WHAT IS CORROSION ? Definition - Material Properties

 

Corrosion

It is the destruction of metals by chemical or electro chemical action.

 TYPES are

1. Sweet corrosion
2. Sour corrosion
3. Oxygen Corrosion
4. Electro chemical corrosion

 

Sweet CORROSION: It occurs in oil or gas wells where no iron sulfide and no odor occur. Corrosion in gas condensate wells is caused by CO2 and water forming carbonic acid. Carbonic acid reduces the PH value of water and which makes very corrosive in steel. Corrosion in gas condensate is not when moisture is not present. When moisture is present, the co2 dissolves and contribute corrosion. Corrosion in the tubing of gas condensate wells usually takes in the form of deep pitting.

Ring worm corrosion: type of corrosion fond in tubing of gas condensate wells.

Sand cutting or erosion: This occurs by mechanical action in combination with corrosive reaction. By increase in gas velocity, the rate of corrosion increases rapidly.

Sour corrosion

It is caused by hydrogen sulfide. When moisture is present H2s is corrosive similarly co2 or o2 is also present. Sulfide corrosion starts slowly and the rate increases with time. The sulfide adheres to the steel surface as a black powder or scale. It results in deep pitting.

Hydrogen can also form molecular hydrogen which leads to blisters and cracks.

O2 Corrosion: the o2 corrosion depends on many factors these are temp, erosion and presence of electrolyte (Water).O2 corrosion are severe in seawater than in fresh water. Corrosion increase with an increase, basic chemical reaction is O2 combines with water to form rust and rust is the common form of all corrosion.

Control

By corrosion inhibitors (sodium chromate, sodium nitrate, polyphosphate, silicates etc.)(Thickness survey)

 

Alloy such as SS, brass, bronze and Monel are good resistant for corrosion, protective coating

 

Mechanical properties

• Strength
• Ductility
• Hardness
• Toughness
• Fatigue resistance
•  

Strength

 Ability of the material to with stand the applied load. Tensile strength, shear strength, torsional strength, impact strength and fatigue strength

Ductility

Property of material to deform or stretch under load with out failing. Low ductile metal fails suddenly in a brittle manner (glass ets)

Hardness

It is the ability to resist penetration.Brinell, vicker and rock well

 

Toughness

It is the ability of the material to absorb energy. (Charpy, crack tip opening displacement CTOD etc)

Fatigue strength

The strength necessary to resist failure under repeated load application. (Motor shaft...fails during operation under various loads)

 

Chemical properties:

 

Common name	Carbon content	Use	Weld ability
Ingot iron	0.03 %max	Galvanizing, deep drawing sheet and strip	Excellent
Low carbon steel	0.15 max	Welding electrodes, special plates, sheets and strips	Excellent
Mild steel	0.15-0.3 %	Structural shapes, plate and bar	Good
Medium carbon steel	0.3 -0.5%	Machinery parts	Fair (preheat and post heat required)
High carbon steel	0.5-1%	Spring,dies,rails	Poor with out post heat and pre heat

 

ALLOY GROUP

 

Common categories of alloys are steel, aluminium and copper.

Plain carbon steel contains primarily iron also small addition of carbon, manganese, phosphorous, sulfer and silicon.

Low alloy steel contains minor addition of other elements such as nickel, chromium, manganese, silicon, vanadium, columbium, aluminium, molybdenum and boron. The presence of these elements decides the mechanical properties.

The last group is known as high alloy steel, stainless and other corrosion resistant are examples of these, SS contains mainly 12% of chromium and 8% nickel, higher resistance to oxidation (rust)

 

SS is divided into five groups

• Austenitic
• Martenstic
• Feriitic
• Precipitation hardening
• Duplex grades.

