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Corrosion Lab

March 18 2001

 

1.0 Introduction

The purpose of this lab is to determine the factors affecting aqueous corrosion, and study failures caused by corrosion. Also to be examined are methods for controlling rates of corrosion, such as polarisation, and passivation.
Passivation is the interruption of the electric circuit via a protective coating forming on the anode surface due to strong anodic polarisation. This current blockage does not allow electrons to be transferred, and thus corrosion is slowed or negated at higher voltages; however, further increase in the voltage will cause current to begin flowing again, as electrons now have enough energy to pass through the protective coating. Furthermore, the adding of salt to the electrolyte will be examined for its effect upon passivation. This experiment will show how increasing voltage potential can actually decrease corrosion.
Galvanic half-cells are a measure of the relative electric potential of various materials. The half-cell potentials can be useful for predicting if, and how severe corrosion will be between materials, usually metals. Half-cell potentials can also be used to determine which of the two metals will be anodic, and thus corrode. This is useful if we wish to prevent corrosion of a chosen metal/material by attaching a sacrificial anode to it. The table of Galvanic potentials will allow for easy selection of appropriate sacrificial anodes.
Polarisation is the change in potential at the anodes towards each other, such that the rate of corrosion will be decreased. The three main types of polarisation are activation polarisation, concentration polarisation, and resistance polarisation. Activation polarisation is related to the energy requirements for the cathode and anode reactions to occur. Increased polarisation causes these reactions to become hindered, and thus current flow and corrosion are decreased. Concentration polarisation is caused when there is a build-up of similarly charged ions at either electrode. The concentration of reactants thus becomes inadequate to carry the current, and thus the current is decreased. Resistance polarisation is caused by electrical resistivity of the electrolyte. High electrolyte resistances prevent current flow, and thus diminish current
 

2.0 Procedure

Refer to Prof. N. Zhou, “ME230- Control and Properties of Materials Laboratory Notes – Winter 2001 – 2A class, University of Waterloo, Pages32 - 36
 

3. 0 Results and Observations

Figure 1 shows the voltage versus current density for 304 stainless steel. Figure 2 shows the voltage versus current density for 409 stainless steel
Figure 3 displays the polarisation of copper and zinc. It is observed that the potential difference between the copper and the zinc electrode decreases until the difference is almost zero. It is also observed that the copper electrode has a greater change in potential than the zinc electrode.

 

4.1 Passivation

The chemical reactions that occur at the anode and the cathode are:
Anode:
Fe  Fe2+ + 2e-
Cathode (in acid):
2H30+ + 2e-  2H2 + H20

At the anode, Fe atoms surrender their electrons to the cathode. The newly formed positive Fe ions disengage from the anodic electrode and dissolve into solution. It is this oxidation of the anode atoms which causes the corrosion of the anode.
Conversely, on the cathode, the excess electrons attract the positive hydronium ions to the surface. The hydronium ions adopt the cathode’s electrons to form hydrogen gas and water. Thus a gas is observed evolving at the cathode, no corrosion of the cathode occurs.

The corrosion rate of 304 stainless steel is 3.34 mils/year (without salt) and 1171 mils/year (with salt). (see Appendix A: corrosion calculations)

The passive corrosion resistance of the 304 stainless steel reduces when NaCl (salt) is added to H2SO4 acid solution (see Figure 1). This is because NaCl increases the electrical conductivity of the electrolyte and allows current to flow more freely through the circuit by decreasing the energy requirements for electrons to travel through the electrolyte.
Since electrons now have more energy, they can more easily pass through the passitive layer on the electrode. Thus the passitive phase of the electrodes is greatly reduced.

The corrosion and oxidation resistance of stainless steels improves as the chromium content increases. The oxide layer that forms with chromium, forms a solid boundary that prevents further oxidation. The addition of nickel to create the austenitic stainless steel grades, strengthens the oxide film and raises their performance in more aggressive conditions. This is also evident from comparing the passive regions of the two stainless steels in Figure 1 and 2: 304 SS has higher Chromium and Nickel content compared to 409. Therefore, it also has a greater passive region when compared to 409 SS.
Two other materials that are passivate in nature include: Aluminium and Lithium.

