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Welcome back to the latest installment in the Omega Update Series. Since we began this series 8 years ago we have covered many metal finishing areas of interest. All Omega Updates can be found on our web site www.OmegaResearchInc.com and are available to our loyal customers and clients free of charge. If you have difficulty in downloading a specific update, give us a call and we will mail you a "print shop" quality copy. A wealth of other metallurgical, metal finishing, and manufacturing processing information is available under the Publications section of our website. Visit it often - the data is free for the asking!
"Corrosion" is a critically important Omega Update. The most important reason metal finishers exist is to improve the corrosion behavior of metals. Indeed there are a multitude of other engineering benefits of metal finishing, such as improved hardness-wear resistance, electrical and electronics applications, high energy physics uses, heat treatment protection and many others. But improved corrosion protection is the very reason metal finishing got started as an industry. Probably the existence of your company and your job is based on the need for corrosion protection of metal parts and components. It is estimated that the economic losses attributable to corrosion in the United States alone in the year of 2005 will amount to over $27 billion dollars. So as we've said before, go pour a cup of coffee, sit yourself down and read aboutCORROSION - The Dark Side of Metals!
Many times people think that corrosion "...eats away..." metal, kind of like "metal mites" chomping away at a tasty piece of metal. In reality, corrosion is the destructive attack of a metal by a chemical or electro-chemical reaction with the environment. It usually is an oxidation mechanism; i.e. reduction reactions do not cause corrosion in the majority of commonly used metals.. By oxidation we simply mean that metallic atoms of the base metal react with oxygen from either gaseous (atmospheric) or liquid media. The result of this oxidation process is a metallic salt. The color of metallic salts formed from corrosion can be reddish-orange rust on iron and steel, or in the case of aluminum or magnesium, white powder. Other metals and alloys will of course have different corrosion products with varying color and appearance.
Since corrosion involves chemical change, it will be helpful to be familiar with some chemistry in order to understand corrosion reactions. It is not the intention of this Omega Update to condense years of chemistry into one short article - this simply cannot be done. We will present here only a highly simplified discussion of corrosion and some basic chemistry. However, since most corrosion reactions are electro-chemical, we will talk a little about some electro-chemistry. Furthermore, since the structure and the chemistry of metals and metallic alloys most often determine corrosion behavior, it will also be helpful to be somewhat familiar with some fundamentals of physical metallurgy.
Our last two Omega Updates have delved into the metallurgy of Steel and Aluminum. Pull those two Omega Updates out of your files to review, or look them up on Omega's website www.omegaresearchinc.com under publications if you have misplaced your hard copies.
MOST ACTIVE - or Least Noble*
LEAST ACTIVE - or More Noble*
An important feature of this chart is the relative position of the different elements top to bottom. If you took a piece of zinc (near top of chart), and touched it to graphite (bottom of chart) and introduced sea water as the electrolyte, you would instantly have a battery producing electricity. Old fashioned dry cell batteries are zinc-carbon! In a nut shell this chart provides corrosion guidance to designers and users of metallic parts. If you have two parts widely separated in the galvanic series chart - you must provide a barrier to prevent a electrolyte completing the circuit and causing corrosion. In todays high tech aircraft designs using carbon epoxy composites, many times these carbon composites must be attached to aluminum fittings. The potential for disaster is present if extensive corrosion prevention barriers are not used to prohibit electrical or galvanic contact. On the back page is a more detailed galvanic series chart that you might find as a useful reference.
Since this article is concerned with Corrosion and not its "kissing cousin" electroplating, we need to turn back onto our main highway of discussion today. There are many types of corrosion: a good listing would include:
Uniform attack is just that, a general chemical reaction over the surface, which is most often seen in pure metals. Pitting attack many times manifests itself in alloys having secondary phases or particles. Fretting is the rubbing of adjacent parts at low amplitudes-high frequencies in the presence of a corrosive media. Cavitation is a phenomena resulting from the impact of a liquid droplet in conjunction with a corrosive media. Exfoliation corrosive attack happens on metallic alloys heavily worked (rolled, extruded, etc) in a singular direction. The corrosion progresses along grain boundaries heavily elongated. Perforation is concentrated corrosion within areas that have a geometry tending to hold a corrosive media. Intergranular attack is similar to exfoliation, in that the corrosion occurs along grain boundaries in the metal and not generally across a grain. Stress corrosion cracking is the combination of corrosion in concert with stress, producing accelerated cracking attack, usually along grain boundaries. Tarnishing, sometimes called tinting, is a mild form of oxidation corrosion superficial in nature. High temperature oxidation corrosion is found in high temperature applications, such as boilers, burner assemblies, gas turbine components, etc. Since corrosion is a chemical reaction based phenomenon, and higher temperatures accelerate corrosion, unique corrosion can occur here.
