faq - corrosion protection principle
TRANSCRIPT
A B Joe H
0806331355
Tugas 4 – Prinsip Proteksi Korosi
Classification of corrosion protection methods
Active corrosion protection
Passive corrosion protection
Permanent corrosion protection
Temporary corrosion protection
Active corrosion protection
The aim of active corrosion protection is to influence the reactions which proceed during corrosion, it being possible to control not only the package contents and the corrosive agent but also the reaction itself in such a manner that corrosion is avoided. Examples of such an approach are the development of corrosion-resistant alloys and the addition of inhibitors to the aggressive medium.
Passive corrosion protection
In passive corrosion protection, damage is prevented by mechanically isolating the package contents from the aggressive corrosive agents, for example by using protective layers, films or other coatings. However, this type of corrosion protection changes neither the general ability of the package contents to corrode, nor the aggressiveness of the corrosive agent and this is why this approach is known as passive corrosion protection. If the protective layer, film etc. is destroyed at any point, corrosion may occur within a very short time.
Permanent corrosion protection
The purpose of permanent corrosion protection methods is mainly to provide protection at the place of use. The stresses presented by climatic, biotic and chemical factors are relatively slight in this situation. Machines are located, for example, in factory sheds and are thus protected from extreme variations in temperature, which are frequently the cause of condensation.
Temporary corrosion protection
The stresses occurring during transport, handling and storage are much greater than those occurring at the place of use. Such stresses may be manifested, for example, as extreme variations in temperature, which result in a risk of condensation. Especially in maritime transport, the elevated salt content of the water and air in so-called salt aerosols (salt spray) may cause damage, as salts have a strongly corrosion-promoting action.
Pembagian Proteksi Korosi dibagi
menjadi 6 tipe:
(1) MATERIAL SELECTION
(selection of proper material for a particular
corrosive service)
Metallic [metal and alloy]
Nonmetallic [rubbers (natural and synthetic), plastics,
ceramics, carbon and graphite, and wood]
Metals and Alloys
No Environment Proper material
1 Nitric acid Stainless steels
2 Caustic Nickel and nickel
alloys
3 Hydrofluoric acid Monel (Ni-Cu)
4 Hot hydrochloric acid Hastelloys (Ni-Cr-
Mo) (Chlorimets)
5 Dilute sulfuric acid Lead
No Environment Proper material
6 Nonstaining atmospheric
exposure
Aluminium
7 Distilled water Tin
8 Hot strong oxidizing
solution
Titanium
9 Ultimate resistance Tantalum
10 Concentrated sulfuric
acid
Steel
[2]Protective Coatings / Wrapping
Provide barrier between metal and environment.
Coatings may act as sacrificial anode or release substance that
inhibit corrosive attack on substrate.
Metal coatings :
Noble – silver, copper, nickel, Cr, Sn, Pb on steel.
Should be free of pores/discontinuity coz creates
small anode-large cathode leading to rapid attack at
the damaged areas.
Sacrificial – Zn, Al, Cd on steel. Exposed substrate
will be cathodic & will be protected.
Application – hot dipping, flame spraying, cladding,
electroplating, vapor deposition, etc.
Surface modification – to structure or composition by use of directed energy or particle beams. E.g ion implantation and laser processing.
Inorganic coating : cement coatings, glass coatings, ceramic coatings, chemical conversion coatings.
Chemical conversion – anodizing, phosphatizing, oxide coating, chromate.
Organic coating : paints, lacquers, varnishes. Coating liquid generally consists of solvent, resin and pigment. The resin provides chemical and corrosion resistance, and pigments may also have corrosion inhibition functions.
[3]Alteration of Environment
Typical changes in medium are :
Lowering temperature – but there are cases where
increasing T decreases attack. E.g hot, fresh or salt water
is raised to boiling T and result in decreasing O2 solubility
with T.
Decreasing velocity – exception ; metals & alloys that
passivate (e.g stainless steel) generally have better
resistance to flowing mediums than stagnant. Avoid very
high velocity because of erosion-corrosion effects.
Removing oxygen or oxidizers – e.g boiler feedwater
was deaerated by passing it thru a large mass of scrap steel.
Modern practice – vacuum treatment, inert gas sparging, or
thru the use of oxygen scavengers. However, not
recommended for active-passive metals or alloys. These
materials require oxidizers to form protective oxide films.
