Aluminium Corrosion & Its Different Types

Aluminium is the second most abundant metal on earth and due to its excellent properties, is one of the most widely used metals today. Therefore, it is useful to be aware of the conditions that shorten the lifetime of these metals.

任何金属的腐蚀significantly impact its functional strength causing structural damage like cracks, partial fracture, and total material failurein extreme cases. In this article, we shall take an in-depth look into aluminium corrosion to help understand the different types of corrosion that can affect the metal.

什么是铝腐蚀？

铝腐蚀是铝分子逐渐衰减到其氧化物中降解其物理和化学特性的。

By nature, aluminium is a reactive metal but it is also a passive metal.

这意味着,尽管新兴铝反应with oxygen and water in the environment, the resulting compound will form a layer on the surface protecting the material underneath from further corrosion. This non-reactive oxide layer sticks well to the surface and does not flake off easily, similarly to不锈钢。

与诸如故意的过程不同laser etching,铝阳极氧化, or brightening, corrosion is a slow process and will occur over many months or years. What makes aluminium unique though is that there are many different types of corrosion pathways. Understanding these different corrosion phenomena is the first step in applying control measures to reduce or completely prevent their occurrence.

Types of Aluminium Corrosion

Atmospheric corrosion

The most common form of aluminium corrosion. Atmospheric corrosion of aluminium occurs as暴露于自然元素的结果。由于它在大多数地方发生的可能性，大气腐蚀形成了狮子在所有类型的腐蚀合并中对世界铝造成的总损害的份额。

大气腐蚀可以分为三个子类别。这些是干燥，湿和潮湿的，具体取决于服务环境的水分水平。

由于水分含量会根据您的地理位置而变化很大，因此某些区域会比其他地区观察到更大的腐蚀。

Other environmental factors that affect the extent of atmospheric corrosion are wind direction, temperature and precipitation changes. Concentration and variety of pollutants in the air, closeness to large water bodies, etc. also play a significant role.

Atmospheric corrosion may be exacerbated if the design does not allow for drainage of moisture. Creating pockets of water for rain and condensation, for example, are harmful design flaws.

Galvanic corrosion

Galvanic corrosion, also known as dissimilar metal corrosion can affect aluminium whenit is physically or through an electrolyte connected to高贵的金属。The noble metal can be any metal that has lesser reactivity compared to aluminium.

Reactivity of a metal depends on its position in the electrochemical series. The severity of corrosion will be worse if the other metal is further away from aluminium in the electrochemical series.

The intensity of corrosion is highest at the intersection, where the two metals meet, and reduces as we go further away from this interface.
For example, if aluminium and brass are in contact or even close to each other and placed in seawater,a galvanic cell形成了。然后，铝部件将作为阳极（正末端）腐蚀。

这可能是一个问题，在船只中，黄铜配件可能接近铝配件，而它们都沉浸在海水中。电子从铝流向黄铜通过海水。

This type of galvanic cell may be inadvertently formed in other service environments and lead to galvanic corrosion. Galvanic corrosion can be更快than normal atmospheric corrosion.

Pitting corrosion

Pitting corrosion is asurface corrosion phenomenon of aluminium metal characterised by small holes (pits) on the surface。Usually, these pits do not affect the strength of the product. Rather, it is an aesthetic issue but can lead to failure if surface appearance is critical.

Pitting corrosion generally occurs in regions where salt is present in the atmosphere, as the presence of chloride阴离子负责。硫酸盐盐也会在一定程度上引起腐蚀。在存在碱性和酸盐的情况下，观察到蚀腐蚀的最坏情况。

为了使腐蚀发生，合金的电势必须在电解质（盐溶液）电位上方。在晶界和第二相颗粒处的表面缺陷的存在是蚀腐蚀的前体。

Crevice corrosion

缝隙腐蚀是一种形式localised corrosion process in materials。Overlapping materials or unintentional design mistakes can lead to the formation of crevices. As a result, collecting seawater into those pockets can lead to crevice corrosion.

即使是螺栓和结构之间的小缝隙也足以使这种类型的腐蚀开始。随着时间的流逝，材料的铝溶解并沉淀到海水中。这种离子铝吸收来自周围空气和氢氧化离子的氧气，从电解质产生氢氧化铝。

This oxygen reduction makes the crevice acidic in the presence of chlorides which accelerates the rate of corrosion.

Intergranular corrosion

When it comes to aluminium, the grain boundary is electrochemically different compared to the alloy microstructure. This causes an electrochemical potential set up between the two and an exchange of electrons takes place.

There are multiple variations of intergranular corrosion based on thermochemical treatments and metallic structures. It is also found to different degrees indifferent series of aluminium alloys。例如，6XXX系列合金相对较不容易受到这种类型的铝腐蚀。

The anodic path will vary with different alloy systems. While in the 2xxx series it appears as a narrow band on either side of thegrain boundary，在5xxx系列中，它表现为沿晶界的连续路径。

像点腐蚀一样，intergranular corrosion begins from a pit。However, it propagates far quicker along susceptible grain boundaries.

Exfoliation corrosion

Exfoliation corrosion is a special type of intergranular corrosion found in aluminium alloys that have marked directional structures. This is predominantly evident in经历过的铝制产品hot or cold rolling processes。

It occurs along elongated grain boundaries in the microstructure. The term exfoliation comes from the fact that the corrosion product is more voluminous and gives the impression of lifting from the material surface.

