CORROSION PREVENTION BY ELECTROCHEMICAL METHODS
Introduction
Corrosion can be prevented by application of electrochemistry principles. This basically
falls into two distinct areas, sacrificial anodes and cathodic protection by impressed currents.
Sacrificial Anodes.
In this preventative technique, corrosion is allowed to occur on a piece of metal that is
extraneous to the structure, for example, a zinc attached to a steel boat hull. The zinc corrodes in
place of the steel hull. The principles behind this process were discussed previously and will not
be repeated except to show the relevant Evans diagram.
E
(V)
Cathodic Reaction 1
Anode Reaction 3
Ecorr 1+2
Cathode Reaction 3
Total Cathode 1+3
Anodic Reaction 2
log Current Density
2
A/cm
icorr 1+2
In this case the anode#3 was protected from corrosion by anode reaction #2. One initial
principle is that the sacrificial material must have a potential lower than the material it is trying
to protect.
Simple examples of the application of this protection technique include:Galvanized bolts, automobile steel and mail boxes, zincs placed outboard engines and steel boat
hulls, aluminum blocks on oil rigs, etc.
Typical coatings which work on this principle are:zinc on steel, aluminum on steel, cadmium on steel.
It should be remembered that cadmium is only slightly below steel in the galvanic series.
As such it does not have much "Throwing power" which is the ability to protect over large
distances.
The consumption rate of the anodes was measured at Key West. The table below lists
some these rates.:Material
Rate(lb/amp.yr)
Zn
24
Mg
17.5
Al-Zn-Sn
20-7
Al-Zn-In
8
Note that the rate of consumption depends on the material and the current flowing.
To design for protection the approximate current per square foot required for
protection should be known. Tables exist for this data. For example:Environment
Immersed in Seawater
Stationary 1-2 (0.1-0.2)
Low Vel 1-3 f/s(0.3-1m/s)
Med Vel 3-7 f/s(1-2 m/s)
High Vel Turb flow
Buried Underground.
Soil resistivity .m
Hot sulfuric acid tank
Fresh water flowing pipes
Water heaters slow flow
Pilings in tidal seawater
Reinforcing steel
Current Density for Protection
mA/m2(mA/ft2)
Well Coated
2-20(0.2-2)
2-5
5-7
250-1000
Poor Coating
20-30(2-3)
5-20
10-30
250-1000
0.5-5
1-2
5-15
0.5-1
Uncoated
50-100
150-300
250-1000
15-40
0.1-0.05
500,000 (50,000)
50-100 (5-10)
10-30 (1-3)
60-80 (6-8)
1-5 (0.1-0.5)
From this type of data the amps.yr data can be calculated to determine the size,
separation and replacement time for sacrificial anodes.
Testing indicated that the corrosion rate of buried galvanized pipe varied from 0.6 to
2
19.5oz/in depending on the soil type. In buried conditions the type of soil must be known so that
accurate predictions can be made.
Cathodic Protection by Impressed Current.
The objective here is to ensure the component requiring protection is maintained in its
cathodic region by the application of a voltage or cathodic current. The system is shown
schematically below:-
DC rectifier
- ve
+ve
electrons
Structure to protect
Anode in impressed
current system
An anode is involved. In some cases the anode can be a consumable anode and manufactured
from a cheap material such as scrap metal. In other cases, the anode should not be consumed if
possible. Such a case is for cathodic protection of steel in reinforced concrete. Anodes cannot
easily be replaced. The table above provides the required currents for protection. Typical anodes
with consumption rates are shown below:-
Curr Den A/m2
Environment
Anode Material
Seawater
Pb-6%Sb-1%Ag
Pb-6%Sn-2%Ag
Pt on Ti,Nb or Ta
Graphite
Fe-14.5Si-4.5%Cr
Lead
Scrap steel
Aluminum
160-220
160-220
540-3200
10-40
10-40
Loss(lb/amp.yr)
0.03-0.2
0.03-0.06
0.008-0.016
0.5-1.0
0.5-1.0
0.1-0.25
20
10-12
The steel and aluminum are consumable anodes and so must be replaced at intervals. The
platinum anode is usually a coating on another metal in the form of a mesh. An example will be
shown in class. This mesh can be placed below the surface of concrete above the reinforcing
steel. Usually experts are called in to design cathodic protection systems as a phenomena called
stray currents can occur.
DC rectifier
- ve
+ve
electrons
This happens when a short circuit path is available between the anode and the cathode so
that is carries current. An example would be the close proximity of another metal conductor to
both the anode and cathode. The short circuiting component then corrodes instead of the anode.
Anodic protection.
An impressed current technique can be applied if the material passivates in the particular
environment. in this case the structure is made more anodic by drawing electrons out of it until it
enters the passive region.
There are advantages to this such as the cost of running the system is cheaper. However,
the disadvantages are high such as more complicated control system, and a non safe system if the
power fails or becomes uncontrolled. If the environment changes then the system may not
passivate the same way.
As a result, anodic protection is not very popular.
Inhibitors.
Inhibitors are used to reduce and block corrosion. They work by several different
mechanisms, some of which will be presented here.
Adsorption inhibitors.
Adsorption inhibitors protect by adsorption on to the metal or metal oxide film exposed
to electrolyte. Organic inhibitors are aliphatic and aromatic amines (N compounds), thiourea( S
compounds) and aldehydes (O compounds). All these have a charged state, for example aliphatic
amines have ammonium cations present, R3NH+. The S and O compounds have a negative
charge on them. Thiourea bonds strongly to a metal by sharing its electrons with the metal
surface. This blocks solvating water molecules and also stops hydrogen gas molecule formation.
N and O compounds are less adsorbed on the metal surface than the S type compounds.
They tend to select active anodic sites. The larger the molecule the greater the inhibition as they
displace solvating water molecules.
Poisons.
These type of inhibitors block either of the hydrogen ion reduction or formation of
hydroxyl ions cathodic reduction reactions. The hydrogen ion reduction reaction is inhibited by
the group V metals or metalloids such as P, As or Sb. As2O3 is added at about 0.25M. The
combination of hydrogen atoms to hydrogen molecules is blocked in a reaction of the form:AsO+ + 2Hads + e- -> As + H2O
Alternatively:As2O3 + 6Hads -> 2As + 3H2O
Scavengers
Scavengers act to remove the oxygen preferentially before it can be used in the cathodic
reactions. Two popular examples are hydrazine and the sulfite ion.
N2H4 + 5/2 O2 -> 2NO2- + 2H+ + H2O
SO32- + 1/2 O2 -> SO42-
Filming Inhibitors.
The addition of specific ions with high redox reaction potentials will produce local
reactions to form protective films. Two ions of this type are the chromate and nitrite ions.
They have redox reactions:NO2- + 8H+ + 6e -> NH4+ +2H2O Eo = +0.9V
2CrO42- + 10H+ + 6e -> Cr2O3 + 5H2O Eo = +1.31V
Both these reactions induce iron to dissolve in the ferric state with 3+ rather than in the
ferrous state as 2+. The ferric oxides are stable on the surface and block further corrosion.
Fe3+ + 3H2O -> Fe2O3 + 6H+
Vapor phase.
These tend to be nitrites, carbonate and benzoate filming inhibitors attached to parachutes
of an organic cation. An example is dicyclohexyl ammonium nitrite. The inhibitor evaporates
onto the metal surface.