One can see that this problem of corrosion has many personalities. It happens not only on a large visible scale but also a microscopic scale. When we talked about electrochemical corrosion, we learned that all three elements of a galvanic cell must be present; that is, an anode, cathode and an electrolyte. One might ask, "....I have a single piece of aluminum alloy exposed to the weather and it is corroding, so why does this
happen if it's not touching another piece of metal to form a galvanic cell?.. " The answer lies in the
physical metallurgy of the aluminum alloy. As an alloy it will have other elements in its chemistry other than just aluminum. These other elements are there to give the aluminum strength and other properties. Some typical additions are copper, zinc, manganese, silicon etc. These elements can be fully dissolved in the aluminum, or as precipitant particles of different chemistry. You will have within the piece of metal a zone of two different metals or metallic compounds, hence the potential for an in-situ anode and cathode. All it takes is water or some other electrolyte to start galvanic corrosion within the metal.
TYPES OF PROTECTION
#1) The best protection is knowledge! The more we know about corrosion, the better we can deal with it.
#2) Any method that can prevent aggressive chemicals from coming into contact with active metals will inhibit corrosion. Aggressive chemicals can be as simple as water and can cause not only general or uniform attack, but also electro-chemical attack. Application of some type of barrier ; i.e. prime, paint, oils or other coatings or plating is needed.
#3) Disable any galvanic couple or cell by removing or isolating any one of the three critical elements: remove the anode, remove the cathode or remove the electrolyte and you disable the galvanic couple. It only takes the removal of just one element!
#4) Provide sacrificial protection if possible. Electro-chemically this is called cathodic protection. Utilize the galvanic series to your advantage. As an example, let's use cadmium plating on top of steel. First, the cadmium tends to act as a barrier to prevent water, moisture etc. from contacting the steel. As you can see, cadmium is higher or more active on the galvanic series. Therefore, if it is electrically paired up with steel(cad plated), and (/'corrosion happens, it will happen preferentially on the cadmium plate. The cadmium plate will corrode (oxidize) before the base steel corrodes. The same is true for zinc plating (galvanizing). Amazing examples of this taken to great lengths are magnesium anodes in water heaters, magnesium anodes bolted to ocean going ship hulls, and magnesium anodes bolted to buried underground steel pipelines. The magnesium is so galvanically active that it will slowly oxidize or dissolve away when used to provide sacrificial protection to the steel liners of our water heaters, ship hulls and buried pipelines.
We only have space and time to talk only about some specifics of corrosion protection for iron-steel materials and aluminum base materials. These are the two most used metals in our society today.
COATINGS and PLATING FOR CORROSION RESISTANCE
Black Oxide: This coating is by nature an oxide of iron, therefore corrosion resistance is naturally improved by increasing either the artificial oxide coating thickness or oxidation state. The oxide states of iron are FeO, Fe2O3 , & Fe3O4 . Special proprietary bath chemistries are available to improve the thickness & hardness of the oxide coating, and may also have proprietary compounds added for enhanced corrosion resistance. A supplemental oiling is highly recommended as this coating is not intended for good stand-alone corrosion resistance.
Cadmium Plate: With little argument, cadmium provides the best corrosion protection for aircraft-aerospace components. It is not only an excellent barrier coating, but more importantly a sacrificial coating. Since it is anodic to the base steel, it will corrode preferentially before the base steel does. The by-product of cadmium oxidation, cadmium oxide, occupies far less volume than ZnO, does not tend to plume and is more "waxy" in composition, rather than the powdery residue from zinc corrosion. The required performance of cadmium as a corrosion protection system is based not only on the ability of the cadmium to protect the base steel from red rusting, but also to be essentially free from cadmium oxide products during the duration of any accelerated test. Supplemental chromate coatings will improve the corrosion performance of the cadmium itself. As with any topical plating, proper thickness is essential to meet the corrosion resistance requirements.
Zinc Plate: The corrosion protection properties of zinc parallel those noted above for cadmium; however, the white zinc oxide products tend to be greater in volume. The cosmetics of this may not be attractive to the user. Supplemental chromate conversion coatings definitely will help minimize zinc oxide products. Clear chromate and newer non-chromate conversion coatings have far less desirable corrosion characteristics.
Nickel Plating: Electrolytic nickel plate and autocatalytic(Electroless Nickel) can provide some corrosion protection for steel substrates, but only as a barrier coating. This is because nickel is more noble than steel, i.e. the steel will corrode before the nickel will. Therefore upmost attention must be paid to laying down a nickel plate free from cracks, pits, pores or cracks. Higher stress (bright) nickel can have more stress cracking, hence less corrosion protection. Medium phosphorous electroless nickel coatings typically meet general purpose requirements for corrosion resistance, while high phosphorous coatings have enhanced salt spray corrosion resistance.
Chrome Plate: More attention is being given these days to using chrome plate for corrosion protection. However, chrome is cathodic to any base steel, therefore it behaves only as a barrier coating in like fashion to nickel type plating. Conventional chrome plate normally has enough residual tensile stresses from deposition that "mud cracking" is present. This inherently provides poor barrier protection as the cracked chrome plate easily wicks any corrosion solution or electrolyte down to the base steel. Many chrome plated hydraulic components exhibit good corrosion protection as this "mud cracking" captures oil or grease within the cracks and pores. Newer technology chrome plate called Thin Dense Chrome or TD Chrome can provide enhanced barrier corrosion protection as this type of chrome is deposited with less stress and no cracking. We see this type of chrome plating on aerospace ball and roller bearing applications. The improved corrosion performance, coupled with higher wear resistance and better metallurgical lubricity, can result in some instances in a 35% improvement in corrosion fatigue life of the bearing. Commercial or decorative chrome plate with a copper and nickel strike under the chrome greatly improves corrosion resistance as can be expected from the soft ductile substrate strikes.
