想要控制车轮和底盘零配件的腐蚀，就需要了解材料之间的相互作用、电镀法及各种防护措施和技术的相对有效性。目前采用的一种防腐蚀措施为使用有机和无机涂料，另一种措施是防止电镀电流穿过绝缘层和机械设计。为了选择合适的技术并充分利用该技术来减缓腐蚀，有必要在实验室中重复并精准控制腐蚀影响因素。在生产中实施某工艺后，需要持续测试和认证其质量。实验室腐蚀试验用于腐蚀控制技术的开发、生产的材料认证及投产后的质量认证。
盐雾试验
盐雾试验是在100多年前开发的，第一个已知标准ASTM B117在1914年提出。随着时间推移，对技术进行了优化，并将它作为复杂的环境测试循环的一部分实施。此技术已经有很多变化，但是大部分都涉及喷浓度一般在0.5%至5%之间的氯化钠溶液。所述溶液可以通过添加醋酸或硫酸酸化，但是一般情况下pH值为中性的。产生喷雾的传统方式为将溶液泵送到一个雾化喷嘴中，在喷嘴中与压缩空气组合，产生漂浮在试验箱中的非常小的液滴，直到沉降到试样上。此类喷雾通常称为“雾”或“薄雾”。
加速腐蚀测试中使用的多种常见盐的潮解相对湿度
一些更新的方法需要喷出能够以更大的沉降率直接落在试样上的更大的液滴。喷雾类型约200-600ml溶液/平方米试样区域/小时，具体视不同方法而定。直接喷淋有时称为“淋浴”，可能在约20分钟喷淋期间输出5-10升/平方米，或者说为在相同期限内使用喷雾时输出溶液量的100倍。
湿/干循环试验
盐雾试验并未重复真实的大气腐蚀，因为它们始终保持试样完全湿润。暴露在环境中的材料会进行周期性的湿和干阶段。湿与干条件之间的转换加速了腐蚀并改变了腐蚀产物的化学结构。因为这个原因，湿和干循环腐蚀试验在20世纪60年代被开发出来。优先被普遍应用的试验称为Prohesion，它在ASTM G85的附件5中被标准化。在此试验中，喷稀释的氯化钠和硫酸铵溶液一小时，然后使用强制空气循环干燥一小时。
In the 1980s, the automotive industry developed a test that included salt spray, forced air drying, and exposure to saturated humidity, or what some standards call "wetting." The tests simulated vehicles driving on roads treated with deicing salt, parked outside during the day to dry out, and rewet at night when the relative humidity increased. Such tests are described in GM 9540P, Volkswagen PV1210 and JASO M609. The GM method was updated and republished as GMW 14872, but all three methods are currently in use.
PV1210 Cyclic Corrosion Test
Chamber RH (%) relative humidity of the Test Chamber (%)
Chamber temp (°C) Chamber temperature (°C)
Setpoint setting value
Actual actual value
Fog fog
Dry off dry
Humidification
GMW14872 Cyclic Corrosion Laboratory Test
One problem with these laboratory tests is that there is poor reproducibility between laboratories and poor reproducibility. This is due to the drying rates achieved under different laboratory conditions and in different Test Chambers. JASO M609, which requires very fast transitions in wet and dry conditions, most likely solves this problem. However, this method cannot accurately simulate the outdoor environment, as discussed below. Despite this problem, this method is still popular with other car companies in Japan and Asia.
The key role of relative humidity
Over the past two decades, corrosion test method development has focused on more accurate relative humidity control. This is related to the deliquescence of the salt. Deliquescence refers to the property of any soluble salt to absorb enough moisture from the air to form a liquid solution when the relative humidity is high enough. Sodium chloride has a deliquescent relative humidity (DRH) value of 76%, which is close to the 80% relative humidity rule above. However, this rule may be too simple. Corrosion of steel begins at about 50% relative humidity and increases at a rate as relative humidity increases. When the relative humidity value is slightly lower than the relative humidity of sodium chloride deliquescence, the corrosion rate of steel is high.
The second factor to consider is galvanic corrosion. Cars and trucks are manufactured from a variety of metals and alloys, and galvanic corrosion is a serious problem. Steel, aluminum and magnesium alloys, and galvanized coatings are all common, and as long as there is an electrolytic film on the surface, a galvanic couple will exist between them. However, the relative humidity is slightly below 76%, which is sufficient for corrosion to occur, but not for the cohesive electrolytic film on the surface to complete the circuit. As a result, cathodic protection of more precious metals (that is, steel when it forms a galvanic couple with aluminum, zinc, or magnesium) may not be effective at 70% relative humidity.
Hyundai Automobile Corrosion Test Method
In today's automotive industry, the trend for corrosion test methods is to include controlled relative humidity cycling between approximately 50% and 95%. Linear transitions are designed to improve repeatability and reproducibility while slowing down the drying process, achieving peak corrosion rates over a longer period of time than in conventional wet/dry testing, and controlling the time relative humidity is below and above 76%. The older JASO M609 method discussed above emphasized rapid transitions, leaving very little time for this important relative humidity region.
These new methods are ACT 1 (Volvo Car and Ford L-467 standards), ECC 1 (Renault), VDA 233-102 and GMW 14872. The new VDA test and ECC1 continue to use the salt spray method, but other tests provide salt spray by the spray method. SAE CHINA has recently developed a new standard called CATCH, which combines useful elements of all these methods to achieve realistic results with a high level of acceleration. It uses the spray method of salt mist.
