Round bar specimen after tensile stress testing

Round bar specimen after tensile stress testing

Steel has been a part of humanity’s legacy for millennia and more. As early as 300 BC, we have examples of steel being used for more than weapons. The product has been refined, improved, and integrated into so many uses that people are likely not aware of how much steel is a part of their lives. Beyond the industrial applications of wire, farming tools, construction, and vehicle manufacture, people around the world continue to find innovative use for steel. This is a testament to its versatility and the boundless power of the human imagination: A substance that, after centuries of existence, continues to find its way into more and more aspects of our culture and productivity.

ultimate tensile strength

Two vises apply tension to a specimen by pulling at it, stretching the specimen until it fractures. The maximum stress it withstands before fracturing is its ultimate tensile strength.

Within the modern kitchen, more appliances and utensils are being constructed of steel. Ceramic
products have to measure up against their steel counterparts to prove their value. Restaurants around the world insist upon steel equipment for the creation of fine dining experiences. Famous chefs put their names and their reputations on cookware made from it. This application isn’t limited to the culinary arts: Sculpture, painting, crafts, and music have incorporated various types of the material to amazing effect. Steel stages are designed for music, theater, movies and television in every imaginable way. The steel guitar, introduced over a century ago, remains a fixture in the music industry. Some guitarists have discovered the unique tones possible by having modern electric guitar bodies constructed from steel instead of wood. Memorable stages designs have traveled the world, due to the ability to break down, transport and reconstruct them.

When the subject of stainless steel cars is brought up, many think only of the DeLorean, made famous in the Back to the Future trilogy of movies. Far more examples exist now, as enthusiasts have remade cars from the 1920’s, along with new factory models of trucks, green cars, and luxury sedans, using the material as the eye-catching aspect and exterior body. Cell phones have traditionally been constructed with plastic cases, but that line of merchandise has its own offerings in the shiny, rugged option. Jewelry, watches, automobile parts, cups, and bikes have all gotten their own showcase of steel.

The options and innovations of yesterday and today are only the beginning. As technology and manufacturing continues to evolve, so will the possibilities of creating new art and machinery. It may not be too far ahead of today when glass containers will be featured more prominently with steel, along with plates, household features such as stairs and counters, and so many other items. Companies like Innovative Steel & Alloys are creating alloys with unique mechanical properties that reduce cost but are a strong as the materials that came before, perhaps even better than ever. With an eye towards the future and the health of our planet, steel will remain the backbone of our civilization.

Iron Ore to Iron

image001We must think about where metals come from first, and they are mined from the earth in raw form called an ore.  They could be a copper ore or an iron ore or some other ore.  All ores come from the earth.  Not all ores are equal either.  For instance Lake Copper is almost in pure form when mined from the earth in an open pit mine in the Great Lakes Region of the United States of America.  Most ores have impurities and these are almost impossible to remove 100% of the impurity; in some cases the impurity enhances the metals characteristics.image003

After mining the ores are delivered to a steel mill except for some non-ferrous metals that use electro-chemical means to make the non-ferrous metal.  Aluminum is an example.

When the ore is delivered to the steel mill they are prepared by crushing and grinding to a smaller size to increase the surface area to the volume.  Unfortunately, the ground iron ore must be sintered.  Sintering is done be partial pre-cooking the iron ore in the presence of coke oven gases.   The ore is now called sintered image005and this is used to charge the blast furnace.

Now the sintered iron ore is prepared to be heated to a very-very high temperature where the ore melts in the presence of gases.  Knowing what temperatures the components in the ore melt at is critical.  Lets consider iron ore.

Iron ore’s principal component in the mined rock is Fe2O3. This iron oxide also known as iron-oxide-III is plentiful enough to be commercially feasible to make steel.  In the rock we will have small amounts of silica, sulfur and other undesirables.  Some can be removed in the steel making process and others we tolerate.

The second step, after mining and crushing the ore into manageable sizes is to make iron in the steel making process from the iron ore laden Fe2O3.  We normally use a blast furnace to strip away the O3 (oxygen) in the Fe2O3 laden ore by adding a great amount of heat in the presences of coke, which is a pure form of carbon (C), and limestone (CaCO3) principally calcium carbonate.  Limestone principally removes the silica from the iron ore in the steel making process.  By using coke and limestone, which is heated to high temperatures with great amounts of compressed air in the presences of the iron ore we cause two-types of chemical reactions.  We reduce the oxygen out of the iron ore and transform this oxygen by oxidation to (carbon dioxide) CO2.  Stripping away O3 (oxygen) is called reducing.