 

4.2 Galvanic Couples

 

Alloy 1

Alloy 2

REF half cell 1

REF half cell 2

Full-cell

STD half cell 1

STD half cell 2

brass Cu-Zn

Zn

-0.103

-0.962

0.867

0.139

-0.720

1100 Al

Zn

-0.718

-0.962

0.265

-0.476

-0.720

304

5052 Al

-0.475

-0.737

0.300

-0.233

-0.495

439

5052 Al

-0.477

-0.737

0.330

-0.235

-0.495

439

409

-0.476

-0.457

0.020

-0.234

-0.215

Ni

1020

-0.450

-0.430

0.015

-0.208

-0.188

1020

Zn

-0.430

-0.962

0.606

-0.188

-0.720

Table 1: Full cell potentials of galvanic couples and half-cell potentials  

 

Galvanic Series

Volts

brass Cu-Zn

0.139

1020

-0.188

Ni

-0.208

409

-0.215

304

-0.233

439

-0.235

1100 Al

-0.476

5052 Al

-0.495

Zn

-0.720

Table 2: Galvanic series in order of decreasing potential 

Full Cell Potential  (V)

Ecathode - Eanode (V)

0.867

0.859

0.265

0.244

0.300

0.262

0.330

0.260

0.020

0.019

0.015

0.020

0.606

0.532

Table 3:  Difference between the cathode and anode half-cell potentials


From table 3 it is observed that the difference between the cathode and anode half-cell potentials shown, is nearly equal to the measured full-cell potentials. Thus, full-cell potential may be determined by
Ecell = Ecathode - Eanode

From the observed data, brass and zinc have the greatest potential difference. Thus, assuming all other factors equal, the brass and zinc full cell will demonstrate the most corrosion. From the observed full-cells, the Ni-1020 had the least potential difference, and therefore would display the least corrosion; however, the metals 304 and 439 theoretically have less potential difference, therefore they should display less corrosion than even Ni-1020, should they ever be placed in a full-cell together.

Of the materials tested, zinc would be the best sacrificial anode since is has the lowest standard half-cell potential. Zinc is also relatively inexpensive, and thus it is commonly used on the hulls of ships. Magnesium also has a relatively low standard half-cell potential, so it serves well as sacrificial anodes. Also, in the case of protecting copper pipes, scrap iron is a good sacrificial anode.
 

4.3 Polarisation

Polarisation is the change in potential of an anode or cathode. The change in potential affects the current in the cell, which in turn affect the rate of corrosion. There are two major types of polarisation: activation polarisation and concentration polarisation.

Activation polarisation is related to the energy required to cause the anode or cathode reactions to occur. If the degree of polarisation is increased the reaction will occur with greater difficulty and thus the rate of corrosion will be reduced. Small differences in composition and structure in the anode or cathode material causes differing degrees of activation polarisation [1].

Concentration polarisation occurs when the concentration of reactants at the electrode surface is inadequate. When the reactants are consumed at either electrode, the reactants must be replenished to allow the reaction to continue. Once the electrochemical reaction begins, a high concentration of ions builds up at the electrodes. If the diffusion rate of these ions is low, the reactions at either the anode or the cathode may be stifled since fewer electrons are released at the anode or captured at the cathode. Thus a change in potential at the electrodes is experienced, which in turn reduces the current density and the rate of corrosion [1].

From figure 3 it can be seen that the copper electrode shows the greater change in potential. This suggests that the copper electrode is under cathodic polarisation. The reduction at the cathode causes plating on its surface, which acts as an insulator to electrons. This can inhibit electron transfer and thus reduces the current density, which in turn reduces the corrosion rate [2].

