DEFINITION OF RUST AND CORROSION
Corrosion; It means that metals and alloys undergo chemical wear and therefore their physical properties deteriorate as a result of ion transfer under the influence of the environment they are in. Since non-metal materials are also affected by environmental conditions, today, when we say “Corrosion”, we understand the general deterioration of all products that are industrial and construction materials due to environmental effects. By the way, we can use the word “Rust” to mean the product formed as a result of corrosion.
The most important factors that increase corrosion are impurity in the metal structure, local differences in alloys, production conditions of the metal, temperature and humidity differences, local concentration differences of gases or salts dissolved in the environment with which the metal is in contact.
All kinds of materials made of metal are subject to corrosion to a greater or lesser extent during use. All mechanical properties of the metal that is corroded change and its strength decreases as the corrosion progresses. Steam boilers, oil and natural gas pipelines, nuclear reactors, bridges, deep well pipes, ships and fixed and working metallic parts of all kinds of motor vehicles are the places where corrosion occurs the most and creates great danger. Thus, corrosion emerges as a major problem in every situation. The production of corrosion-resistant materials, surface coatings, additions made to the environment to reduce the effectiveness of corrosive environments and replacement of parts that are corroded to a degree that they cannot perform their duties with new ones are considered as economic losses directly caused by corrosion.
MEASURING CORROSION
In calculations related to corrosion, the unit mil/year is generally used. Since 0.001 inch is approximately 0.025 mm, this unit corresponds to a penetration value of 25 microns/year on the metal surface.
As a general rule, a corrosion rate of 0-2 mils/year (0-50 microns/year) is considered very good for a metal, 2-20 mils/year (50-500 microns/year) is considered good, 20-50 mils/year (500-1250 microns/year) is considered moderate and greater than 50 mils/year (1250 microns/year) is considered poor.
1 Mil / year = 0.001 inch / year = 25 microns / year
0 – 2 MIL / YEAR = 0 50 MICRONS YEARS > VERY GOOD
2 – 20 MIL / YEAR = 50 500 MICRONS / YEAR > GOOD
20 – 50 MIL / YEAR = 500 1250 MICRONS / YEAR > MEDIUM
50 500 MIL / YEAR = GREATER THAN 1250 MICRONS / YEAR POOR
TYPES OF CORROSION
1. Uniform Corrosion: Uniform corrosion is the most common corrosion, which causes thinning of the metal surface as a result of homogeneous abrasion. Although metal loss is higher than other types of corrosion, it is the least feared type of corrosion because the corrosion rate and life of the material can be easily calculated.
HOMOGENEOUS DISTRIBUTED WEAR AND
CORROSION CAN BE CALCULATED FAST. MATERIAL
LOSS AND LIFE CAN BE CALCULATED.
*IT IS THE LEAST FEARED TYPE OF CORROSION.
2. Galvanic Corrosion: This is the type of corrosion that occurs in different metals that are in contact with each other in the same environment. In order to be protected from this type of corrosion, during design and manufacturing, it is necessary to avoid pairing metals that are far from each other in the galvanic series as much as possible.
DIFFERENT METALS IN THE SAME
ENVIRONMENT SHOULD BE AVOIDED IN DESIGN AND DESIGN.
EXAMPLE: CORROSION BY ALUMINUM, IRON OR BRASS SCREWS
3. Pitting Corrosion: Although the metal loss in this type of corrosion is much less than in uniform corrosion, it is one of the most feared types of corrosion because it is widespread and difficult to control. As the corrosion event is concentrated in small areas, numerous pits are formed on the metal surface. The metal is quickly pierced and becomes unusable. This type of corrosion generally occurs in neutral environments containing chloride and bromide ions. Environments containing chlorides of reducible metal ions are very dangerous in terms of pitting corrosion.