Step 1

(Coke Oven Gas) C + Compressed Air O2  →  2CO (Flammable Carbon Monoxide)

Step 2

Fe2O3 (Iron Ore) +  3CO + Heat  →  2Fe (Iron) + 3CO2 (Carbon Dioxide)

image007The iron is produced at temperatures of over 3,500oF and has carbon content when the iron leaves the blast furnace of greater than 4% to about 5%. This is similar to cast iron but not refined enough to be cast iron.  Now the impurities must be removed in the form of slag, which floats on top of the molten (liquid) iron.  Slag’s principal component is calcium silicate or CaSiO3, which is where the impurity silica in the iron ore is made into to for removal from the steel making process.  This is where the limestone does the work in the blast furnace.  Remember,  all these reactions are occurring at the same time in the furnace.

Step 4

CaCO3 (Limestone) + Heat  →  CaO (Calcium Oxide) + CO2 (Carbon Dioxide)

Step 5

CaO (Calcium Oxide) + SiO2 (From Iron Ore)   →  CaSiO3 (Calcium Silicate in the Slag)

Another impurity, which affects toughness that needs lowering from the iron is high amounts of sulfur/sulphur (S) and this is done by using magnesium (Mg).  The Magnesium is blended in with the blast furnace charge.

S + Mg  →  MgS (Inclusion)

Now the slag can be drained from the top of the liquid iron, then the iron is drained from the bottom in to a special vehicle that can transport and hold the super hot iron liquid.

Making Steel from Iron

image009The molten iron is now placed into another vessel, from the from the special transportation vehicle, that is called a BOS or Basic Oxygen Steelmaking vessel (called a converter) that is a special furnace to remove more impurities and melt-in some recycled steels by using calcium oxide (CaO) and compressed oxygen that is injected on and into the liquid iron.  This is an oxidation chemical reaction.

The BOS is where the decision is made to make low carbon steel, medium carbon steel or high carbon steel.  The BOS process has the capacity to reduce carbon to almost 0.0% (C).

Carbon plays an important role in metals except non-ferrous.  We have provided a spectrumimage011 of carbon and how this fits with carbon steel and cast iron.  As you can readily find in this spectrum the higher the carbon the more the metal begins to get one step closer to cast iron.  And the lower the carbon the closer the metal becomes pure iron.

Let us get back to how carbon is manipulated.  Carbon to removed by the oxidation process where Oxygen O2 is blown, which is called blowing, in the BOS and captures a large portion of the 4% to 5% carbon.  In fact that longer the O2 is blown in the lower the carbon content.  This process can be done in less than an hour.

Sulfur and Phosphorous are two important impurities are further removed that are removed in the BOS by using left over calcium oxide (CaO) from the blast furnace.  The reaction of CaO and compressed air with the liquid iron and the remaining impurities creates heat to further the reaction.

When the liquid does not have Si in great amounts then Magnesium can be used to removed the sulfur/sulphur (S) as follows –

Mg + S → MgS

2Mg + O2 → 2MgO

When Si (Silicon) is in present in sufficient amounts then CaO is used to reduce the impurity of sulfur as follows –

CaO + FeS → CaS + FeO

These impurities form into slag and float on the top of the liquid steel.  The liquid steel is taped from the bottom and are drained off forming a primary steel or raw steel into a special vehicle looking like a rail car with a big bowl or ladle (equipped with electrodes for heat) for making steel to specification for final end usage and this also where alloying takes place.

In the rail car with the ladle, the primary steel is still very-very hot.  Here is where Manganese (Mn), Silicon (Si), including more carbon to increase or refine the level of carbon content, Cr (Chromium) for stainless steel, and to add other alloys like Copper (Cu).  This steel can be further refined by adding Aluminum is to remove O2 that was trapped or left behind from the BOS making process and is done as follows –

Al2O3 → 2Al + 3O

image013Another way of removing the left over O2 is to shake the ladle while under a vacuum.  This is called degassing.  Both of these are specified by rimmed steel, semi-killed steel or killed steel.  Where killed steel refers to the removal of all oxygen that could react with carbon once the steel is in plate form or solid form.  Semi-killed is where the oxygen is not completely removed in that when cooling to a solid there is CO degassing of the ingot that benefits the ingot by minimizing ingot shrinkage.  Rimming is similar to semi-killed steel except that the CO degassing occurs during the enter solidification process of the ingot such that there are not gas bubbles or porosity.

