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Introduction A PURPORTED
DEBATE AMONG MIDDLE AGE PHILOSOPHERS AND THEOLOGIANS REVOLVED AROUND,
"HOW MANY ANGELS CAN DANCE ON THE HEAD OF A PIN?" THE OBJECTIVE OF
THIS PROJECT WAS MUNDANELY ANALOGOUS: "HOW MANY ACTIVITIES OR
EXPERIMENTS RELEVANT TO THE CHEMISTRY CLASSROOM CAN BE ENVISIONED ON THE HEAD
OF A STEEL NAIL? ANSWER
MANY MORE THAN ARE INCLUDED HERE! No matter what conceptual real-life analogies
we choose, what mathematics we apply, what media we use, which experiments we
routinely conduct, or even what degree of sophistication our approach, as
high school chemistry teachers we normally cover the same broad topics of
study in our classrooms. Why not try a fresh approach
to your standard analogies and experiments using Steel an alloy
of particular interest to students living in a city that calls its football
team the "Steelers" ; a city with an economic and historical
development that revolved around this very substance for many years! |
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Objective The
overall purpose of this set of laboratory activities is to use a specific
material, steel, in meaningful high school chemistry experiences. The
corrosion of steel is an electrolytic process that can be studied easily
within the parameters of a high school chemistry classroom. The experiments
described herein provide some very tangible applications of; experimental
design, phases of matter, crystal structures, oxidation/reduction reactions,
Standard Reduction Potential, electrochemistry, LeChatlier's Principle, G,
Free Energy and others as discussed throughout this study. How many of theses concepts are
drawn upon and taught with the following experiments, depends on the
objectives which the teacher desires to fulfill. From the simplest
perspective, the following experiment can function as no more than good
practice in experimental design; practice observing steel corrosion,
hypothesizing as to the underlying factors responsible for corrosion, setting
up a multivariable experiment and gathering the data The data is analyzed for
evidence of "trends" without interpretation of results in
terms of any underlying causal factors. A
more complex treatment of the experiment would require that the student have a good grasp on
electrolytic processes and the function of an electrochemical cell;
mechanisms by which corrosion of steel occurs. This would enable the student
to more effectively identify and "interpret" data trends in light
of a known mechanism. The most complex treatment of
the experiment would include all of the above plus knowledge of the
composition of the different steels being used (SAE # 1008, 1045,
4140, 4340) and some knowledge of the various microstructures that
may occur in these steels. This would allow for interpretation of data trends
in light of the microstructure of the steels involved (how the various
alloying elements in the crystal structure or the %C phase mix might
possibly interact with the electrolytic mechanism of steel corrosion.) |
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Background Initial
discussion with students will likely indicate that many know steel contains
the element iron. Also, from everyday life, many will no doubt be familiar
with the "corrosion" of iron appearing as a red, low-density
substance commonly called "rust." A few students may even suggest
that rust is the compound, iron (III)oxide (under most conditions rust is
actually iron(III) hydroxide.) When it comes to hypothesizing what it is that
causes steels to rust, a few students should know that moisture definitely
plays a part (if they've ever left their bike out in the rain, etc.) To hypothesize about
corrosion, students must first be familiar with the simplest definition of
steel as an iron-based carbon alloy (containing up to approx. 2%
carbon.) Let them examine firsthand some partially corroded iron and steel
objects; demo objects should be drawn from a variety of environmental
conditions (rust on an "indoor" object such as scissors; rust on an
object often left "outdoors," such as a garden tool; a picture of
cars rusting in a junkyard etc.) After examining the objects, students should
at least be able to hypothesize that rust might be related to the
environmental conditions surrounding a steel object. Experimenting
with steel requires comprehension of the term alloy. According to the Metals
Handbook, 8th ed., published by the American Society for Metals it
is," A substance having metallic properties and being composed of two or
more chemical elements of which at least one is an elemental metal."