METAL LOSS IS LOW, COMMON AND DIFFICULT TO CONTROL. IT IS THE MOST
DANGEROUS TYPE OF CORROSION . A NUMBER OF PITS ARE FORMED IN NARROW AREAS (CALLED PITTING OR PITTING). METAL IS PERFORATED IN A SHORT TIME AND CANNOT BE USED. IT IS FREQUENTLY SEEN IN NEUTRAL ENVIRONMENTS WHERE CHLORIDE AND BROMIDE IONS ARE PRESENT. ENVIRONMENTS CONTAINING CHLORIDE OF REDUCEABLE METAL IONS (NaCl, KCl, CaCl2, MgCl2) ARE VERY DANGEROUS.
4. Crevice Corrosion: In this type of corrosion, the event is concentrated on narrow areas. It starts in gaps that cannot be eliminated during the assembly of machine parts. As these gaps widen, the effectiveness of corrosion decreases. The precipitation of solid particles in the environment on metallic surfaces and low-quality protective coatings prepare suitable ground for this type of corrosion. For this reason, solid particles accumulated in the assembly gaps of machine parts must be constantly removed.
IT IS FREQUENTLY ENCOUNTERED IN NARROW GAPS THAT CANNOT BE ELIMINATED DURING ASSEMBLY (MACHINING SURFACES OF FRAME PARTS…) AS THE
GAPS WIDE, CORROSION EFFICIENCY DECREASES. PRECIPITATION OF SOLID
PARTICLES IN THE ENVIRONMENT ON THE SURFACE AND POOR QUALITY COATINGS.
5. Selective Corrosion: This is the type of corrosion that causes a certain metal to dissolve by concentrating on it in alloys. In this type of corrosion, there is a great decrease in the durability of the material, but there may be no change in its external appearance, other than a change in color. An example is the loss of silver in dilute nitric acid in a gold-silver alloy.
6. Intercrystalline Corrosion: In this type of corrosion, although there is no significant change in the external appearance and weight of the material, its mechanical strength is greatly reduced. Because the corrosion event is concentrated near the crystal boundaries of the material, while the crystals maintain their integrity and shape, the intercrystalline bonds are destroyed. This type of corrosion is especially seen in Austenitic textured Chromium-Nickel Steels and Aluminum-Copper alloys.
MATERIAL SIZE AND WEIGHT REMAIN CONSTANT.
CORROSION OCCURS AT MOLECULAR DIMENSIONS AT CRYSTALS BOUNDARIES. CRYSTALS PRESERVE THEIR
INTEGRITY AND SHAPE. INTERCRYSTAL BONDINGS ARE WEAKEN . ESPECIALLY SEEN IN AUSTENITIC CHROME – NICKEL STEELS AND ALUMINUM – COPPER ALLOYS.
7. Stress Corrosion: Stress corrosion is a type of corrosion that occurs when systems operating under mechanical stress are also in contact with corrosive environments. High-pressure vessels, steam boilers, pump shafts and rotors operate under this type of corrosion threat.
The corrosion process begins with cracks on the material. The cracks progress into the material and eventually fractures occur. The temperature of the environment increases the corrosion rate.
8. Erosive Corrosion: This type of corrosion is seen on the surfaces of materials that are in contact with corrosive environments that move at high speeds. Pipelines where gases and liquids are pumped, pump bodies and blades, valves, turbine blades operate under the threat of erosive corrosion. Corrosion occurs as a result of the removal of the surface layers of the material by the flow. The corrosion rate increases with the flow rate of the medium and if the flow is turbulent, the material corrodes rapidly. The rate of corrosion progress is weaker in laminar flows. Equipment such as elbows, valves, flange parts and sudden changes in the path of the flow are the factors that increase the speed of this type of corrosion. In addition, if there are solid particles in the environment, they accelerate the removal of oxide layers by hitting the surface of the material, thus causing the corrosion rate to increase.
Frettage: It is a type of corrosion seen on metal surfaces moving back and forth on each other under load. The protrusions on the metal surfaces moving on each other are scraped during the movement and the interfaces are oxidized. The process continues by repeating itself as the formed oxide layers are scraped off, the corroded parts exhibit a pitted structure surrounded by oxides.