Another way of making secondary steel is by using 100% recycled steel.  This is done in an Electric Arc Furnace.

Next the steel is processed in to a ingot or casted.  The difference between the two is ingot is a batch process of a very large piece of steel.

The caster is a continuous casting process that makes very long slabs, blooms, and billets.  These are made on a caster machine that forms the molten steel as it flows down the caster machine slowly cooling and cut in long lengths.

The ingot process is the oldest form of secondary steel making.  There will be greater varieties in the metallurgical properties in the ingot.

Microscopic Structure of Carbon Steel

Metals are made up of very small building blocks that are called grains or crystals.  These image015building blocks are stacked tightly.  The size of these grains (like wood grains for the carpenter or cells for the biologist) is very small and normally are around 0.1-mm to 0.01-mm.  In a piece of carbon steel can have population of 1000-crystals per inch.

Each grain in steel represents a location, in the steel slab/bloom, billet, etcetera, that froze at the same time like ice crystals in the metal solid.  There are billions and billions of then in a small piece of steel.  When the metal freezes to a solid they all form and grow at the same time and stop growing when they meet each other at the grain boundary.

Cast Iron

image017Cast iron is a brittle like material that cannot absorb impact from a hammer; therefore, cast iron is little or no toughness when compared to low carbon steel.  Cast iron has a fraction of the tensile strength of low carbon steels.  When this material fails it will not deform in a noticeable way and appears to snap apart or break in a manner consistent with a snap.  Therefore, there is no early warning of a failure.  Cast iron is economical and when used right it is an excellent choice for many applications.

Cast iron is normally made from induction furnaces or cupola furnace.  A cupola is like a image019small blast furnace for refining the iron to desired properties.  Pig iron and scrape steel are used much that way primary steel is made. Pig iron is blast furnace iron.  The word pig means ingot form (never making it to the BOS) or (possible original source) small pig size ingots.  The scrape steel determines the desired property.

id-10074357Carbon is normally in a graphite flake form that is pure carbon and acts as a natural defect in the material.  The iron is so saturated with carbon that graphite forms (free carbon) and causes the cast iron to be weaker and behave as if it is weak.  Much smaller amounts of carbon is combined with iron Fe in the form of FeC or a hard brittle iron carbide called Cementite named after an individual who took up the study of metallurgy in 19th century.

Cast iron is made up of principally Fe, Si, and C.   The carbon content for gray cast iron is around 3.5%-C and Silicon is lowered just below 2.0% Si from the higher pig iron levels.  As you can deduce most of the cast iron is in the form of Fe (Iron) and at around 94% Fe.

Machining cast iron is possible without a cutting lubricant because the graphite is a natural lubricant.  Cast iron does not corrode readily on machined surfaces unless the graphite film is removed or compromised.

Gray Cast Iron – Gray Iron has a lower tensile strength and lower ductility. Very brittle.

Nodular Cast Irons

Malleable Cast Iron – Malleable Cast Iron had reasonable good ductility and is desirable for low temperature service and small components like fittings.

Ductile Cast Iron – Ductile Iron has high strength and reasonably good ductility. This material has twice the tensile strength Gray Cast Iron.

There are other specialty cast irons like austenite gray cast iron and inoculated cast irons.  Cast iron can be alloyed much like carbon steels including Chromium (Cr),  Nickel (Ni), etcetera.

Carbon Steel

Carbon steel the most commercially used of all the steels.  As the name applies this steel is about carbon steels and based on their Carbon (% C) content. Carbon steels belong to a broad classification also known as Plain Carbon Steels and categorized as follows –

  • image021Low Carbon Steels  0.01% C to 0.30% C
  • Medium Carbon Steels  0.30% C to about 0.60% C
  • High Carbon Steels  0.60% C to 1.00% C

We further classify them as to oxygen content (deoxidation) and how this is accomplished, image023for instance rimmed steel, semi-killed steel, and killed steel.  This is a very important aspect that must be addressed in ordering plain carbon steels.