Have several examples of alloys available for students to examine, such as a
brass ring. Steel is "iron-based" so that is its dominant metal;
and carbon up to 2% is the other most common alloying element in steel. Do students think that
"steel" always means the same substance? Go back to the term
"alloy." An alloy is a "mixture" of substances; not a
compound! Chemistry students should be able to differentiate a compound from
a mixture. Calling on what they know about mixtures (constituents
can be mixed in any proportion, separable by physical means), have them
conjecture whether or not all steels are the same (of course, they are not.)
There are many types of steel containing different proportions of the
alloying elements; that is, steels vary widely in composition. For students who have no
difficulty with the above ideas, go on and build on their knowledge of
mixtures, introducing them to the concept of the "microstructure"
of steels. Students should be aware of two mixture types, heterogeneous and
homogeneous, from their previous study of the Composition of Matter. They
know that a heterogeneous mixture is "multi-phased" and that a
homogeneous mixture is termed a "solution" which is the same phase
throughout. Do students think these ideas also apply to the mixtures called
alloys; particularly the alloy steel? Indeed, they do! Not only do steels
vary in chemical composition; the same steel can have either single-phased or
multi-phased forms depending on ( among other things) its thermodynamic
history. According to Noel F. Kennon in a recent book edited by Leonard E.
Samuels called Light Microscopy of Carbon Steels , at normal
temperatures many steels are heterogeneous two-phased mixtures of ferrite
(iron atoms arranged in a body-centered cubic crystal structure) mixed with
cementite ( carbon in the form of the compound, iron carbide, Fe3C).
At higher temperatures, the same steel can transform into a homogeneous
mixture, that is a solid solution, of carbon atoms dissolved in austenite
(iron atoms arranged in a face-centered cubic crystal structure.) (6-9, 29) After
students realize that steels have many different compositions and
microstructures, explain that steels can be classified according to their
compositions by a Society of American Engineers (SAE) number such as 1008,
1045, etc. If teaching a more advanced course, if enrichment is desired, or
if your students can handle the material, at this point the body-centered
cubic crystal structure and the face-centered cubic crystal structure can be
illustrated with models and associated with the phase of steel to which they
apply. (Van Vlack 59 -84) Building on these ideas; that
steels are alloys containing iron, that iron "corrodes" into iron
(III) hydroxide, that steel objects seem to corrode more when moisture
(water) is present, and that not all steels are the same, have students "hypothesize"
factors they think play a role in the corrosion of steel and to what degree
(do some factors have a greater effect on steel corrosion than others?) Have
them "brainstorm" an experimental set-up designed to test their
ideas about steel corrosion. Any experimental design should include the above
ideas. For mainstream chemistry classes, the foregoing ideas will be enough
to handle and test. The discussion on steel microstructure and crystal
structure should be eliminated. Advanced Chemistry Classes and
Gifted Chemistry Classes, however, should be required to understand the
"mechanism" by which steel corrodes. They should call upon the
equations for the corrosion of steel when designing any experiment on
corrosion. Activities or discussions on these topics should be conducted in
second and third lab sessions or during regular classes. "The Selection of Mild Steel
for Corrosion Service," (Metals Handbook 1:257 ) states,
"the corrosion of iron and steel has become accepted as a phenomenon
that is essentially electrolytic, especially where attack depends on the
simultaneous presence of moisture and oxygen." This means that for
corrosion to occur, moisture (water)containing charged particles, an
electrolyte, must be present. Electrons are carried away from the neutral Fe
atoms in steel, forming iron (II) ions, Fe2+ to a substance with a
higher potential to gain electrons. In a sense, this forms a local
"electrochemical cell" on the surface of the steel. (Corrosion in
Action 15) Students who understand the function of and the nomenclature
associated with the electrochemical cell will have a much better grasp on the
corrosion process. Unless students are very comfortable with electrochemistry
and the electrochemical cells, it would be good to spend several class
periods diagramming electrochemical cells; building several simple cells in
the lab. (Petrucci 471 - 490) Students must at least know that
oxidation, electron loss, occurs at the electrode in a cell called the anode
(Fe functions as an anode when it gives up electrons from a steel surface and
corrodes.) Reduction ,electron gain, occurs at the electrode in a cell called
the cathode and normally O2Ý functions in this capacity on the
steel surface. Knowing the process is electrolytic, students hypothesize that
water solutions containing ions, such as salt water, will have a greater
effect on the speed of corrosion than will pure water. Solution type, as well
as steel type, now becomes a factor to be considered in the corrosion
experiment. Consider the electrochemical equations
associated with corrosion. Even in the absence of O2, it
has been demonstrated that ferrous metals will go into solution in water
until a state of equilibrium is reached according to the following equation Fe +
2 H2O = Fe2+
+ 2OH- + H2 Iron water ferrous hydroxide hydrogen gas
ions ions The
ferrous and hydroxide ions result from the single displacement reaction that
reduces H+ in the water to hydrogen gas; which is sometimes
"plated out" on the surface of the metal. If the conditions
are such that the equilibrium constant for this reaction is low
enough and the vapor space small enough, the reaction will
stop before there is saturation with respect to ferrous hydroxide.