IT IS A TYPE OF CORROSION OBSERVED ON METAL SURFACES THAT MOVE BACK AND FORTH UNDER LOAD. THE PROMINENCES ON THE SURFACES ARE SCRAPED DURING THE MOVEMENT AND THE INTERMEDIATE SURFACES ARE OXIDIZED. THE EVENT IS REPEATED WITH THE SCRAPING OF THE OXIDE LAYERS. THE PARTS SUBJECTED TO CORROSION SHOW A PITTLED STRUCTURE SURROUNDED WITH OXIDE. IT IS ALSO POSSIBLE TO CLASSIFY CORROSION TYPES AS FOLLOWS:
Since corrosion is the chemical wear of different metal materials as a result of ion transport due to the environment they are in, and therefore the deterioration of their physical properties (change in strength values), it is possible to divide the corrosion of metals into Chemical and Electrochemical.
Corrosion generally starts at high speed in unprotected environments and surface conditions and its speed gradually decreases, but it continues to progress. Because the chemical reaction product that occurs during corrosion, (2Fe2 + 302 2Fe2 03) (mostly the oxide of the metal) forms a protective layer on the material. As a result of the destruction of this film by mechanical effects, corrosion starts again at high speed, as a result, the material wears away in a short time, its physical dimensions, physical and chemical properties change. Changes in physical dimensions and mechanical wear are also called mechanical material erosion. The simultaneous progress of corrosion and erosion is an undesirable situation and leads to great economic losses.
A) ELECTRO-CHEMICAL CORROSION
Electrochemical corrosion occurs when two different metals, at normal temperatures and in environments containing liquids, exchange electrons excessively in an electrolytic (such as a battery, etc.) and an electric current is generated, and as a result, one of the metals (anode) is corroded (dissolved) between the anodic and cathodic regions.
There are 3 types of electrochemical corrosion.
A.1) ELECTROCHEMICAL corrosion caused by the effect of acids:
Most metals (those with a standard oxidation potential greater than 0) are corroded while dissolving with the release of hydrogen gas. Those with an oxidation potential less than 0 such as Gold (Au), Copper (Cu), Silver (Ag) do not dissolve under the effect of acids. In other words, they do not corrode. However, some metals are not affected by acid after corrosion has started, as the products formed as a result of corrosion cover the surface. For example; While theoretically lead should be dissolved in sulphate acid (H2SO4), the lead sulphate (corrosion product) that forms forms such a successful corrosion layer on the metal surface that once this layer is formed, it is not expected for the lead metal to be affected by sulphate acid again.
A.2) E/C corrosion formed by galvanic couple effect:
In galvanic pairs formed as a result of two metallic materials with different dissolution voltages coming into contact with each other in a solution, one of the metals forming the pair acts as the ANODE and the other as the CATHODE. The anode dissolves and corrodes.
A.3) E/C corrosion formed by the effect of different weathering:
It occurs due to the differences in oxygen concentration formed at different points of a single metal. The oxygen-poor parts act as the anode, and the rich parts act as the cathode. As a result, the oxygen-poor parts corrode and combine with the humidity of the air (oxygen). This situation occurs as a result of the condensed moisture formed by fingerprints or temperature and pressure changes in closed packages on unpainted metal surfaces sticking to various pores on the metal surface.
B) CHEMICAL CORROSION
The corrosion of a metal by the effect of gases in its surroundings is called chemical corrosion. This event is a combination with oxygen or, in the simplest sense, a combustion event. It is usually the effect of oxygen gas on the surface of the metal (Iron and Iron alloys) and the formation of Iron oxide (Fe203-PAS). This oxide film layer formed on the surface is called rust and penetrates deeply into the surface of the material. The formation of rust increases with the effect of CO2 in the air and heat. Since the red blood-colored RAS formed at low temperatures is formed in a porous, brittle structure, it does not provide a protective layer on the Iron (Metal) surface, and continues until the metal is exhausted. Therefore, it is necessary to pay attention to the places where it is used in the corrosion of iron and its alloys. Among the factors that accelerate corrosion, DUST, GASES IN THE ATMOSPHERE (H2S – Hydrogen Sulfide) combine with metals and cause the formation of METAL SULFIDE.