Carbon is still the single most important factor in plain carbon steels because it effects the strength and other mechanical properties including hardness. Plain carbon steels are also alloyed to a certain degree, but they are not called alloy steels because they are under 2% when the alloying element are totaled.  Mn (manganese is an important ingredient in carbon steels.

Low Carbon Steel

Low-carbon steels  0.01% C to 0.30% C and are the most widely used in boilers, piping, image025tanks, pressure vessels, automobile body panels and wire.  They are hot rolled and cold rolled.  We can split them in to a lower carbon range for the light gauge sheeting where carbon content is around 0.10% C with a corresponding Manganese at 0.40% Mn.  At the upper range is plate or boiler plate with above 0.20% C to 0.30% C with a corresponding Manganese of 0.80% Mn to 1.5% Mn.

 

Medium Carbon Steel

image027Medium-carbon steels contain 0.30% C to 0.60% C and Mn (Manganese) from 0.60 to 1.65%. Medium carbon steels are used for quenching and tempered including product forms of shafting, gears, crankshafts, forgings, rails, and railway wheels assemblies.

High Carbon Steel

High-carbon steels contain from 0.60% C to 1.00% C with Manganese contents ranging from 0.30% Mn to 0.90% Mn.  Normally, these steels are used in springs and similar product shapes that require very high strengths.

Carbon steels with carbon content in the range of 1.25% C to 2.0% C are specialty carbon steels.

Alloy Steelsimage029

Steels that are alloyed are either –

  • High Alloy Steels
  • Low Alloy Steels

High alloy steels have four-application classes-

  • Stainless Steels (Corrosion Resistance) for stress corrosion cracking (SCC).
  • High Temperature Steels (+)1000F  These are steels that must have good resistance to high-temperature creep and ruptures.  Also important to be resistive to oxidation and corrosion.  Stainless steels also fit this class except ferritic.
  • Low Temperature (-)300F  This class of application is suited best for stainless steels of the austenitic type.  Low carbon high alloy steel do not perform well at -40F unless steps are taken to alter the steel characteristics, and regardless of purity and chemical character (-) 300F is where performance is unacceptable.  Austenitic type is very suited for this -300F temperature with alloying.
  • Wear Resistance Steels – These are done by diffusing gases like carburizing, sulfiding, siliconizing, nitriding, and boriding to mention a most methods.  Other methods are through alloying and coating the high alloy steels.
  • Electro-magnetic Steels – These are transformer and generator plain carbon steels including iron cores.  Permanent magnetic also fit this class.  Silicon (Si) is an important alloy.
  • Tooling Steel – These are cutting tools, forming dies, and shearing tools; they can be hardened and will have a high carbon content.  Tools like chisels can have carbon (C) content up to 1.10% and razor blades has high as 1.40% C.  Tools will have different chemical composition for low speed tooling (including pneumatic powered) and high speed tools where abrasion is important.

Low alloy steels, typically plain carbon steels that have only two-alloys elements but can be as high as five-alloying elements.  the majority of the alloying is less tan 2% and in most cases under 1%.  Nickel (Ni) can be as high as 5%, but this is an exception and may be found in transmission gearing. In the chemical analysis you will find many more elements but these are incidental to the making of the steel as opposed to alloying to for specific property in the steel. of normally less than 2%

Stainless Steels

image031Silverware is what we think of when discussing this metal because it does not rust, has luster, and does not stain thanks to the Cr (chromium content).  We generally do not think of stainless steel in other ways.  Stainless steel can be categorized as either –

  • Ferritic Stainless Steel
  • Austenitic Stainless Steel
  • Martensitic Stainless Steel

These three categories can be grouped in to two-classes-

  • Straight Chromium Alloys
  • Chromium Nickel Alloys

The straight chromium alloys are numbered as 400-series and 500-series.  These are the ferritic and martensitic stainless steels.  500-series stainless steels are martensitic.  The 400-series stainless steels are both ferritic and martensitic (overlaps into the 400-series).

400-series stainless steels have a Cr content range of 10.5% to 30%.

500-series stainless steels have a low Cr content range of 4% to 6%.

Ferritic stainless steels have a notable property of good to excellent corrosion resistance when combined with heat such as automobile exhaust.  This is weldable and has good mechanical properties.

Martensitic stainless steels are notable in steam turbines and mild corrosion resistance with increased toughness and other mechanical properties.