("The Selection of Mild Steel for Corrosion Service" Metals
Handbook 8th 1: 257) Chemistry students should be very familiar with
single replacement reactions and recognize the ions involved. (However, be
careful here! My students normally learn that Fe does not replace hydrogen
ion from water at room temp...and indeed the kinetics are against it...but
then it is not customary to use water devoid of oxygen in the high school
chemistry lab.) The corrosion of steel is an
excellent application of LeChatelier's Principle. LeChatelier was a French
industrial chemist and professor particularly interested in problems related
to metallurgy. His principle states that a system in dynamic equilibrium
subjected to a stress will shift in such a way as to remove that stress.
(Wilbraham 502) The presence of iron and water will push the reaction to the
right (in order to use up the reactants) and enhance corrosion; whereas the
excess presence of either product, hydroxide ions, OH -, or H2
gas, should cause the above corrosion reaction to go into reverse, and thus
slow down the corrosion. (The reaction shifts left to remove the excess If O2 is in the water
(as it most always is), then it normally enhances corrosion. Recall, if Fe
acts as the "anode," giving up electrons in the local
electrochemical cell that enables corrosion, then O2Ý can function
as the "cathode," taking on those electrons, because a potential
difference exists between them.(Corrosion in Action 20) Oxygen is reduced
according to the following equation;O2 + 2H2O + 4e-
= 4OH- (Petrucci 484) Oxygen present in solution also
can enhance corrosion in other ways. "Oxygen, if present, reacts with
the hydrogen film (that may build up on the steel surface), removes it from
the surface of the metal in the formation of water and also reacts with the
ferrous ions to form ferric ions in such a concentration as to exceed the
solubility product of ferric hydroxide, and hence a precipitate of ferric
hydroxide is formed. Both reactions are involved in the corrosion
process." ("The Selection of Mild Steel for Corrosion Service"
1:257). An additional equation shows why, as corrosion progresses, the water
around the corrosion turns from yellow to orange to red. As mentioned, the O2
goes on to convert the ferrous ions (yellow in solution) to the
characteristic ferric ions (orange/red in solution) associated with
"rust" : 4 Fe2+
+ O2+ 2H2 O = 4 Fe3+ + 4 Students
apply LeChatelier's Principle and postulate that any O2 present in
solution will drive the reaction forward, enhancing corrosion. Oxygen level
is indeed an important variable in any experiment on the corrosion of steel! Finally, good experimental design
often incorporates indicators as to the reactions being tested. Evidence that
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What will you Need? Materials: Steel Samples (cut in rectangular
solids 1/2" x 1/2" x 1and 1/2") 13 samples of Low Carbon Steel
SAE# 1008 13 samples of Medium Carbon Steel
SAE# 1045 13 samples of Medium Alloy Steel
SAE# 4140 13 samples of Medium Alloy Steel
SAE# 4340 (Suggestion; Mark each block with
Steel SAE# using magic marker) 13 1000mL beakers 26 glass rods (cut to fit bottom
of above beakers) 2 Liters Distilled H2O 2 Liters Saline Solution ( 1M
NaCl, .01 M KBr, .01M KI) 2 Liters .005M H2SO4
(pH =2) 2 Liters 100ppm CaCO3
solution 4 wash bottles (one for each
solution above) 4 berol pipets (one for each
solution above) |
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Procedures: See Pictures #1 & 2 Appendix A 1.
