TECHNICAL INFORMATION

            CORROSION TECHNOLOGY

        RUST AND CORROSION – CONTEMPORARY METHODS FOR THE

             PROTECTION OF UNPAINTED METALLIC SURFACES AGAINST

             RUST AND CORROSION

• DESCRIPTION OF RUST AND CORROSION                  
• MEASUREMENT OF CORROSION
• CORROSION TYPES
• ELECTROCHEMICAL CORROSION
• CHEMICAL CORROSION
• CORROSIVE ENVIRONMENT AND COUNTERMEASURES TO BE TAKEN AGAINST CORROSION
• SELECTION OF APPROPRIATE RUST PROTECTION AGENT

•    DESCRIPTION OF RUST AND CORROSION

Corrosion is chemical erosion of metals and their alloys as a result of the effect of the environment to which they are exposed, and by extension, decomposition of their physical characteristics. Since nonmetallic materials are also affected by environmental conditions, today, when we refer to “corrosion”, we understand the general decomposition through environmental effects of the entire range of products qualified as industrial and construction materials. In the same context, we can use the words “rusting/rust” as the “product” that emerges as a result of corrosive action.

Impurities in the metallic structure, local disparities in alloys, manufacturing conditions of the metal, temperature and humidity differences, local concentration of solvent gases or salts in the atmosphere to which the metal is exposed are the most important factors that intensify the effects of corrosion.

All types of materials manufactured of metals will more or less suffer from corrosion. Entire mechanical characteristics of a metal are altered after corrosion has set in, and its strength diminishes as corrosion progresses. Steam boilers, petroleum and natural gas pipelines, nuclear reactors, bridges, deep-well piping, ships and static and kinetic metal parts all types of motor vehicles are equipment most severely attacked by and constantly under serious threat of corrosion. Thus, the corrosion phenomenon emerges as an enormous problem encountered under all circumstances.  Manufacturing corrosion-resistant materials, surface claddings, additives to reduce corrosion effects in the environment and replacement of components that have been attacked by corrosion to a degree that has rendered them out of commission, are considered as economic losses directly caused by the corrosion.

• MEASUREMENT OF CORROSION

In corrosion-related calculations, generally used is the unit mil/year that corresponds to 0.001^th of an inch. Since one inch approximates 0.025 mm, the above unit refers to a surface penetration of 25 microns/year in the metal,

as a rule of the thumb, corrosion penetration in a metal of 0-2 mil/year (0-50 microns/year) is considered very good; 2-20 mil/year (50-500 microns/year) as good; 20-50 mil/year (500-1250 microns/year) as average, and more than 50 mil/year (1250 microns/year) as poor.

  1 mil/year – 0.001 inch/year – 25 micron/year

0-2 MIL/YEAR = 0-50 MICRON YEAR > VERY GOOD

20-50 MIL//YEAR = 50-500 MICRONS/YEAR > GOOD

20-50 MIL/YEAR = 500-1250 MICRONS > AVERAGE

50-500 MIL/YEAR = 1250 MICRONS/YEAR AND ABOVE > POOR

• CORROSION TYPES

1. Equally Dispensed Corrosion:  Most commonly encountered type of corrosion is equally dispensed corrosion that results in uniform reduction of the surface film of a metal. Although the loss of metal is in excess than that of other types of corrosion, since the rate of corrosion progress and life span of the material can be calculated, this is the least feared type of corrosion.

Ø  IN UNIFORMLY DISPENSED EROSION,

Ø  CORROSION RATE CAN BE CALCULATED

Ø LOSS OF MATERIAL AND ITS LIFE EXPECTANCY CAN BE CALCULATED , LEAST FEARED TYPE OF CORROSION

2. Galvanic Corrosion: This is a corrosion type that occurs between different metals in the same environment in contact with each other. A measure for protection against this type of corrosion is to refrain as much as possible during design and production to couple metals that are distant from each other in the galvanic chart.

IN DESIGN AND PRACTICE OF MATERIALS TO BE USED IN THE SAME ENVIRONMENT,

Ø  IT MUST BE REFRAINED FROM COUPLING METALS DISTANT FROM EACH OTHER IN THE GALVANIC CHART.

EXAMPLE: CORROSION IN AN ALUMINUM PLATE BY THE ACTION OF IRON                       OR  BRASS BOLTS

3. Pit Corrosion: Although metal loss in this type of corrosion is much less than that in uniformly dispensed type, its contagious and difficult-to-control characteristics render it one of the most feared types of corrosion. Due to concentration of corrosion in localized areas, a multitude of pits appear on the surface. The metal is soon perforated and becomes unusable. This type of corrosion (pitting) generally occurs in neutral environments containing chloride and bromide ions. Metals in atmospheres containing chlorides of reducible metal ions are particularly susceptible to pit corrosion.

