# Computer Vision and Image Processing Approaches for Corrosion Detection
## 2 Background
### Corrosion  
The strength of a metallic material, its operation, physical appearance, and mechanical properties can be affected by corrosion. However, the degradation of metallic materials is inevitable and they are susceptible to the surrounding environment [5]. The adjacent environment can be classified into wet corrosion and dry corrosion categories. Wet corrosion happens because of the formation of reactive electrochemical cells in the presence of liquid. Examples of wet corrosion include galvanic corrosion, erosion corrosion, and crevice corrosion. Moreover, dry corrosion refers to the reaction of metallic material with oxygen in the air, without the presence of liquid. Examples of dry corrosion include hydrogen attacks, molten-salt, and oxidation. The most corrosive environments are coastal areas and marine environments. The combination of sodium chloride (salt), oxygen, and moisture can rapidly increase the corrosive level in both environments. Moreover, seawater is more corrosive because of its higher penetrating power and conductivity through the metallic surface films. Furthermore, acid, polluted air, hot air, and hot water are more corrosive than the normal environment. Recently, the high toxicity level in the wastewater of domestic areas, urban sewage plants, and industrial plants has become a major concern as it can lead to a corrosive environment due to the presence of dissolved and suspended compounds, organic and inorganic chemicals, pathogenic and non-pathogenic compounds, etc. [6]. Other than environmental exposure, such as wet corrosion and dry corrosion, corrosion can also be categorized in accordance with attack morphology. There are eight categories of corrosion morphology, including general attack, erosion corrosion, intergranular corrosion, stress-corrosion cracking, galvanic corrosion, pitting, crevice corrosion, and selective leaching [7]. The uniform electrochemical reaction that occurs on the surface of a material is known as a general attack. It can be contained and is predictable. Erosion corrosion occurs due to the accelerated corrosion process resulting from the high velocity of corrosive fluid on the metal surface. Abrasion and mechanical wears are also involved. Intergranular corrosion is a specific form of corrosion that targets the boundaries between grains in a material. It creates a small, corroded area. Corrosive mediums and tensile stress can result in stress-corrosion cracking. A crack is produced from brass season cracking and steel caustic embrittlement. Galvanic corrosion occurs because of the potential difference between two submerged metals in a corrosive solution [8]. The metal with resistance to corrosion acts as a cathode whereas the other metal acts as an anode. The anode metal will corrode more than the cathode metal. Pitting is a type of corrosion that is highly destructive and causes localized damage, resulting in the formation of holes in the metal surface. Due to the accumulation of corrosion products, it can be challenging to identify these areas of damage. Crevice corrosion is a localized corrosion with small amounts of stagnant solution. It always attacks the holes, gaskets, and lap joints. Selective leaching is a type of corrosion that occurs when a particular element from an alloy is extracted [9]. This happens to aluminum, zinc, cobalt, chromium, and iron. In relation to ship structures, the weakening caused by corrosion and fatigue can decrease the durability of vessels and ships, leading to potential structural failures [10]. The primary factors causing corrosion in ship structures include the single metal element (such as the element at the ship's hull), two different metals (such as those used for the metallic coatings and rivets), and the concentration element (such as the presence of electrolytes and metal). Moreover, ship structures can be categorized into atmospheric zones and immersed zones of stiffeners and plates.