Mark 3 beakers "Distilled Water" (A
Freshwater Environment) Additionally,
label the first beaker "Minimum O2", second
"Dissolved O2", and a third "Max O2." 2.
Mark 3 beakers "Saline Solution" (A Marine Environment) Additionally,
label the first beaker "Minimum O2", second
"Dissolved O2", and a third "Max O2." 3.
Mark 3 beakers "H2SO4 solution, pH =2" ( A
Heavy Industrial Environment) Additionally,
label the first beaker "Minimum O2", second
"Dissolved O2", and a third "Max O2." 4.
Mark 3 beakers " 100 ppm CaCO3 solution" (A Hard water
Area) Additionally,
label the first beaker "Minimum O2", second
"Dissolved O2", and a third "Max O2." 5.
Mark the last beaker "Open to the Atmosphere/ Dry Conditions." 6.
Arrange beakers on a table in groups by solution type ( Distilled,
Saline, H2SO4, 100ppm CaCO3.) Additionally
arrange beakers within each group by increasing O2 level: Minimum
O2, Dissolved O2, Maximum O2 The
effect of both solution type and oxygen level will be tested. 7.
Sit the "Dry Conditions/ Atmospheric Exposure" beaker by itself. 8.
Place two of the glass rods horizontally in the bottom of the "Dry
Conditions" beaker and arrange the four types of steel in order of
increasing SAE # 1008, 1045, 4140, 4340. * See Picture #3 These
will serve as the experimental control. Any corrosion due to interaction
between normal atmospheric conditions and specific steel type should show up
here. Also, sure that the initial
depth of each sample or the initial mass of each sample is marked on a folder
assigned to each beaker prior to the experiment if you plan to use initial
values in the calculation of extent corrosion. * See Measurement of
Variables 9.
Place two glass rods horizontally in the bottom of each remaining beaker. 10.
As mentioned, prepare a manila folder marked "Distilled
Water/ Minimum O2." Remove the four steel samples from
this folder. In the beaker marked "Distilled Water/Minimum O2"
arrange these samples in order of increasing SAE # 1008, 1045, 4140,
4340. 11.
Proceed in the same manner as # 8 and #10 above for each of the remaining
beakers, making sure to match the proper set of steel samples from the manila
folders to the correct conditions under which they shall be tested as marked
on the beaker. |
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Learning Outcomes At the conclusion of these laboratory
experiences (dependent on sophistication of approach) students should be able
to do some or all of the following: 1.
set up and conduct a multivariable experiment with appropriate control,
systematically gather and classify data, and analyze data for any dominant
trends therein. 2.
describe steel as an iron-carbon alloy containing up to approx. 2% carbon
(and always traces of manganese). It can also contain several other elements,
called alloying elements, added to affect its properties. (Samuels 29-30). 3.
indicate that there are many types of steel with great variations in their
microstructures; these being dependant on the initial composition of the
steel and how the steel was processed both thermodynamically and
mechanically. ("Compositions of Standard Steels." and "Hot
Finished Carbon Steel." Metals Handbook. 8th ed., Vol 1: 61-63) 4.
show a general awareness of the range of microstructures that can occur in
steels and be able to describe several of these. 5.
describe, using a body-centered cubic crystal model, the most common
microstructure of many steels at normal temperatures. This is essentially a
two-phased heterogeneous system consisting of ferrite ( iron atoms arranged
in a bcc array) mixed with carbon In some from of an iron carbide compound,
Fe3C, called cementite. (Samuels
6-9) 6.
describe, using a face-centered cubic crystal model, another microstructure
that can occur in steels (at elevated temperatures.) This is a one phase,
homogenous mixture or a "solid solution" of carbon atoms
"dissolved" in austenite (iron atoms arranged in a fcc array.)Ý The
carbon atoms are found in the interstitial spaces (spaces in the crystal
structure between the iron atoms.) ( 6, 29) 7.