Phosphate Coatings: Phosphate coatings are usually zinc, manganese or iron phosphates deposited onto steel or iron. These coatings provide slightly enhanced corrosion resistance, although they should not be expected to provide stand alone corrosion protection without supplemental oil, wax, prime or paint. These coatings are highly porous in nature and are not good as a barrier coating.
Silver Plating: Silver is many times used today as an engineering coating for enhanced resistance to galling and fretting. Since silver is more noble than all other engineering alloys, the only protection silver can provide is barrier protection. Silver plating can be susceptible to pin holes, and therefore, breaching of this barrier can have dire consequences.
Copper Plating: Copper is many times used as a decorative or cosmetic plating or for engineering purposes. Copper also is more noble than the majority of useful alloys such as steel, and therefore any corrosion protection is gained solely as a barrier coating.
Passivation: A passive metal by definition is one that is active in the galvanic series, but which corrodes at a very slow rate. Passivity is the property underlying the useful natural corrosion resistance of many structural metals such as stainless steels, aluminum and nickel. Typical methods of passivation involve the chemical cleaning of the surface of a part to remove any tramp or residual material, then exposing the metal to a passivating chemical, e.g. chromate or nitrite solutions for iron base alloys, or by anodic polarization at high current densities, e.g. in sulfuric acid. The passive film is very, very thin in nature. There are two commonly accepted explanations for passive films. In the first theory, the passive film is a diffusion barrier layer of reaction products, e.g. metal oxide or other compound which separates the metal from its environment, and which slows down the rate of reaction. This is normally referred to as the oxide film theory. The second theory holds that passive metals are covered by a chemisorbed film such as oxygen. Such as layer displaces the normally adsorbed water molecules and slows down the rate of anodic dissolution. This theory is called the adsorption theory of passivation. In reality, both can be empirically and analytically proved.
COATINGS FOR ALUMINUM ALLOYS
Chemical Conversion Coatings - Chem Film: These type of coatings are conversion type coatings in that they are the result of chemical reaction-interaction of the solutions chemical compounds with the surface of the aluminum. They are extremely thin in nature, measured in microns. It is not an oxide type layer like anodize. Chem film can provide not only good stand alone corrosion protection of the aluminum alloy surface, but also enhanced surface adhesion of any follow up prime or paint top coating. Since the coating is not only thin, but relatively soft in nature -it is easily scratched and damaged. Also the applied chem film needs to be "aged" some period of time (24 hours or more) to promote complete reaction of the compounds and the aluminum substrate. Chem film is the method of choice to provide surface protection of components requiring good electrical transmission (low resistance) properties, i.e. black boxes, avionics etc.
Anodize: Anodizing is an electrolytic process whereby the surface of the aluminum part is chemically cleaned, stripped and then intentionally oxidized to a precise thickness and morphology. Anodizing is a barrier-type corrosion protection system. The chemical formula for the anodize layer is Al2O3 and as anodized, is very porous, almost "swiss cheese" in microscopic appearance. Common types are chromic and sulfuric acid, denoting the chemistry of the electrolytic processing bath. Anodizing is a process that penetrates down into the aluminum, and also as part of the process, grows upward away from the surface. A rough rule of thumb is a 50/50 split between growth and penetration. Some nominal thickness ranges obtainable from anodizing would be 0.0005" to 0.004". An essential part of anodizing is the sealing process which causes the porous aluminum oxide to swell and "seal" itself, i.e. the pores are closed or sealed off. Common types of sealing are hot water, dichromate compounds and acetate compounds. Different aluminum alloys such as 2024, 6061, 7075 etc. respond differently to anodizing and can display very different corrosion resistance after anodizing. A variant of conventional anodizing is called hard anodizing, which can provide improved wear resistance in like fashion to hard chrome plating. It is done in sulfuric acid baths. The anodize is aluminum oxide and can display hardness properties of around 1500 Vickers. Hard chrome plating can produce a Vickers hardness in the 850-1100 range, so you can see hard anodizing is commonly used in wear applications such as hydraulic cylinders. Hard anodize can also be impregnated later with low friction compounds such as Teflon or other PTFE chemicals.
Omega Research brings a wealth of aircraft industry experience to the field of corrosion control and testing. Clients are encouraged to contact Omega with their specific problems or questions for a detailed analysis from our engineers. Omega Research Inc. is internationally known for our engineering expertise in the field of corrosion testing. We are ISO 17025 registered, and A2LA and Nadcap accredited. From this brief discussion of corrosion and corrosion control, you can see why there is a separate field of science called corrosion engineering. We hope you have been stimulated to ponder these issues faced by the metal finishing industry - to make metals and their alloys more durable and long lasting in our society.
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