这种类型的铝腐蚀在表面上方膨胀，侧向膨胀了产品中的应力。反过来，这最初会在表面迁移到大部分产品之前在表面上产生楔形作用。严重的分层发生，材料削弱。表面降解结果如蚀，剥落和起泡。

The 2xxx, 5xxx, and 7xxx series are more prone to exfoliation corrosion due to their highly directional grain structures. This makes the grain boundaries far more sensitive to intergranular corrosion.

The susceptibility to exfoliation corrosion can be modified by using热处理方法to redistribute the precipitates.

一般腐蚀

当腐蚀在铝产物表面几乎均匀地发生时，它是均匀或一般腐蚀的。

这种类型的腐蚀可能会随着不断暴露于高酸性或碱性培养基的产品而发生。当产物在电解质中时，它也可能在具有高电化学势的情况下发生。一个典型的例子是在酸性溶液中生锈。

Uniform corrosion is the result of the continuous shifting of anode and cathode regions in contact with the electrolyte which manifests as a uniform corrosive attack on the surface.

In high and low pH solutions, the oxide layer is also unstable and does not protect the metal underneath. The thickness of the material reduces and it will eventually dissolve completely.

The attack is not completely uniform and there will exist peaks and valleys. The absence of small deep corroded areas is enough to term it as a general corrosion example.

Deposition corrosion

Deposition corrosion occurs when adissimilar metal gets deposited on the aluminium surface leading to serious localised corrosion。

Imagine water flowing through a copper tubing. When the water flows through, it picks up copper ions. These copper ions are now in a solution. When this solution comes into contact with an aluminium surface or vessel, it deposits these copper ions onto it.

These ions now form a subtle galvanic cell which corrodes the aluminium through pitting if the ions are lower in the electrochemical or galvanic series.The larger the difference between aluminium and the deposited ion in the galvanic series, the worse the corrosion。

Even a concentration of 1 ppm copper ion solution is known to perform serious corrosion on the aluminium surface.

The metals that can cause deposition corrosion of aluminium are referred to as ‘heavy metals’. Some important heavy metals are copper, mercury, tin, nickel, and lead.

The corrosion caused by this method is more pronounced in acidic solutions as compared to alkaline solutions. This is because these ions have low solubility in alkaline solutions.

Stress corrosion cracking (SCC)

Stress corrosion cracking (here on out referred to as SCC) is一种形式的晶间腐蚀，可能导致铝零件的总失败。

Three conditions need to be fulfilled for this corrosion to occur.易感合金is the first of them. Not all aluminium alloys are equally prone to SCC. High yield strength alloys are more likely to suffer from stress corrosion cracking.

The second condition is that theservice environment must be humid or wet。The third condition is theexistence of tensile stress in the material。This tensile stress is responsible for the opening of the crack and its propagation through the metal.

SCC过程有两种类型。第一个是晶间应力腐蚀裂纹（IGSCC），其中裂纹沿晶界传播。第二个是透射应力腐蚀裂纹（TGSCC），其中裂纹通过晶粒体而不是沿边界传播。

腐蚀腐蚀

侵蚀腐蚀铝是由我造成的pingement of a铝制机身上的高速水流。

Two factors that aggravate erosion-corrosion arethe velocity of the water and its pH level。水中碳酸盐和二氧化硅含量的存在可以进一步增加腐蚀速率。

In pure water, aluminium corrosion proceeds at a sluggish rate. But when the pH level exceeds 9, this rate increases. In acidic water, corrosion is faster.

通过控制侵蚀腐蚀可以预防the above factors. Either reducing the water velocity, or maintaining water quality, or both can significantly reduce erosion-corrosion. Improving the water quality means maintaining the pH level as close to neutral as possible (<9), and reducing silica and carbonate content.

Corrosion fatigue

It is a well-known fact that fatigue can cause complete failure of a product if left unchecked. In the case of aluminium,fatigue cracks can act as initiation sites for pitting corrosion。

Corrosion fatigue in aluminium occurs when it isrepeatedly subjected to low stress for long periods of time。The crack initiation and propagation takes place with greater ease in a corrosive environment like seawater and salt solutions.

Corrosion fatigue cannot proceed without the presence of water in the atmosphere. It also remains largely unaffected by stress direction as the crack propagation is mostly transgranular. Thus, the stresses do not affect its propagation unlike in the case of SCC.

Filiform corrosion

Filiform or wormtrack corrosion is initiated as pitting corrosion. Itstarts at points where the paint has peeled off the surface of the aluminium。The reason could be scratches or bruises on the surface that expose the underlying metal surface.

丝状腐蚀发生并在存在氯化阴离子和高湿度的情况下很容易扩散。尽管它以盐水蚀的腐蚀作用，但传播的方式是缝隙腐蚀的方式。

The head of the wormtrack is acidic and has high chloride content. It absorbs oxygen and acts as the anode. The latter part of the wormtrack acts as the cathode and the reaction ensues.

Filiform corrosion can be通过防止表面免受损坏并使用油漆或蜡缩小所有小间隙来阻止。如果实用，则必须降低环境的相对湿度。

微生物诱导的腐蚀

微生物诱导的腐蚀or MIC is由微生物/真菌引起的腐蚀。This type of corrosion is noticed in fuel and lubrication oil tanks.

在石油中存在水的情况下，微生物和真菌可以繁衍生息。这些生物中的一些能够消耗石油和排泄可能导致腐蚀的酸of the aluminium vessel used for storing.

This acid causes pitting corrosion in the aluminium vessel, eventually leading to leakage.

To prevent this, theoil must be purified as far as possible to remove water content。Draining water at regular intervals from fuel tanks after purification is also necessary. If improving the fuel quality is not an option, using fungicides it possible to prevent germination.