explain that the corrosion of steel is an electrolytic process in which
electrons are transferred away from the Fe in steel through a solution
containing charged particles, called electrolytes. ("The Selection of
Steel for Mild Corrosion Service." 1: 257) 8.
define an oxidation half reaction as one in which electrons are lost 9.
define reduction half reaction as one in which electrons are gained 10.
describe an electrolytic process as one in which electrons will flow from an
area of high charge density to an area of low charge density through an
electrolyte; a moist interface containing charged particles. 11.
describe the components of a "half-cell" as a metal strip called an
electrode immersed in a solution of its own ions 12.
construct a simple electrochemical cell (also called a voltaic or galvanic
cell) by connecting two "half-cells" to a voltmeter ( after placing
a salt bridge between them.) Give evidence that electrons are flowing between
the two half cells. 13.
identify the parts of the electrochemical cell ( anode where oxidation
occurs) and cathode (where reduction occurs), electrolyte, salt bridge, etc. 14.
use the Standard Reduction Potential Table to explain why electrons flow
between the anode and the cathode in the electrochemical cell constructed.
Connect this to emf. 15.
explain corrosion on the surface of a piece of steel in terms of an electrochemical
cell. 16.
give the equations which define corrosion in steels and explain how each
reactant and product affects the corrosion reaction in terms of LeChatelier's
Principle. 17.
explain the design of an experiment to test the environmental factors that
affect the rate of corrosion in steels and tell why each environmental factor
is included in the experiment. 18.
in terms of what is known about steel corrosion, hypothesize as to the
results of this corrosion experiment; both as to steel type and as to
specific environmental conditions. 19.
define "galvanic couple" in terms of the electrochemistry of
metals. 20.
explain what is meant by steel acting "anodic" and what is meant by
steel acting "cathodic." 21.
explain how steel can be made to act "cathodic" and what is meant
by "cathodic" protection. 22.
tell what indicator you would use to test for steel areas acting
"anodic" and forming Fe2+ ions; and what color you
expect 23.
tell what indicators you would use to show that steel is acting "cathodic"
and what color to expect 24.
explain why the potential on stainless steel should differ from that on low
carbon steel, and describe an experiment that would show this |
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Sample Results EXPERIMENTAL
RESULTS Qualitative Data
Grid (Observations) Corrosion
of Steels
SAE# 1008,
1045, 4140, 4340 Observations Date______________________ Time _____________ Solution Type Minimum O2 Dissolved
O2 Max
O2 (sealed) (wet/dry) Saline orange/red deposit thick red deposit
film of red/org on bottom (1mm) on bottom (5-6mm) susp on bott thick(3mm) dep
on thick, furry ,drk red 1008(blk pts top of all
blks, furry corr 1008(most)-1045
4140-1045- red/orange
on sides 4340-4140(least)
4340(least) solution:
yell/or particles solution:org/ ppt red Acidic__________________________________________________________________ pH=2 bottom clear orange deposit slight H2SO4 4140
bubbles on surface bottom (1mm) org chnk no corr-4340 few bub blks covered thin or under 1008 slight
corr-1045(no bub film 1008
(most) 4340- top
(sl corr)-1008 lght
1045-4140(least) Orang cor yell
top (most corr) solution:
clear/or top 1008- solution:
clear/yell tinge fine or deposit on top 4140-4340 _ -1045 (no) Basic___________________________________________________________________ 100ppm white/or deposit deep or/red depost white/org CaCO3 bottom bottom under
1008 thk org
"icing" thk
corr 1008- 4140
most cor all
blk, esp 4140 4140-1045-4340 then 1008 clear yellow sol clear or sol white powder (suspended
org part top) top1045,4340 Distilled________________________________________________________________ Water fine
or powder thick red deposit flecks of 4140 mold-like
furry furry or needles most, then1008 1008,4140
most 4140 most,
oth medium Quantitative Ultrasound Measurements
Index to Corrosion: Sum of depth lost
on a 3-Point Test
