Corrosion#Resistance to corrosion

As corrosion protection is referred to measures to avoid damage that can be caused by corrosion of metallic components. Since an absolute corrosion resistance can not be established, the aim of the protective measures implemented generally on reducing the rate of corrosive attack so far that damage to the component may be avoided during its lifetime.

Corrosion damage

If, by a corrosive attack to impairment of the functionality of a component, then one speaks of a corrosion damage. The term corrosion is no longer used today only for metallic materials, but also for glass, plastic, construction materials, etc.. Engaging the medium is referred to as a corrosive agent. According to DIN EN ISO 8044 include the following types of corrosion can be distinguished:

• Surface corrosion in which the surface is uniformly damaged
• Trough corrosion, in which the surface is uneven badly damaged
• Destroyed pitting, only small areas of the surface
• Crevice corrosion, in which the material surface is in narrow columns, such as in welds, attacked

To prevent this chemical process, there are various methods and procedures which are generally referred to as corrosion protection. Below is a closer look at the metallic corrosion protection.

We distinguish active and passive corrosion protection. Based on the concept of " constructive wood protection" can be achieved by suitable design protection against corrosion even in metallic materials.

Passive corrosion protection

Passive corrosion protection includes all measures which achieve a shielding effect against corrosive media. This can be achieved, inter alia, by a suitable coating or coating of the material as well as structural measures (for example, avoidance of fluid retention, roofing, etc.). Examples of optionally active pigmented ( eg, zinc dust, phosphate, zinc chromate or red lead even earlier ) coating materials ( ie liquid, paste or powder coated materials) are synthetic resins (eg EP or PU), plastics ( z. , PVC or plastic sheeting ), oils, paint, rubber or hard paraffins. Examples of ( inorganic or metallic ) coatings are a conversion layer by a phosphating, an anodic coating, a hard anodized aluminum, chromating or other conversion coatings with more non-metallic character as well as hot-dip galvanizing, which in turn form a protective passivation layer and in addition protect the substrate as a sacrificial anode if damaged.

Big and long pipelines from steel for water transport are preferably lined for corrosion protection with an inner coating of cement mortar. By the admixture of suitable plastics, the corrosion resistance can be further improved. The main advantages of this type of coating are:

• Low cost
• Also largely resistant to corrosive water up to a pH of about > 4.0; this resistance does not apply to deionized water.
• Self-healing of small cracks that go up to the iron surface

Also electroplating or chemically produced, the metallic covering of tin, gold, nickel, copper, chromium or nickel - phosphorus alloy coatings such as ( electroless nickel ) cause corrosion protection. Also find the galvanizing broad application. Its protective effect is based on metal layers on its capacity, even not to corrode ( or noble metals such as nickel, a spontaneous self-passivation ), or to the passivation of the base metal by forming a dense oxide film on the surface that serves as corrosion protection. Some metals are capable of "automatically " to form a covering layer which protects the base material, such as patina on copper or zinc.

A certain corrosion protection can provide tinplate - Tin here is used as a coating material, so that food can be so packed. However arise after some time, when the can is open, tin ions, which are toxic to eg cress. Therefore, the can should be additionally painted.

Passive corrosion protection has the disadvantage that the layers must be absolutely tight - otherwise place of pores may place increased corrosion ( local elements ).

Less noble layers protect the workpiece and act as sacrificial or protective anode - the layer dissolves preferentially and thus receives as long as possible, the function of the component. A classic example is the galvanizing of steel, but also the protection of hydraulic structures such as ships, locks, sheet piling, boat parts and rails by attaching sacrificial anodes of zinc, aluminum or magnesium alloys.

Active cathodic protection

Active cathodic protection with impressed current

Another possibility for the cathodic corrosion protection - short PPS - can be achieved by means of impressed current and impressed current anodes. For oil pipelines distance to be, for example, at intervals of a few kilometers in a few hundred meters sunk transverse to the line electrodes in the soil, which are fed with a reverse voltage. The other pole is located on the pipeline so that this voltage gradient exactly the galvanic element from soil and metal line balances that moves specifically in the order of a few volts. As it depends on the chemical composition of the soil, it must be examined and the fed back tension to be adapted to the local conditions.

In bridge, especially at highway bridges, the PPS is performed by means of impressed current anode. For this purpose, a grid of coated titanium anode is applied to the surface to be protected and injected with shotcrete about 2-3 cm. The shotcrete serves as the electrolyte. The stream is introduced via a rectifier in the reinforcement, thus reaching the cathodic protection. The position is reviewed on an ongoing basis with an automatic monitoring system.

In addition, electrodes for cathodic corrosion protection of copper titanummanteltem ( titanium clad copper) as well as silver-silver chloride.

Iron is available in the electrochemical series of positive than zinc, ie, zinc is less noble than iron and is in the galvanic cell is the anode and the cathode is iron. Iron as the more noble metal is therefore as long cathodically protected until the zinc is corroded away.

• Protection of a pipeline system by corrosion inhibitors are added to the water cycle
• To determine the corrosion resistance of coatings standardized environmental tests are performed. Here, for example, surfaces slightly injured with Ritzprüfgeräten, and then exposed to a salt spray.

Active cathodic protection without external power

With the active corrosion protection without external power pursues one goal, a metal that often comes in contact with water, for example, to protect against rust. To this end, use is made of a less noble substance that is sacrificed for the quasi metal. For this, a circuit is established which is supplied from the redox reaction of the oxidation of the anode. The current of a few milliamperes is guided through a simple circuit. The reaction is as follows ( the example of Fe / Mg): As soon as magnesium or iron comes in contact with the water, it is oxidized to Mg2 and Fe2 . Compliance with the electrochemical series creates a potential difference between the magnesium and the iron of 1.9 V ( standard potentials at 25 ° C, 101.3 kPa, pH = 0; ion activity = 1). Since magnesium has a much higher potential than iron -0.41 V with a potential difference to the hydrogen of -2.362 V, magnesium is oxidized at the anode and reduces the iron to electron emission. This reaction proceeds only very slowly, but can be accelerated by changing conditions. The incident on the water electrons now split on this 2OH - in H2 and. The iron does not change, since it can absorb the emitted electrons from the magnesium again. The magnesium dissolves, however, after a certain time, and the rust protection for the iron must be replaced if necessary. The magnesium anode is centered, so the mounted in the center usually circular to be protected container, so that the potential at all surfaces of the container takes about the same value ( see photo). Consequently, the anode is mounted such that the container ends is approximately the same distance as is the circular container walls. Consequently, the anode being shorter than the container is low, thus approximately the same distance from the container bottom at the end is, as to the container walls. Therefore, the anode is isolated built into the box, otherwise a very high protection current would flow at the installation site, the rest would be but little container protected. The circuit is then closed via a cable, in which a flow meter for direct current can be looped, which measures in the milliampere range (see photos above).

Example of a corrosion process

The following is a simple test for corrosion protection using the example of an iron nail is briefly described:

Figure 2: iron with magnesium anode in aqueous solution

Figure 3: Iron and copper anode in aqueous solution

In corrosion protection by means of a sacrificial anode, anodes are used made ​​of different materials depending on the application. The sacrificial anode needs to be connected with the conductive metal to be protected in order to achieve protection.