone resistance of rubber can be divided into three levels: 50~500pphm natural rubber, NBR, SBR, butadiene rubber and isoprene rubber. 1000~2000pphm chloroprene rubber and butyl rubber. 10000pphm or more CSM, PU, silicone rubber and KFM.
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