Instrumental Uncertainty
Values x
10-4 in +/-
3 x 10-4 in Minimum
Oxygen Level ( Immediately Sealed) 1008 1045 4140 4340 Saline 13 8 22 7 pH
=2 4 11
4 4 Sulfuric
Acid Basic ppm 100 9 9 6 17 Calcium
Carbonate Distilled 12 28 0 7 Ultrasound Measurements Index to Corrosion: Sum of depth
lost on a 3-Point Test Instrumental
Uncertainty
Values x 10-4 in +/-
3 x 10-4 in Dissolved Oxygen Level (Liquid
Maintained at 400mL Level) Oxygenated & Stirred Daily) 1008 1045 4140 4340 Saline 6 23 0 0 pH
=2 3 16 9 10 Sulfuric
Acid Basic ppm 100 3 24 0 6 Calcium
Carbonate Distilled 8 4 0 2 Ultrasound Measurements Index to Corrosion: Sum of
depth lost on a 3-Point Test
Instrumental
Uncertainty
Values x 10-4 in +/- 3 x 10-4 in Maximum Oxygen Level (Bathed
Daily in Given Solution) Wet/Dry Interface 1008 1045 4140 4340 Saline 38 12 0 13 pH
=2 22 16 2 1 Sulfuric
Acid Basic ppm 100 0 13
1 0 Calcium Carbonate Distilled 21 6 0 6 |
ÝÝÝ |
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Other Data/Information INTERPRETATION OF
EXPERIMENTAL RESULTS (CONCLUSIONS) FEEDBACK FOR FUTURE EXPERIMENTS After conducting this
experiment, several conclusions were immediately apparent. First off, from
a "qualitative" standpoint, the experiment was indeed a success.
More viable data can be gleaned "visually" from the observations
made during this experiment than would at first appear to be the case. For
example, steel type 4140 began to corrode in the distilled water, max oxygen
(wet/dry interface) environment on the second day of the experiment;
progressing rapidly from the yellow Fe(II) initial corrosion stage, through
the "intermediate orange" Fe(II) and Fe(III) mix stage, to the
reddish Fe(III) hydroxide stage within the duration of this two week
experiment. On the other hand, steel type 4340, in the same environment, took
a week before showing any signs of corrosion and hardly progressed through
the light yellow Fe(II) stage in two weeks. High school students could make
such observations and record as quantitative data the number of days it takes
for a steel type in a given environment to show any corrosion at all; the
number of days only yellow corrosion is present; the day on which orange
corrosion first appears (such as 7th or 8th) etc. Looking up the compositions
of steels 4140 and 4340 and relating these compositions to the recorded
experimental observations is an exercise in connecting the external
properties of materials with their internal compositions. The compositions of
these steels are as follows: (Ladle chemical composition limits %)(Metals Handbook, 8th
ed 61) SAE 4140
.38-.43%C
.75-1.00%Mn .040%P .040%S .20-.35% Si 0% Ni .80-1.10%Cr
.15-.25% Mo 0%V 4340
.38-.43%C
.60-.80% Mn .040%P .040%S .20-.35% Si 1.65-2.00% .70-.90% Cr .20-.30%Mo 0% Ni Both steels are considered
to be medium-carbon alloy steels and their compositions are quite similar.
The one component noticeably different between 4140 and 4340 is the nickel
content; 4140 has none! Analyzing the data drawn from experimental
observations that in distilled water (wet/dry interface) steel 4140 corrodes
much faster and to a greater extent than steel 4340, it would not be
unreasonable to conclude that Ni as an alloying element definitely must play
some role in retarding the corrosion of steel; at least in that specific
environment. Nor are such conclusions the
extent to which the "qualitative" approach can be taken. Students
have ample experimental data to go on and access observations about the corrosion
of 4140 and 4340 in all the other solutions tested (keeping the wet/dry
interface constant.) The data shows that in basic solution and in acid
solution, the same is the case; 4140 corrodes earlier and to a greater extent
than 4340. But just when students are sure that the Ni content difference
provides corrosion protection in every solution, observations show this is
not the case in saline solution! However, the fact that 4340 seems corrosion
protected in the three or the four solution types, must indicate that Ni
alloying at least often retards corrosion in the wet/dry scenario. Because this experiment has so
many aspects to it, students can analyze the data further and check on the
corrosion behavior of 4140 and 4340 at oxygen levels other than max (wet/dry.)