  

AISI TYPES	C	Mn max	Si Max	Cr	Ni	Other
		Austenitic steels
304	0.08	2	1	18-20	8-12	-
304L	0.03	2	1	18-20	8-12	-
310	0.25	2	1	24-26	19-22	-
316	0.08	2	1	16-18	10-14	2-3 mo
321	0.08	2	1	17-19	9-12	5*Cmin Ti
		Martenstic
403	0.15	1	.5	11.5-13	-	-
410	0.15	1	1	11.5-13.5	-	-
420	0.15	1	1	12-14	-	-
		Ferritic
430	0.12	1	1	14-13	-	-
446	0.2	1.5	1	23.-27	-	0.25 max
		Precipitating hardening
15-5	0.07			15	5	3 cu
17-4	0.07			17	4	4 cu
17-7	0.09			17	7	1 al
		Duplex
329	0.08			25	4.5	1.5 Mo
3RE60	0.03			18.5	5	2.7
44LN	0.03			25	6	1.7

 

Phosphorus is 0.06% max in types 304,304L,310,316,321 sulfur 0.03% max in types 304,304L310,316,321,403,410,420,430 and 446

 

Carbon

Important alloying element in steel up to 2 %( LESS THAM 0.5% in welded steel) it can exist in dissolved iron or in a combined iron carbide.incresed amount can increase hardness and tensile strength but it will reduce weld ability.

Sulfur

It is an undesirable impurity present in steel. Amounts exceeding 0.05% cause brittleness and reduce weldability.0.1 to 0.3% increases mach inability. Of steel.

Phosphorus

It is an undesirable impurity in steel.mostly up to 0.04% in most CS.it hardened steel, it may tend to cause embrittlement.it is added  0.1% to improve both strength and corrosion resistance.

Silicon

It is usually (0.2%) present in rolled steel used as de oxidizer. in steel castings 0.35 – 1.00% which dissolves in iron and tends to strengthen it. weld metal usually appr 0.5% silicon as a de oxidizer. some filler metals may contain 1% to provide enhanced cleaning and deoxidation for welding on contaminated surfaces. when these filler metals are used for welding clean surfaces, the resulting weld metal strength will be markedly increased. The resulting decrease in ductility could present cracking problems in some solutions.

MANGANESE ( 0.3%)

1. Assist in the deoxidation of the steel

2. Prevent the formation of iron sulfide inclusion

3. promotes greater strength by increasing harden ability of the steel 1.5% in CS

Chromium

Powerful element in steel. To increase Hardenbility, Improves corrosion resistance, if excessive result in crack during weld.

Molybdenum

It is a strong carbide former less than 1.0%.To increase hard ability and elevated temp strength. Added in austenitic stainless steel to avoid pitting corrosion.

Nickel

Added to increase hard ability, improve toughness and ductility of steel. It is frequently used to improve steel toughness at low temperature.

Aluminum

Added as de oxidizer, to improve toughness

Vanadium

Used to increase hardenability.above 0.05% tendency for the steel to become embrittled during thermal stress relief treatments.

Nobium (columbium)

Used to increase the harden ability of steel. it may combine with carbon in the steel to result in decrease in hardenability.also it will increase weld ability

Dissolved gases

Hydrogen H2, nitrogen N2 and oxygen O2 all dissolve in molten steel can embrittle steel if not removed.

Aluminum alloy.

NON-FERROUS ALLOY-Available in both wrought and cast forms. Good strength, light weight high thermal expansion and good electrical conductivity and corrosion resistant.

Nickel

Is a silvery metal, has excellent resistance to corrosion and oxidation at high temp.nikel iron chromium and copper. High tempo alloys and corrosion resistance alloys have nickel up to 60% to 75 % .such as Monel 400, inconel and hastalloy C-276.

 

Ferritic stainless steels have certain useful corrosion properties, such as resistance to chloride stress-corrosion cracking, corrosion in oxidizing aqueous media, oxidation at high temperatures and pitting and crevice corrosion in chloride media. These steels contain above approximately 13% Cr and precipitate a prime phase in 350°C to 540°C range, and the maximum effect is at about 470°C. Because precipitation hardening lowers temperature ductility, it must be taken into account in both processing and usage of ferritic stainless steels, especially those with higher chromium content

Structures of these steels are kept completely ferritic at room and high temperature by adding titanium or columbium, or by melting to very low levels of carbon and nitrogen, or both. Such microstructures provide ductility and corrosion resistance in weldments. Molybdenum improves pitting corrosion resistance, while silicon and aluminum increase resistance to high temperature oxidation. The representatives of this group include ASTM designations 409 and 439.

