Pitting occurs when a small hole or cavity forms in the metal, usually as a result of the depassivation of a small area. This area becomes anodic, while some of the remaining metal becomes cathodic, causing a localized galvanic reaction. The deterioration of this small area penetrates into the metal and can lead to failure. This form of corrosion is often difficult to detect because it is usually relatively small and can be covered and hidden by compounds generated by corrosion. The most corrosion-resistant materials are those where corrosion is thermodynamically unfavorable. Gold or platinum corrosion products tend to spontaneously break down into pure metal, which is why these elements are found in metallic form on Earth and have been appreciated for a long time. More common “base” metals can only be protected by more temporary means. Corrosion is described as the chemical and electrochemical destruction of metals or alloys by the surrounding wet and humid environment. Corrosion is an oxidation process because it occurs in the presence of oxygen. Almost all metals corrode, but the corrosion rate varies depending on the metal. Glass disease is the corrosion of silicate glasses in aqueous solutions. It is controlled by two mechanisms: diffusion-controlled leaching (ion exchange) and hydrolytic dissolution of the glass network. [14] Both mechanisms are highly dependent on the pH of the contact solution: the ion exchange rate decreases with a pH of 10−0.5 pH, while the hydrolytic dissolution rate increases with a pH of 100.5 pH.

[15] The ISO 719 standard test is not suitable for glasses whose alkaline components are poor or non-extractable, but which are nevertheless attacked by water, e.g. quartz glass, B2O3 glass or P2O5 glass. Corrosion is the deterioration and loss of a material and its critical properties due to chemical, electrochemical and other reactions of the exposed surface of the material with the environment. Special alloys, either low-carbon or with added carbon “getters” such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to avoid the similar phenomenon of “knife line attack”. As the name suggests, corrosion is limited to a very narrow area next to the weld bead, which is often only a few micrometers wide, making it even less noticeable. The chemistry of corrosion is complex; It can be considered an electrochemical phenomenon. When corrosion at a certain point on the surface of an iron object, oxidation takes place and this site behaves like an anode. The electrons released at this anodic site move through the metal and go to another location on the metal, reducing oxygen to this point in the presence of H+ (which would be available via carbonic acid (span{display:block}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em}]]>H2CO3), which are formed by the dissolution of carbon dioxide from the air into water under conditions of humid air from the atmosphere.

hydrogen ions in water may also be available due to the dissolution of other acid oxides in the atmosphere). This spot behaves like a cathode. For efficient CP, the potential of the steel surface is polarized (pressed) more negatively until the metal surface has a uniform potential. With a uniform potential, the driving force of the corrosion reaction is stopped. In galvanic CP systems, the anode material corrodes under the influence of steel and may need to be replaced. Polarization is caused by the flow of current from the anode to the cathode, which is driven by the electrode potential difference between the anode and cathode. The most common sacrificial anode materials are aluminum, zinc, magnesium and related alloys. Aluminum has the highest capacity, and magnesium has the highest drive voltage and is therefore used where the resistance is higher. Zinc is a universal goal and the basis of galvanizing. The imbalance between the space and the rest of the material contributes to high corrosion rates.

Crevice corrosion can occur at lower temperatures than pitting corrosion, but can be minimized by proper joint design. In a given environment (a standard medium is aerated, seawater at room temperature), a metal is either nobler or more active than the others, depending on the force with which its ions are bound to the surface. Two metals in electrical contact share the same electrons, so the “tug-of-war” on each surface is analogous to the competition for free electrons between the two materials. When the electrolyte is used as a host for the flow of ions in the same direction, the precious metal absorbs the electrons of the active metal. The resulting mass flow or electric current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called the galvanic series and is useful for predicting and understanding corrosion. Glass is characterized by high corrosion resistance. Due to its high water resistance, it is widely used as a primary packaging material in the pharmaceutical industry, as most drugs are kept in an aqueous solution. [13] In addition to its impermeability, glass is also robust when exposed to certain chemically aggressive liquids or gases. Most ceramic materials are almost completely immune to corrosion.

The strong chemical bonds that hold them together leave very little free chemical energy in the structure; They can be considered as already corroded. When corrosion occurs, it is almost always a simple dissolution of the material or a chemical reaction and not an electrochemical process. A common example of corrosion protection in ceramics is lime, which is added to lime-lime glass to reduce its solubility in water; Although not as soluble as pure sodium silicate, normal glass forms submicroscopic defects when exposed to moisture. Due to its fragility, such errors lead to a dramatic reduction in the strength of a glass object during its first hours at room temperature. The formation of an oxide layer is described by the Deal Grove model, which is used to predict and control the formation of an oxide layer in different situations. A simple test to measure corrosion is the weight loss method. [9] The process involves exposing a properly weighed piece of metal or alloy to the corrosive environment for a period of time, then cleaning to remove corrosion products and weighing the part to determine weight loss. The corrosion rate (R) is calculated as follows: If corrosion has been detected, the only safe way to repair it is to remove it. Abrasion, the details of which depend on the metallurgy of the corroded part, followed by a corrosion inhibitor such as a zinc-chromate primer, another primer and finally paint, eliminates a slight surface corrosion.

Passivation refers to the spontaneous formation of an ultra-thin film of corrosion products, known as passive film on the surface of the metal, which acts as a barrier to further oxidation. The chemical composition and microstructure of a passive film differ from the underlying metal. The typical passive layer thickness on aluminum, stainless steels and alloys is less than 10 nanometers. Passive film is different from oxide layers that form during heating and are in the micrometric thickness range – passive film recovers when removed or damaged, while the oxide layer does not. Passivation in natural environments such as air, water and soil with moderate pH is observed in materials such as aluminum, stainless steel, titanium and silicon. Galvanic corrosion occurs when two different metals have physical or electrical contact with each other and are immersed in a common electrolyte, or when the same metal is exposed to an electrolyte with different concentrations. In a galvanic pair, the most active metal (the anode) corrodes at an accelerated rate and the nobler metal (the cathode) corrodes more slowly.