The most recognizable form of stainless steel is the 300-series or the Cr-Ni (Chromium Nickel Alloy).   This the austenitic stainless steels and have Ni contents of >6% to over 24% and a Cr contents of 16% to 30%.  Austenitic have excellent corrosion resistance but should not be exposed to temperatures above 800F.

A lesser known austenitic stainless is the 200-series with Cr content in the range of 16% to 19%. the second chief constituent element is Mn that substitutes for Ni.

Non-ferrous metals are any metal that lacks iron.  These including the nonferrous metals-

  • Copperimage033
  • Aluminum
  • Magnesium
  • Monel
  • Nickel
  • Zinc
  • Inconel
  • Lead
  • Tin
  • Tantalum
  • Titanium

image035These are the most common nonferrous metals.   There are other more precious nonferrous metals like gold, silver, and platinum.

Nonferrous metals are chosen chiefly, other than ordination, for corrosion resistance, electrical properties, thermal properties, and strength & elasticity (young modulus).  Of all these properties corrosion resistance is the most relative issue.  The most important factor is cost when compared to ferrous materials.

  • Copper – Electrical, Thermal, and Corrosion Resistance
  • Aluminum – Strength, Electrical, Thermal, & Corrosion Resistance
  • Magnesium – Strength and Corrosion (poor) Properties
  • Monel – Part of the Ni family with Cu and a small amount of iron good for high temperatures
  • Nickel – Corrosion Resistance
  • Zinc – Corrosion Resistance and Corrosion Properties
  • Inconel – Like Monel except Cr (Chromium) is substituted for Cu; has high impact resistance and resistive to oxidation
  • Lead – High corrosion resistance and excellent shielding against high frequency rays (x-ray and gamma)
  • Tin – Has the most wide usage of all the non-ferrous metals
  • Tantalum – Surgical, Tooling, Hardness, Wear Resistances and Strength
  • Titanium -Corrosion Resistance, Thermal Properties and Strength
  • Chromium – Corrosion Resistance, Used in Super Alloying, and high Temperature Applicationsimage037

These metals when compared to plain carbon steel have comparable features such as-

Volume verse Strength: plain carbon steel generally stronger

Weight verse Strength: plain carbon steel generally stronger except Al and Mg

Non ferrous metals are used when the cost verses benefit is better than plain.

Tool Steels

image039These are steels that must be suitable for cutting materials including other steels and forming other steels with dies and making hammers of all kinds.  Tool steels are known to have 0.6% Carbon and around 0.25% Silicon with 0.25% Manganese.

Lets review what we learned from Alloy Steels since tool steels fits into this category-

These are cutting tools, forming dies, and shearing tools; they can be hardened and will have a high carbon content.  Tools like chisels can have carbon (C) content up to 1.10% and razor blades has high as 1.40% C.  Tools will have different chemical composition for low speed tooling (including pneumatic powered) and high speed tools where abrasion is important.

 

There are five categories of tool steels and these are-

  1. image041Carbon Tool Steels – Case hardening type and are effective when the tool will not be used over a temperature of 500F.
  2. Alloy Tool Steels – These tools are chisels, punches, shears, rivet tools and are considered to be moderately alloyed to achieve this purpose.
  3. High Carbon, Chromium and Air Hardened Die Steels – great wear resistance tools.
  4. Hot Work Steels – This category is forging, hot drawing, extrusion and die casting.  These remain hard at relatively high temperatures.
  5. High Speed Steels – These are tools that remain hardened even when red-hot.  These have hard carbides in the micro-structure of the tool.

Heat Treating (Heat Treatment)

image043This is a combination of heating the cooling of steels to obtain a specific outcome or metallurgical property.  Forging and the like is not considered heat treating.

There are three-common forms of heat treating processes and they are –

  • Normalizing – This is used on ferrous metals to condition the part for grain refinement or grain uniformity.  To achieve this requires the part or metal to be heated such that the building blocks (crystal lattices) reorientate themselves or make like new.  This choice of language is meant to be figuratively, but not literally.
  • image045Annealing – This heat treating process is the most versatile or has the most wide spread applications.  For instance this can be used to soften the metal to altering the electrical properties and to stress relief annealing.  Unlike normalizing there is no transformation temperature specified although the procedure may call for this temperature.  Annealing is a term used to describe a heating, holding and cooling process to achieve desired metallurgical results.
  • Stress Relieving – Always done below the transformation temperature of the metal to minimize the welds residual stress.  The temperature is held for a certain amount of time (could be 1-hour) until the residual stresses are minimized, then cooled very slowly to prevent new stresses from setting up in the metal.  This can also be done on non-ferrous metals.

