General
Aim (altered)- The effect of salt concentration on corrosion rate of magnesium and zinc in
stagnant and aerated solutions.
- Possible extension; Compare results to different metals such as
steel and aluminum.
How to
measure corrosion rate:
The rate of corrosion is the speed at which
a metal deteriorates in a specific environment
In order to
calculate the rate of corrosion, the following information must be collected:
•Weight
loss (the decrease in metal weight during the reference time period)
•Density
(density of the metal)
•Area
(total initial surface area of the metal piece)
•Time (the
length of the reference time period)
To do-
-
Identify topic and central
concept of interest
-
Refine
ideas to generate a hypothesis
-
Complete
risk assessment for equipment/materials list
Components
to measure:
-
Concentration
of salt (three different levels)
-
Corrosion
rate, comparably, of magnesium and zinc (see page 365/371)
-
Difference
in rate in stagnant and aerated solutions. Dissolved oxygen (aerated/stagnant
solutions)
Possible
quantitative measurements:
-
Corrosion
rate in aerated/stagnant solutions
-
Compare
standard electrode potentials with results for magnesium and zinc
-
Graph
amount of oxygen with rate of corrosion, then compare for both metals.
-
Concentration
of salt and level of oxygen vs corrosion rate
Magnesium,
before I use it, will already have oxidized to a certain degree depending on the
amount of oxygen the strip has been exposed to. Therefore, there will be a
protective layer of black surrounding the metal, preventing any further
oxidation to occur. Before I begin my experiment, I need to scrub this oxidized
layer off to ensure further oxidation in the salt water has occurred.
a. Some factors which influence metal corrosion and the rate of corrosion are the:
(1) Type of metal;
(2) Heat treatment and grain direction;
(3) Presence of a dissimilar, less corrodible metal (galvanic corrosion);
(4) Anode and cathode surface areas (in galvanic corrosion);
(5) Temperature;
(6) Presence of electrolytes (hard water, salt water, battery fluids, etc.);
(7) Availability of oxygen;
(8) Presence of different concentrations of the same electrolyte;
(9) Presence of biological organisms;
(10) Mechanical stress on the corroding metal; and
(11) Time of exposure to a corrosive environment.
(2) Oxygen concentration cells.
The solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under either metallic or nonmetallic deposits on the metal surface and under faying surfaces such as riveted lap joints. Oxygen cells can also develop under gaskets, wood, rubber, and other materials in contact with the metal surface. Corrosion will occur at the area of low oxygen concentration (anode) as illustrated in Figure 2-6. Alloys, such as stainless steel, which owe their corrosion resistance to surface passivity, are particularly susceptible to this type of crevice corrosion.
c. Salts.
Most salt solutions are good electrolytes and can promote corrosive attack. Some stainless steel alloys are resistant to attack by salt solutions but aluminum alloys, magnesium alloys, and other steels are extremely vulnerable. Exposure of airframe materials to salts or their solutions is extremely undesirable.