No matter whether the solution is sealed from the atmosphere (low ox) or open
to the atmosphere at 400mL line, in basic solutions or distilled water, 4140
is still the most corroded and 4340 the least or close to it. The conclusion
that the effect of alloying with Ni retards corrosion still holds credence. However,
when the acid and saline solutions are checked, a remarkable trend
surfaces...4140 is the least corroded at any oxygen level in the harsh saline
and acid environments! Do these environments actually interact negatively
with the presence of Ni so as to overcome its corrosion retarding effects; or
does Ni do something to the microstructure of the steel so that it is
actually "softer" than 4140 and therefore more susceptible to
attack in the harsher environments of salt and acid? Students can investigate
all the topics and trends generated by this one experiment alone! Qualitatively,
a few pleasant surprises even occurred. The most striking of these was
observing all four types of steel sitting in pH 2 sulfuric acid (sealed)
for ten whole days before a sign of corrosion appeared on any of them...not a
tinge of yellow! These steel samples were covered with gas bubbles from the
onset of the experiment. Even if not a shred of "quantitative" data
was derived from this experiment (and that is just about the case as will be
later discussed), a high school class could be given the corrosion equation
under "low oxygen" conditions (see page 7 of this paper) and asked
to explain the observed phenomenon. Students could be asked to think in terms
of the H+ ion abundant in acid conditions and directed to what they know
about the Reduction Potentials between Fe and H+. The potential to be reduced
is 0.000ev for H+ and - .440 e for Fe (Petrucci 476) showing that Fe will
give up electrons to H+. Students could be asked to explain the origin of the
bubbles surrounding the steel samples as well as the identity of the gas in
the bubbles ( H+ ion is reduced to hydrogen gas which appears as bubbles on
the steel when the Fe in steel is oxidized.) Using the corrosion equation
under "low oxygen" conditions, students should explain, via
LeChatelier's principle, why hydrogen bubbles clinging to the surface of
steel will retard corrosion. Specifically, "Where there is no
electrolytic coupling, the film of hydrogen will be on the surface of the
iron or steel and, in the absence of oxygen, will hinder further corrosion by
insulating the metal from the water and by its own tendency toward a back
reaction. The formation of such a gas film on metal is called
'polarization'." (" The Selection of Mild Steel for Corrosion
Service" Metals Handbook, 8ed. 1: 257 )*See Picture #6 (Compare
saline/front with acid (hydrogen bubbles)/back) It is of interest to
note the connection, again, between steel composition and the formation of
hydrogen bubbles on the surfaces of the steel types in the sealed acid
solution. Bubbles formed fastest and in greatest amount around steel type
4140! This was the only steel type to retain all of the bubbles for the
duration of the experiment and the only steel type to show no corrosion.