Heat treating can be accomplished by in a factory or shop or in the field at a remote location.  The types of furnaces can be large gas fired furnaces that can hold many tons or portable electric wraps around piping but not limited to electric. Torch or a flame can be used in the field to achieve heat treating.

Heat treating can be confusing to follow because the same equipment maybe interchangeable and the technique can be the same, but the objectives are different as are the procedures.

Laboratory Metallurgy & Metallurgist

rust_by_struckdumbMost engineers were not that involved in the science of metals around 1900, but the time 1950 arrived this field of study was very important to the engineer, for the engineer had to select material for the design, repair, or maintenance of the machine.

For the engineer understand the metal being examined, the engineer has to know what testing and evaluations are valid. Lets review this now-

Testing Technique – Obtaining significant results from field and laboratory equipment.  Knowing their limitations and applicability.

 

sheet-metal-screws-washer-bMeasurements – How much accuracy is required to achieve the result.  An example is measuring 0.15% carbon content does not require accuracy out to 0.01500% since this amount has no significant effect on the low carbon steel properties.  Another point is how many reading are necessary to obtain a useful average that provides significant results.

Evaluation of the Results – This is requires training, education and experience to evaluate the results of all the test results with macrographs and micrographs.  The conclusion is presented in the final report in a condensed manner that is readily understood for application in the function and life of the machine or pressure part.

Some testing to consider-

  • Visual – Macrographs and Micrographs
  • Chemical Content of the metal
  • Hardness measurement of the metal
  • Tensile Strength with Yield Point
  • Impact Testing
  • Nondestructive Testing (NDT)

Good testing technique does not require state of the art equipment to give excellent relative results from the field or laboratory.  To evaluate requires training, education and experience and may only require a visual examination by a few macrographs or require micrographs, chemistry, impact testing, and nondestructive testing (NDT).

Failure Analysis

A Study of Why the Component does not full fill its Purpose 

iStock_000005258227XSmallThere are two perspectives on this subject.  The first is a large-scale study and the second is the micro study of the component or equipment.  They are as follows-

Forensic engineering study (large-scale failure analysis) relates to the big picture in evaluating the reasons for the failure and should include contributing factors and more than just a root cause.  The forensic work may use Finite Element Analysis (FEA), MORT (management oversight risk tree) or other investigation tools like Change Analysis, reconstruction and others to understand the forensic engineering aspects.  In addition, forensic engineering may employ metallurgical work when necessary.

Failure analysis at the micro level is a narrow study of how the component failed.  In our discussion we are interested in using metallurgy to study and understand the failure as this relates to the pieces of metal that failed.  The conclusions from a metallurgical study may possibly provide an accounting, providing enough information is available, of what the metal experienced through the failure from beginning to end.

Together, metallurgical failure analysis and forensic engineering study can provide a more complete picture of the failure.

A metallurgical failure analysis may be all that is needed to solve the failure of why the component failed.  The metallurgical failure analysis will include some or all of the following to full fill this aspect of failure analysis-

  • whatwedosteel1Microscopic examination of the grain structure (microstructure)
  • Hardness Test
  • Tensile Testing with yield and elongation
  • Chemical Analysis
  • Impact or Toughness Evaluation
  • Fatigue Studies
  • Fractography (the study of the fractured surface)
  • Corrosion Evaluation

As a finally note…a metallurgical failure analysis may include field work but not all tests can be completed in the field.  We hope that this overview helps you understand the perspective of failure analysis better.

Basics of Corrosion on Carbon Steel

Corrosion is a Chemical Reaction

The typical causes are influenced by the ‘half and half-not’  Simply, stating that when there are differences there is a potential for corrosion.  In chemistry we refer to this as electro-chemical reaction but in but in the world of metals corrosion becomes more complicated but operates on the same fundamentals as electro-chemical reactions.  Here are some examples of differences that will cause basic corrosion-

Electrical differences between the two-metals.  This is sometimes referred to as the galvanic series.  Magnesium and zinc are at one end or the most negative side and graphite and platinum are at the other end or the most noble (positive) side.