Ø   A CORROSION TYPE (PITTING) CAUSING LOW METAL LOSS RATE, BUT IS HIGHLY DANGEROUS DUE TO ITS CONTAGIOUS AND DIFFICULT-TO-CONTROL CHARACTERISTICS. 

Ø  IN THIS TYPE OF CORROSION ATTACK, THE METAL IS SOON PERFORATED AND LOSES ITS FUNCTION.

Ø  THIS IS ENCOUNTERED OFTEN IN NEUTER ENVIRONMENTS CONTAINING CHLORIDE AND BROMIDE IONS.

Ø  METALS IN ATMOSPHERES CONTAINING CHLORIDES OF REDUCIBLE METAL IONS (NaCl, KCl, CaCl, MgCl) ARE PARTICULARLY SUSCEPTIBLE TO THIS TYPE OF CORROSION.

4. Interval Corrosion:  The corrosion of this type of is concentrated on small areas. It often starts in discontinuities that could not be eliminated in the erection of machinery parts. As these gaps are widened, the efficacy of corrosion diminishes. Deposition of solid particles in the environment on metallic surfaces and protective coatings of low quality prepare a suitable medium for corrosion. Therefore, solid particles accumulated in the gaps between machinery parts during erection must be rid of constantly. 

Ø  THIS TYPE OF CORROSION IS ENCOUNTERED IN SMALL GAPS THAT COULD NOT BE ELIMINATED DURING ERECTION OF MACHINERY (e.g. JOINING SURFACES OF CHASSIS PARTS).

Ø  THE EFFECT OF CORROSION IS REDUCED AS THE GAP WIDENS.

Ø  DEPOSITION OF PARTICLES IN THE ENVIRONMENT ON THE SURFACE AND COATINGS OF LOW QUALITY SHOULD BE AVOIDED.

5. Selective Corrosion: This is a corrosion type that concentrates on a particular metal in an alloy that decomposes that metal. In this type of corrosion, an apparent change, other than its color, may not be observed in the material, despite a great reduction in its strength. An example to this phenomenon could be given as the loss of silver in a gold-silver alloy (electrum) in diluted nitric acid.

6. Inter-crystalline Corrosion: In this type of corrosion, although no important change occurs in the appearance and weight of the material, its mechanical strength is drastically reduced.  This is because, as the corrosion concentrates on the boundaries of the crystals of the material, while crystals retain their integrity and form, the inter-crystalline bonds are destroyed. This type of corrosion is found particularly in austenitic chrome-nickel steels and aluminum-copper alloys.     

Ø  DIMENSIONS AND WEIGHT OF THE MATERIAL REMAIN CONSTANT

Ø  WHILE CORROSION OCCURS IN ITS CRYSTAL BOUNDARIES AT MOLECULAR DIMENSIONS.

Ø  CRYSTALS RETAIN THEIR INTEGRITY AND FORM WHILE.

Ø  INTER-CRYSTALLINE BONDS ARE WEAKENED.

Ø  THIS PHENOMENON IS FOUND PARTICULARLY IN AUSTENITIC CHROME-NICKEL STEELS AND ALUMINUM-COPPER ALLOYS.

7. Tensional Corrosion:  This is a corrosion type that is the result of systems operating under mechanical tension while being simultaneously exposed to a corrosive environment. High-pressure vessels, steam boilers, pump shafts and rotors operate under the threat of this type of corrosion.

Corrosion appears in cracks on the material that progress deeper in time, and finally causes fracture of the part. High ambient temperatures accelerate the rate of corrosion.

8. Erosional Corrosion: This is a corrosion type observed in materials in rapid kinetic action while being exposed to a corrosive environment. Pipelines through which gases and liquids are pumped, pump bodies and vanes, turbine blades operate under the risk of erosional corrosion. Corrosion appears in the surfaces of the material eroded by the flowing medium. Rate of corrosion is directly proportional to the velocity of the flowing medium, and if that flow is turbulent, the rate of corrosion is accelerated. In laminar flow, the corrosion rate is slower. Parts that cause sudden changes of direction of flow such as elbows, valves and flanges are factors that increase the rate of corrosion. Moreover, collision of solid particles, if present in the flowing medium, reduces the thickness of oxidized film on inner surfaces, and thus accelerates corrosion.