Students can go on to investigate connection between resistance to corrosion
in acid and the 4140 composition. Hydrogen
bubbles initially formed around all the steel samples in the acid solution
open to the atmosphere. They did not start to corrode until the 3rd day of
the experiment; even sitting in acid. Corrosion started on some of the blocks
when the bubbles disappeared. Students can be asked to explain the
observation that bubbles on the samples in the solution open to the
atmosphere finally disappeared; whereas the bubbles stayed on the steel
samples for two weeks in the acid sealed from the atmosphere. Here again, a
simple qualitative observation leads to another avenue of investigation. Most
likely, the oxygen initially dissolved in the sealed acid container was
expended; the acid solution open to the atmosphere and stirred continued to
bring oxygen to the surface of the steel. The oxygen removed the hydrogen
film and corrosion proceeded. "Oxygen, if present, reacts with the
hydrogen film, removes it from the surface of the metal in the formation of
water and also reacts with the ferrous ions to form ferric ions......Both
reactions are involved in the corrosion process;" ( 1: 257) See picture
#7 Finally, when the parafilm was
removed from the sealed acid beaker, little bubbles were observed all over
the surface of the water and clinging to the under surface of the parafilm
covering. Could the vapor pressure of the hydrogen gas being released above
the acid solution in the sealed container have increased to the point that it
created a back pressure of hydrogen on the liquid surface and therefore held
the remaining hydrogen bubbles in solution? High school chemistry students
could make that observation from this experiment and be asked to explain it
in terms of Henry's Law ( the solubility of a gas in a liquid is directly
proportional to the pressure of that gas above a liquid.) Within
the analytical treatment of the qualitative observations generated by this
experiment, lie many more investigations and experiments. Not a few concepts
learned in high school chemistry can be drawn upon or reinforced with
analysis of the qualitative data alone. The goal of this project has been
reached; using the study of steel and its composition to generate meaningful
activities and laboratory experiences for the high school chemistry
classroom. As for the "quantitative" data generated by the ultrasound
measurements in this experiment; it was basically meaningless. The
limited time frame in which the experiment had to be conducted was from the
outset a major concern. The experiment was run during the summer for little
more than two weeks. This was sufficient to generate a wealth of qualitative data
and to illustrate many chemistry concepts as discussed above; but it was not
enough time for corrosion to remove a measurable amount of material; at least
in terms of the ultrasound equipment being used. Any mass loss would have no
doubt been equally, if not more so, imperceptible. It should be noted,
however, that the ultrasound equipment being used was not designed to do
"corrosion mapping"; this type of equipment would have given a
digital map of the surface of each steel sample. However, that level of
instrumental sophistication is not within the range of most high school
budgets; and this was an attempt to simulate an experiment for the high
school chemistry class. Three- point
test depth readings were taken on each of the 52 steel samples prior to and
after conducting the experiment. The ultrasound gage was calibrated against
the four steel samples that served as the control; determining the velocity
of sound in each type of steel. All steel samples of a given steel type were
measured before moving to a different steel type and depth readings were
recorded to the ten- thousandths of an inch. The initial and final depths of
each sample are not recorded herein, but only the sum of the depth loss
over the 3-point test for each sample; this was considered to be an index
to the extent of corrosion on each block. The total depth lost for
each sample in each solution and at each level of oxygen is shown on pages 17
- 19. The following is an analysis of that data. The "uncertainty" on any one measurement taken with the
ultrasound equipment being used was +/- 3 ten -thousandths of an inch. This
means that the uncertainty on one depth measurement on each block (final -
initial depth measurement) was +/- 6 ten-thousandths of an inch as
uncertainties are additive when subtracting. The "index to
corrosion" was the sum of the depths lost at three places on the block;
and again, since uncertainties are additive, the uncertainty on each
answer was the sum of the three uncertainties for each depth measurement
or +/- 18 ten-thousandths of an inch! This
means that if there was no change in depth, i.e., the depth change was 0 (no
corrosion) it could conceivably show up as a loss of 18 ten-thousandths of an
inch; and if a block actually lost 18 ten-thousandths of an inch (by
corrosion), it could conceivably show up as losing nothing! So unless the sum
of the depth lost is considerably more than the uncertainty, the value in
this case is basically an index to nothing! And when only 7 data pieces out
of 52 are greater than the uncertainty on their values, it appears that an
instrument of much more precision must be used before any valid conclusion
can be drawn as to depth loss due to corrosion. Now the data is beguiling; it
at first appears to have some plausible trends in it. It is true that some of
the highest values of total depth lost appear in the max oxygen (wet/dry) set
of data especially for steel type 1008. Yet at this same oxygen level in
basic conditions, 4140 is the only steel sample that developed dark orange
corrosion and it is shown as having lost only 1 ten-thousandths of an inch,
while a sample of 1045 steel, coated with calcium carbonate deposit and still
shiny when this was removed after two weeks and showed only a tinge of
corrosion, supposedly lost 13 ten-thousandths of an inch! The ultrasound data
just is not valid for this particular experiment. The
upshot of this is that the corrosion experiment as such should either be run
from six months to a year in order to generate measurable quantitative data
or run for the two-week duration and used qualitatively. In the high school
chemistry curriculum, qualitatively is no doubt the more reasonable and
practical choice! |
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