CORROSIVE ENVIRONMENTS AND PRECAUTIONS THAT CAN BE TAKEN
AGAINST CORROSION Chemical Industry, in addition to environments formed by chemical substances; atmosphere, water, ground, biological environments are the most effective corrosive environments that metals encounter. There are various methods to protect metals from the harmful corrosive effects of these environments. These can be listed as:
– Appropriate design
– Controlling the characteristics of the environment (inhibitors, passivators)
– Surface coatings (Phosphating, Painting, Metallic, Noble, Organic and Inorganic Coatings)
– Cathodic Protection.
The fact that qualified industrial production involves the design and production of very different materials in various fields, from the simplest needs in our daily lives to ultra-modern space vehicles, requires that these products used be protected according to more active and developing technological possibilities. The easiest and cheapest way to protect against rust and corrosion all kinds of machine parts that are processed through various metal processing techniques into finished or semi-finished products, that are precision machined / ground, that should not be painted or covered with other fixed coatings, that are waiting for assembly, that will be stocked or shipped overseas, is organic surface coatings.
Despite much evidence, contrary to popular belief, ordinary oil and grease are not effective rust preventives. (Except under moderate conditions) Although the use of greases as commercial metal protectors dates back to the mid-18th century, during World War II, modern SURFACE PROTECTION COMPOUNDS were developed due to the need to protect large quantities of machined parts against severe heat, humidity and salt contamination. This development in particular was carried out under the leadership of the American VALVOLINE-TECTYL company, which was the first to discover engine oil in the world, and specialization was achieved primarily in temporary rust preventives for military and civilian purposes.
Surface protection compounds that prevent rust and corrosion should be manufactured in various forms such as oil, solvent, wax and water based, and the surfaces to be protected should be functional and suitable for their purpose, inexpensive, able to protect for the desired period of time, highly compatible with environmental and atmospheric changes, and have stable or easy-to-clean qualities beyond just simple protection.
SELECTION OF THE SUITABLE RUST PREVENTIVE
The following conditions should be determined for the selection of the most appropriate and lowest-cost rust preventive protective oil.
1 Type of material to be protected (ferrous or non-ferrous)
2 Mechanical processing quality of the surface (roughly machined, turned, ground etc.)
3 Expected minimum protection period (Approximate or anticipated protection period until assembly)
4 Time for the material to be protected to be packaged after lubrication, packaging type and shape.
5 Whether the protective oil needs to be cleaned from the surface
6 Where the protected material will be shipped and the shipping vehicle (Overseas, TIR, Container etc.)
7 How it will be stored before use, its condition and the environment it will be kept in.
The correct answers to these questions will determine what properties the protective rust preventive oil should have. However, it is certain that the right protective product should be used at the right place and time.
For example:
If protection is to be made with a solvent-based product, it is absolutely necessary to wait for the solvent on the surface to evaporate in order to switch to a complete and airtight packaging. If the waiting time cannot be provided, it is absolutely mandatory to use non-solvent-based, Oil or Water-Based (more expensive) protective products. Otherwise, the water vapor or the solvent vapor that could not be removed in the closed packaging that does not receive air and becomes a humidity chamber due to the increase in temperature will condense as a result of the pressure drop and, with the effect of dissolving the protective oil on the surface, will prepare the ground for sudden and severe corrosion.
Instead of using a protective that is difficult to clean from the surface, costly, time-consuming and destructive to the material, it would be a more appropriate method to use a protective oil that does not require cleaning from the surface and creates a thin, non-drying surface film.
Protective oils are classified among themselves and among other brands according to the results of 5% salt spray test in accordance with American, ASTM B117 and DIN 50021 standards, and 100% humidity cabinet test in accordance with ASTM D1748 and DIN 50017 standards. While the
salt test resistance period can be 200-250 hours in the best resistant Epoxy paints, this value is seen to reach up to 2000-3000 hours in TECTYL protective oils.
Another quality indicator of protective oils is their ability to stick to the metal surface by magnetic penetration by removing water and moisture from the metal surface to which they are applied.
Protective oils should be able to be cleaned from the metal surface with all petroleum or mineral-derived solvents, low-pressure steam or alkaline baths when desired.