From figure 3 there is no evidence of activation polarisation. The mechanism active in the cell is most probably concentration polarisation. As the corrosion proceeds, a high concentration of ions accumulates near the cathode, effectively plating the cathode. This inhibits the transfer of electrons and thus suppresses the electrochemical reaction.

Cathodic protection is used to protect against corrosion by supplying the metal with electrons and forcing it to be cathodic. The most common method of cathodic protection is the use of a sacrificial anode. A sacrificial anode is attached to the desired material, forming an electrochemical cell with the desired material. The sacrificial anode corrodes as a result of supplying electrons to the metal, thereby preventing an anodic reaction from occurring in the desired material, and thus protecting the desired material from corrosion. Cathodic protection is extremely effective for preventing corrosion on ships and pipeline. This type of mechanism is ineffective in protecting automobiles from rusting because air does not act an effective electrolyte between the car body (cathode) and the sacrificial anode.
 

4.4 Corrosion Failures

Microbiological corrosion was observed in a component from an organic spray gun. The component had accumulations that might have caused it to become clogged. The growth of colonies of the organisms led to the development of oxygen concentration cells. The colonies reduce the diffusion of oxygen to the metal beneath. Thus the area beneath the colonies was anodic (low-oxygen region), whereas the unaffected areas were cathodic (high-oxygen region). As the electrons flowed, the metal beneath the organisms corroded, usually in the form of pitting [4].

Microbiological corrosion can be prevented by frequent cleaning, using a protective coating, selecting more resistant materials, or by making the containing environment unfavourable for microbiological growth (e.g. change pH) [4].

Erosion corrosion is the corrosion of a metal, which is accelerated by the movement of a fluid. The motion of the fluid removes any surface rust or other films, exposing more of the bare metal to the fluid and promoting more corrosion [4]. Failure by erosion corrosion was observed in an elbow joint. The piping leading to the joint was corroded and was covered with rust. However the surface at the elbow joint was relatively smooth, and noticeably thinner, because of the constant corrosion and erosion. Continued use would have caused a rupture at the elbow. The erosion failure can be prevented by the selection of alloys with greater corrosion resistance and/or higher strength, by reducing the fluid velocity and by using corrosion resistant and abrasion resistant coatings, and by using better design to minimise turbulence, flow restrictions and abrupt changes in direction of the fluid [4].

 

5.0 Conclusion

In conclusion to passivation experiment it was found that the addition of salt to electrolyte increases the electrical conductivity and therefore increases the corrosion rate. In addition it was found that chromium increase the resistance to corrosion of stainless steels, and nickel strengthens the oxide films that forms with chromium to raise the resistance of stainless steel in mire aggressive conditions.

In conclusion to galvanic couples it was found that galvanic half-cells are a measure of the relative electric potential of various materials. The half cell potential can be used to predict the corrosion between materials. Half-cell potentials can also be used to determine which of the two metals will be anodic, and thus corrode. This information can be used in the selection of sacrificial anodes.

In conclusion to polarisation experiment, it was found that concentration activation is the most active mechanism in a galvanic couple composed of copper and zinc. It was also determined that the electrode with the greatest change in potential is the cathode.

Microbiological corrosion and erosion corrosion are two types of corrosion failure mechanisms that occur in different siutaions. There are many protective measures that can be taken in order to minimise its affects.

 

6.0 References


[1] Askland Dpmald, The science and Engineering of Materials Third S.I. Edition, Chapman and Hall, Oxford, 1996
[2] Prof. N. Zhou, “ME230- Control and Properties of Materials Laboratory Notes – Winter 2001 – 2A class, University of Waterloo, Pages32 – 36
[3] Electrochemistry, www.iesvic.uvic.ca/fuelcell/basics/electrochemistry/default.htm,
March 19, 2001
[4] Corrosion Failures, http://httd.njuct.edu.cn/MatWeb/corrosie/c_bio.htm, March 19 2001