Type 409 with 12% Cr is relatively low-cost and has good formability and weldability. Recommended thickness is limited to approximately 3, 8 mm maximum if ductile-to-brittle transition temperature (DBTT) at room temperature or lower is needed (Figure 1). Its atmospheric corrosion resistance is adequate for functional uses, so applications of this type of steel include automobile exhaust equipment, radiator tanks, catalytic reactors, containerization and dry fertilizer trunks. Type 439 with 18-20% Cr resists chloride stress-corrosion cracking. Resistance to general and pitting corrosion is approximately equivalent to that of austenitic types 304 and 316. This grade is suitable for equipment exposed to the aqueous chloride environments, heat transfer applications, condenser tubing for fresh water power plants, food-handling uses and water tubing for domestic and industrial buildings.

High-chromium ferritic stainless steels

High-chromium ferritic stainless steels - such as types 442 and 446 - have excellent resistance to corrosion and to oxidation in many industrial environments. These alloys are included in ASTM specifications A176-74 (Chromium stainless flat products), A 511 (Seamless stainless steel mechanical tubing), A268-74 (Ferritic stainless steel tubing for general service) and also in ASME code and AISI and SAE specifications.

High-chromium ferritic steels have 18-30% Cr and low content of carbon and nitrogen. Titanium in these alloys prevents intergranular chromium-carbide and nitride precipitation during welding or processing. Types 442 and 446 have excellent oxidation resistance at elevated temperatures. They also have high thermal conductivity, higher yield strength than austenitic stainless steels, and lower tensile ductility.

The commonest austenitic steel is so-called 18/8 containing around 18% Cr and 8% Ni. It has the lowest nickel content concomitant with a fully austenitic structure.
Austenitic steels are prone to stress corrosion cracking, particularly in the presence of chloride ions where a few ppm can sometimes prove disastrous. This is a type of failure which occurs in some corrosive environments under small stresses, either deliberately applied or as a result of residual stresses in fabricated material. In austenitic steels, it occurs as transgranular cracks which are most easily developed in hot chloride solutions. Stress corrosion cracking is very substantially reduced in high nickel austenitic alloys.

Types of stainless steel

There are different types of stainless steels: when nickel, for instance is added the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels

Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.

Stainless steels are also classified by their crystalline structure:

Austenitic stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition is 18% chromium and 10% nickel, commonly known as 18/10 stainless. Similarly 18/0 and 18/8 is also available. “Super austenitic” stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high Molybdenum contents (>6%) and nitrogen additions and the higher nickel content ensures better resistance to stress-corrosion cracking over the 300 series. The higher alloy content of "Super austenitic" steels means they are fearsomely expensive and similar performance can usually be achieved using duplex steels at much lower cost.

Ferritic stainless steels are highly corrosion resistant, but far less durable than austenitic grades and cannot be hardened by heat treatment. They contain between 10.5% and 27% chromium and very little nickel, if any. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.

Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely strong and tough as well as highly machineable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), no nickel, and about 0.1-1% carbon (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic. It is also known as "series-00" steel.

Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50:50 mix although in commercial alloys the mix may be 60:40. Duplex steel have improved strength over austenitic stainless steels and also improved resistance to localized corrosion particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium and lower nickel contents than austenitic stainless steels.

Standard types

The AISI defines the following grades among others:

• 200 Series—austenitic iron-chromium-nickel-manganese alloys
• 300 Series—austenitic iron-chromium-nickel alloys
• Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working.
• Type 303—Free machining version of 304 via addition of sulfur
• Type 304—the most common; the classic 18/8 stainless steel.
• Type 316—the next most common; for food and surgical stainless steel uses; Alloy addition of molybdenum prevents specific forms of corrosion. Also known as "marine grade" stainless steel due to its increased ability to resist saltwater corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants.
• 400 Series—ferritic and martensitic alloys
• Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
• Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
• Type 410—martensitic (high-strength iron/chromium).
• Type 420—"Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel". Also known as "surgical steel".
• Type 430—decorative, e.g. for automotive trim; ferritic.
• Type 440—a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly.
• 600 Series—martensitic precipitation hardening alloys
• Type 630—most common PH stainless, better known as 17-4; 17% chromium, 4% nickel