9. Frettage Corrosion: This is the type of corrosion observed on metal surfaces in reciprocating action against each other. Projections on the metal surfaces are scraped during the motion, and surfaces in contact are oxidized. With the scraping of the oxidized surfaces during the next cycle, the process is continued at repeated intervals. Corroded parts display a structure with pits circumscribed by an oxidized ring. .

Ø  THIS IS THE CORROSION OF THE TYPE OBSERVED IN METAL SURFACES IN RECIPROCATING MOTION UNDER LOAD.

Ø  PROJECTIONS ON SURFACES ARE SCRAPED AND CONTACT SURFACES ARE OXIDIZED.

Ø  THE PERIOD IS REPEATED UPON RE-SCRAPING OF THE OXIDE FILMS IN THE NEXT CYCLE.

Ø  CORRODED PARTS DISPLAY A STRUCTURE WITH PITS SURROUNDED BY OXIDES.

CORROSION TYPES CAN ALSO BE CLASSIFIED IN THE MANNER BELOW:

Since corrosion may be described as chemical erosion of metallic materials and decomposition of their physical characteristics (change in strength values) as a result of ionization of different metals due to the environment in which they exist, it is possible to classify “Corrosion of Metals” as a Chemical and Electrochemical  phenomenon.    

Corrosion, generally under non-protected conditions, starts at an accelerated rate that is gradually reduced in time, despite continuing in its progress.  This is because the product of the chemical reaction during the corrosion process forms a protective film on the surface of the material (generally oxides of the metal, e.g. 2Fe₂ + 3O_{2   ---------->}  2Fe₂O₃).  Upon elimination of that protective film by mechanical action, corrosions starts anew at accelerated speed as a result of which the material is rapidly eroded with alterations in its physical dimensions, and also in its physical and chemical characteristics. Alteration in physical dimensions and the effects of mechanical abrasion may also be termed as mechanical erosion of the material. The joint advent of corrosion and erosion is undesirable, and results in huge economic losses.  

A) ELECTROCHEMICAL CORROSION

Electrochemical corrosion is the result of a reaction between two different metals in a solution (the electrolyte) at about room temperature (e.g. batteries), triggering severe electron exchange between the metals that generates an electric current  which in turn creates cathodic and anodic zones in the solution, whereby one of the metals (the anode) erodes (decomposes).  

There are three types of electrochemical corrosion.

A.1) ELECTROCHEMICAL Corrosion by the effect of acids:

The majority of metals (those with standard oxidation potential more than “0”) are eroded while dissolving by the release of hydrogen gas. Metals such as Gold (Au), Copper (Cu), Silver (Ag) of oxidation potential less than “0” do not dissolve by the attack of acids, and therefore, do not corrode. In turn, some metals become resistant to acids because of the protective layer formed on their surfaces by the effect of initial corrosion. For instance, while lead should normally dissolve in sulfuric acid  (H₂SO₄), the corrosion product lead sulfate builds such an effective layer on the metal that once that layer is formed, the lead is not expected to be affected any further by the sulfuric acid.

A.2) Electrochemical corrosion caused by galvanic effect in couples :        

As galvanic couples created as a result of two metals of different solubility voltage in a solution come into contact with one another, one acts as the ANODE, and the other as the CATHODE. It is the anode metal that dissolves and corrodes.

A.3) Electrochemical corrosion caused by different aeration elements

This is the result of differences in the oxygen concentrations on different spots of a metal piece where oxygen-deficient spots act as anode, and oxygen-rich spots as cathode. In conclusion, oxygen-deficient sections, by reacting with humidity in the air (oxygen), start the corrosion reaction. This phenomenon is often seen in the example of fingerprint marks on unpainted metal surfaces, or observed as condensed vapor in the air sticking to various pores in the metal in a closed packaging due to atmospheric temperature and pressure changes.

B) CHEMICAL CORROSION

Corrosion of a metal by the effect of gases in the environment is termed as chemical corrosion. This is a phenomenon of combination of metal molecules with oxygen, or simply put, a matter of combustion. This generally occurs as a result of oxygen gas reacting with minute pores on the metal surface where it forms a layer of rust, which in iron and its alloys is iron oxide (Fe2O3). This layer of “oxide film” on the surface that penetrates to a considerable depth in the material is called rust. Rust formation rate increases with the CO₂ in the air and high ambient temperature. As the blood-red RUST formed at low temperatures is of a porous and brittle structure, it does not serve as a protective coating on the surface of iron (metals), and continues to increase in thickness until the metal is completely decomposed.  This is why particular attention has to be paid to the location of use of iron and its alloys. Among corrosion accelerators are DUST, ATMOSPHERIC GASES (H₂S – Hydrogen Sulfide) that combine with metals to form METAL SULFIDES.

