Causes of Corrosion Danger

Causes of Corrosion Danger 18.2.1 Formation of Corrosion Cells... 

Sections of the borehole casing threatened by corrosion can be located with the help of the profile measurement technique described in Section 18.3.1. In general, the profile measurement cannot identify which factors are the main cause of corrosion danger. 

As in the case of corrosion at the insulating connection due to different potentials caused by cathodic protection of the pipeline, there is a danger if the insulating connection is fitted between two sections of a pipeline with different materials, e.g., mild and stainless steel. The difference between the external pipe/soil potential is changed by cell currents so that the difference between the internal pipe/ medium potential has the same value, i.e., both potential differences become equal. If the latter is lower than the former for the case of free corrosion, the part of the pipe with the material that has the more positive rest potential in the soil is polarized anodically on the inner surface. The danger increases with external cathodic protection in the part of the pipeline made of mild steel. 

A similar danger of corrosion lies in cell formation in steel-concrete foundations (see Section 4.3). Such steel-concrete cells are today the most frequent cause of the increasing amount of premature damage at defects in the coating of new steel pipelines. The incidence of this type of cell formation is increased by the connection of potential-equalizing conductors in internal gas pipelines and domestic water pipelines [25], as well as by the increased use of reinforcing steel in concrete foundations for grounding electrical installations [26]. 

Fig. 12-1 Danger of corrosion caused by cell formation with steel-reinforced structures and of the pipe/soil potential.

On the other hand, such a long-term approach won t do if one wishes to determine relatively quickly the corrosivities of a number of metals in a certain ambient or one metal in the presence of a series of corrosion inhibitors. Another limitation arises from the fact that the most dangerous corrosion (that which sometimes causes the collapse of bridges or buildings) involves internal cracking of the metal (Section 12.6.5) so that, with little loss to the solution, a dangerous loss of strength in the metal can occur. 

Selective and localized forms of corrosion are more dangerous than uniform surface corrosion, because they are impredictable and are often only recognized after the damage has been done. They are caused by electrochemical attack by the medium, that is, electron transport to a reduction occuring at another site (formation of a local element). Metallic materials are attacked in this way when the attacking medium is liquid and has a certain minimum conductivity. The required conductivity can be very low, and the conductivity of pure water is already sufficient. The majority of corrosion reactions of metals can be classified as oxygen corrosion and acid corrosion (Figure 2.10-3). 

Another danger is the formation of Cr-carbides at grain boundaries. They cause the depletion of the adjacent metal in Cr and thus reduce locally the protection of the alloy. As a consequence, pits may form at these sites at more negative potentials. They may merge together and cause intergranular corrosion. 

Toxicology LD50 (oral, rat) 1070 mg/kg LCLo (inh., rat, 5 h) 200 ppm highly toxic poison by skin contact and ing. mod. toxic by inh. ing. of even sm. amts, can be fatal primary skin and severe eye irritant lachrymator may cause pulmonary edema, death TSCA listed Precaution Flamm. corrosive dangerous fire and mod. explosion hazard seif-reactive iron salts may catalyze potentially explosive thermal decomp. incompat. with water iron metal salts acids alkalis amines alcohols can dec. above 20 C... 

There are several major classes of corrosive substances. Strong acids such as nitric, sulfuric, and hydrochloric acid can cause serious damage to the skin and eyes. Hydrofluoric acid is particularly dangerous and... 

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