Changing concentration – higher concentration of acid
has higher amount of active species (H ions). However, for materials that exhibit passivity, effect is normally negligible.
Environment factors affecting
corrosion design :
Dust particles and man-made pollution – CO, NO,
methane, etc.
Temperature – high T & high humidity accelerates
corrosion.
Rainfall – excess washes corrosive materials and
debris but scarce may leave water droplets.
Proximity to sea
Air pollution – NaCl, SO2, sulfurous acid, etc.
Humidity – cause condensation.
[4] Cathodic Protection
Cathodic Protection: DC current injection towards pipeline,
result: pipeline is “shifted” as cathode.
Sacrificial anode
Impressed current
Galvanic Sacrificial Anode - Principle
Metal Driving voltage (Vd)
Negative Vd = active metal =tendency to corrode
Pipeline: connected with active metal
Anode: Mg, Zn Cathode: pipeline
Corrosion on Mg, Zn hence “sacrificial”
Galvanic Sacrificial Anode-Principle
Material Driving Voltage (V)
Silver (Ag/Ag+) +0.8 (cathodic)
Copper (Cu/Cu2+) +0.34
Water (O2+H20+4e- = 4OH-) +0.401
Hydrogen (H2) 0 (reference)
Iron (Fe/Fe2+) -0.44
Zinc (Zn/Zn2+) -0.76
Magnesium (Mg2+) -2.36 (anodic)
Galvanic Sacrificial Anode-Installation
(Source: Peabody’s Control of Pipeline Corrosion)
Galvanic Sacrificial Anode-Installation
(Source: Peabody’s Control of Pipeline Corrosion)
Impressed Current
Minimum potential: -0.85V Reference electrode: Copper Sulfate
(Source: Peabody’s Control of Pipeline Corrosion)
Galvanic Sacrificial Anode-Design
Calculate total area to be protected (Ap) Determine the current density (ρ) Calculate total protection current (Itot = ρ .Ap) Calculate R per anode : R = f(d,l,ρ) Calculate Ia per anode : (Vd-0.85/R) Calculate total anode needed : Initial : N=Itot/In Lifetime: N = f(mass,lifetime,A/poundyear)
Impressed Current-Principle
• Rectifier
• Variable voltage, variable current
• Higher current density (> 1 A)
• Higher soil resistivity (> 104 Ωcm)
Impressed Current-Installation
• Types of anode: Graphite : big CR (pound/A/year) High Silicon Cast Iron : medium CR Platinum & niobium : small CR
• Rectifier rating: Voltage, Amperes, Watt, Freq. Cooling System : Air, Oil Efficiency
CR: Consumption Rate
Impressed Current - Installation
(Source: Peabody’s Control of Pipeline Corrosion)
Impressed Current - Design
Calculate total area to be protected (Ap) Determine curent density (ρ) Calculate total protection current (Ip) Calculate total anode needed (N) Initial Lifetime
Calculate total anode resistance (Rtot = f(N,ρ,d,L,spacing)) Calculate rectifier specification (Vdc, Idc, Pdc = f(Rtot,Ip)
Different Corrosion protection methods Anodizing
Advantages
A tough surface layer which has very good corrosion protection
properties and very good adhesion to the surface.
Disadvantages
Must be applied after welding or brazing if the joining areas are to be
protected. This can be complicated for large structures. Can not be done
on site.
When to use
For protection against weathering and scratching/abrasion.
Conversion coating
Advantages
A non costly and simple protection of the aluminum surface, which in addition
increases the adhesion of lacquers and adhesives.
Disadvantages
Has limited resistance to mechanical and thermal influence.
When to use
Primarily used as a pre-treatment before lacquering or adhesive bonding.
Lacquering
Advantages
A high quality lacquering system has very good corrosion protection
properties.
Disadvantages
The performance of the lacquering system is very dependent on the quality
of the pre-treatment and application work.
Relatively expensive.
When to use lacquering
Where appearance and/or corrosion performance is very important
Inhibitors
Advantages
Can be tailored to give excellent protection in specific
environments.
Disadvantages
Expensive to use with large amounts of liquid.
May cause increased corrosion if incorrectly used.
When to use inhibitors
For protection against internal corrosion in closed systems,
circulating or non-circulating
Protective adhesive tapes
Advantages
Prevents galvanic contact. Grease filled tapes will seal crevices.
Disadvantages
Costly to apply. May need to be supported in place.
When to use
Buried pipelines