CORROSIVE ENVIRONMENTS AND MEASURES THAT COULD BE TAKEN AGAINST CORROSION

Alongside Chemical Industries and environments engendered by chemicals, the atmosphere, water, soil, biological mediums are environments are of highest efficacy in the formation of corrosion. There are various methods to protect metals from the harmful effects of corrosion, such as:

- Appropriate design,

- Control of environment characteristics (inhibitors, pacifiers),

- Surface coatings (Phosphatizing, Painting, Metallic, Breeding, Organic and

   Inorganic Cladding,

- Cathodic Protection.

In view of the design and manufacturing by qualified industries of products ranging from the simplest utensils used in our daily lives to ultramodern space vehicles having to be built according to the purposes of their utilization from a huge variety of materials addressing a multitude of operation areas, it is imperative to have these products protected by more active methods consistent with the developing technical possibilities.      

Organic surface coatings are the easiest and cheapest kind of protection against rust and corrosion of all types of accurately machined/ground finished or semi-finished machinery parts that have been subjected to a large variety of metal-working processes which are not to be painted or permanently coated by any other method, but have to be kept in line for assembly, or stored or preserved until overseas shipment.    

Contrary to the general belief formed despite many evidences, ordinary oils and grease, with the exception of moderate environment conditions, are not effective rust preventive agents. Although, the use of greases as commercial metal protection agents goes back to the mid-eighteenth century, “SURFACE PROTECTION COMPOUNDS” were developed on the basis of the requirement during the Second World War of protection of large numbers of machined parts against contamination caused by extreme temperatures, humidity and salt.  This development was pioneered by the American firm VALVOLINE-TECTYL, the  discoverer of the original engine oil, that has which specialized on temporary rust protection agents used primarily in military and civilian vehicles.

Rust and corrosion preventing surface protection compounds, manufactured as oil, solvent, wax and water based products, more than simple protection agents, have to carry the characteristics of functional and purpose oriented, cheap products, providing protection for the desired period, that are extremely compatible with environment and atmospheric changes and also indelible or easy-to remove as the situation warrants.

SELECTION OF APPROPRIATE RUST PREVENTION AGENT

 Following prerequisites have to be known in order to select the most appropriate and cost effective rust prevention and protective agent:

1- Type of material to be protected (ferrous or nonferrous)

2- Mechanical processing quality of the surface (rough machined, turned on lathe,

    ground etc.)

3- Expected minimum protection period (approximate or estimated protection

    period until installation).

4- Period to elapse between oiling and packaging of the material to be protected.

5- Whether or not the protective oil has to be removed later from the surface.

6- Destination and transportation vehicle of the protected material (Overseas,  

     motorway, container etc.)

7- Storage method, status and warehouse conditions before use.

Accurate answers to the above questions determine the characteristics of the preservative and rust protection oil. However, the golden rule under all circumstances is to use the correct protection oil at the correct place and time.

For instance:

If protection is to be provided by a solvent-based product, a complete and airtight packaging is possible only after a delay for having to wait for the solvent to vaporize completely. Where to allow for such a delay period is not possible, it is definitely required to use the more expensive “Oil” or “Water” based products. Otherwise, water vapor in the airtight and overheated packaging would be converted into a humidity chamber, and as a result of pressure drop, would precipitate which, by the solvent action of the protective oil coating on the surface would prepare the conditions required for sudden and severe corrosion.

It would be a more proper method to use protection oil with thin, non-solidifying characteristics forming a surface film that would not later need removal from the surface, rather than a difficult-to-clean, expensive and time consuming product.

Protection oils are classified between themselves and other trademarks according to the results of 5% salt spray tests as per A.S.T.M. B117 and DIN 50021 and 100% humidity chamber tests as per A.S.T.M. D1748 and DIN 50017.

While resistance period in salt tests in the most resistant epoxy paints can be 200-250 hours, that period is observed to be extended to 2000-3000 hours in TECTYL protection oils.

Another quality indicator in protection oils is their ability to dispel water and humidity from the metal surface, and stick on the surface by magnetic penetration.

Protective oils, when required, may be removed from metal surfaces by the action of petroleum or all mineral-derivative solvents, low pressure steam or alkaline baths.    

Ramazan Özer

Mech. & Marine Eng.