If the metal is more impervious of the attaching substance that could be a chemical solution that is either acid or alkaline.  Carbon steel likes a slightly alkaline solution.  When the pH of the metal and solution are not different or are the same then there will be no corrosion.

metal_blastfurnacesOxygen has another way of causing a chemical reaction.  In fact, as you may remember early in the tutorial iron or carbon steel all originate from iron ore that is an oxide of iron.  Furthermore, if iron is exposed outside in the weather for 100-year or more it would turn to rust and leach into the soil reverting back to a soft iron ore.

More specifically –

In metals we need to classify a metal as either a cathode or an anode.  A cathode is noble or protected and the anode is the opposite.  The anode is the metal that is corroded or gnawed away.  They must be in contact with each other either directly.  There must be a electrolyte or chemical solution (acid or alkaline).  Sometimes the electrolyte is moisture or soil, and sometimes that soil may also be mineral rich.

In a desert, the sand is mineral poor and moisture deficient; therefore, the electrolyte is removed and corrosion is all most impossible to occur for carbon steel.  Just the opposite occurs in the tropical forest with rich moist mineral bearing soils in a humid environment where the electrolyte is present everywhere and coming in contact with the soil will cause corrosion of the carbon steel.

The electrolyte and chemical solution will have a pH.  With acid the solution is positively charged.  An alkaline the solution is negatively charged.  Remember a pH-7 is neutral.

q2_steel-plate-supplier-steel-supllier-astm-a572-a572-steel-plate-fabricationCarbon steel is a slightly more alkaline than neutral and will not corrode in water that is slightly alkaline unless oxygen is present in the water and or when dissimilar metals are connected with the carbon steel like brass as an example.

Oxidation of Carbon Steel Pipe in Water with Bronze Valve

Pipe

Fe – 2e  → Fe++

Two electrons left the carbon steel, Fe – 2e,  by being electrical connected or touching each other to a more attractive metal, bronze, wants the electrons based on galvanic reaction.  (note: this is why we like to use black iron pipe with black iron valves and fittings) What is left of the carbon steel is the iron with an unusual positive charge that has to leave the carbon steel and enters the electrolyte (water).

Water

Fe++  + OH –  → Fe(OH)2

The positively charged iron mixes with water, which will always strip the H20 apart to obtain the hydroxyl (OH -) ions (Note: The hydrogen ion will head over to the bronze).  The mixing occurs very-very close to the surface of the carbon steel and forms a ferrous hydroxide Fe(OH)2.  In fact, this seems to occur at the surface and there is a lot of ferrous hydroxide Fe(OH)2 forming, but this  ferrous hydroxide is unstable such that it will recombine in the water again and again until it becomes more electro-chemically stable.

Rust

Fe(OH)2 + H2O + O2  → 4Fe(OH)3

Ferric hydroxide 4Fe(OH)3 is rust.  All rust is made this way in water and eventually the reaction becomes embedded in the carbon steel.

Once corrosion starts it can never to stopped 100%.

In closing, we can always expect corrosion, based on the galvanic reactions, between carbon steel and

  • Set of valuable metals ingots isolated on white. 3D photo rendering.Magnesium (Magnesium will corrode)
  • Zinc (Zinc will corrode)
  • Aluminum  (Aluminum will corrode)
  • Bronze  (carbon steel will corrode)
  • Brass  (carbon steel will corrode)
  • Copper  (carbon steel will corrode)
  • Stainless Steels 300-series  (carbon steel will corrode)
  • Titanium  (carbon steel will corrode)

There are many forms of corrosion.  Lets highlight a few, but remember they all are based on a chemical reaction of some sort –

  • Stress corrosion is influenced by stresses in the metal like constant straining from pressure on a pipe or residual welding stress
  • Fatigue corrosion is subjected to fluctuating stresses where the stress is removed or reversed then applied again
  • High-temperature corrosion from sulfidation, de and carburization, and an oxide layer like mill scale
  • Fretting corrosion is influenced by rubbing or parts that are fitted together that vibrate differently
  • Intergranular corrosion is where the metal grain boundaries is chemical attacked.
  • Impingement or Erosive corrosion that is subjected to either flow related condition in the presence of corrodent product or cavitation
  • Leaching is a corrosion process where certain alloys are selectively removed from the metal

We hope that this overview helps you understand corrosion better.