Comparison of standardized steels

EN-standard Steel no.	EN-standard Steel name	ASTM-standard Type
1.4016	X6Cr17	430
1.4512	X6CrTi12	409
1.4310	X10CrNi18-8	301
1.4318	X2CrNiN18-7	301LN
1.4307	X2CrNi18-9	304L
1.4306	X2CrNi19-11	304L
1.4311	X2CrNiN18-10	304LN
1.4301	X5CrNi18-10	304
1.4948	X6CrNi18-11	304H
1.4303	X5CrNi18 12	305
1.4541	X6CrNiTi18-10	321
1.4878	X12CrNiTi18-9	321H
1.4404	X2CrNiMo17-12-2	316L
1.4401	X5CrNiMo17-12-2	316
1.4406	X2CrNiMoN17-12-2	316LN
1.4432	X2CrNiMo17-12-3	316L
1.4435	X2CrNiMo18-14-3	316L
1.4436	X3CrNiMo17-13-3	316
1.4571	X6CrNiMoTi17-12-2	316Ti
1.4429	X2CrNiMoN17-13-3	316LN
1.4438	X2CrNiMo18-15-4	317L
1.4539	X1NiCrMoCu25-20-5	904

Stainless steel finishes

Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final material. Any oxidation that forms on the surface (scale) is removed by pickling, and the passivation layer is created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.

• No. 0 - Hot Rolled Annealed, thicker plates
• No. 1 - Hot rolled, annealed and passivated
• No, 2D - cold rolled, annealed, pickled and passivated
• No, 2B - same as above with additional pass through polished rollers
• No, 2BA - Bright Annealed (BA) same as above with highly polished rollers
• No. 3 - coarse abrasive finish applied mechanically
• No. 4 - fine abrasive finish
• No. 6 - matt finish
• No. 7 - reflective finish
• No. 8 - mirror finish

Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel.

Austenite is a metallic, non-magnetic solid solution of carbon and iron that exists in steel above the critical temperature of 1333°F (about 723°C). Its face-centred cubic (FCC) structure allows it to hold a high proportion of carbon in solution.

As it cools, this structure either breaks down into a mixture of ferrite and cementite (often in special forms such as pearlite and bainite), or undergoes a slight lattice distortion known as martensitic transformation. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel. Quenching (to induce martensitic transformation), followed by tempering (to break down some martensite and retained austenite), is the most common heat treatment for high-performance steels.

Bainite is a mostly metallic substance that exists in steel after certain heat treatments. It forms when austenite (a solution of carbon in iron) is rapidly cooled past a critical temperature of 1333°F (about 723°C).

Martensite is a class of hard minerals occurring as lathe- or plate-shaped crystals. When viewed in cross-section, the crystals appear acicular (needle-shaped), which is how they are sometimes incorrectly described. It most commonly refers to a form of iron and carbon found in very hard steels, for use in such products as springs and piano wire. The crystals have body-centred tetragonal (BCT) symmetry, and result from the rapid cooling of austenite during

Martensite has a very similar crystalline structure to austenite, and identical chemical composition. As such, a transition between these two allotropes requires very little thermal activation energy, and has been known to occur even at cryogenic temperatures.

Bainite is a mostly metallic substance that exists in steel after certain heat treatments. It forms when austenite (a solution of carbon in iron) is rapidly cooled past a critical temperature of 1333°F (about 723°C).

Cementite or iron carbide is a chemical compound with the formula Fe₃C, and an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in metallurgy.

It forms directly from the melt in the case of white cast iron. In carbon steel, it either forms from austenite during cooling or from martensite during tempering. It mixes with ferrite, the other product of austenite, to form lamellar structures called pearlite and bainite. Much larger lamellae, visible to the naked eye, make up the structure of Damascus steel, though the process has been lost to history (see article for information on attempted reconstruction of the process).