A Better Understanding of
Chemical Deicing
Facts and Myths
Scott Koefod
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1
Goals
• Take the mystery out of chemical deicers
• How do deicers work?
• What makes a deicer work better at lower
p
temperatures?
• What is the eutectic and what does it tell us?
• What do deicer freezing point curves tell us?
• How effective are corrosion inhibitors?
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How do we make ice melt?
?
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1
Method 1: Raise the ice temperature
above the freezing point
Temp = 40 0F
Freezing Point = 32 0F
Temp = 20 0F
Freezing Point = 32 0F
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Method 2: Lower the freezing point below
the ice temperature!
Temp = 20 0F
Freezing Point = 32 0F
Temp = 20 0F
Freezing Point = 15 0F
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All deicers work the same way
ice
•
•
•
•
water
Deicer molecules “interfere” with water molecules
Makes it harder for them to lock together and freeze
Freezing point drops
The more deicer dissolves, the more the freezing point drops
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2
How can we control how much we
drop the freezing point?
• Easy!
• More chemical dissolved = more
“interference” = lower freezing point
• Lowest freezing point is limited by how much
chemical will dissolve
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Freezing Point = 32 0F
Freezing Point = 22 0F
+
Freezing Point = 12 0F
+
Freezing Point = 2 0F
+
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Sooner or later no more salt will
dissolve – liquid is saturated
++
F.P. = - 6 0 F
The freezing point lowering from salt bottoms out
Salt cannot possibly melt ice below -6 0 F
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3
Liquid NaCl
40
30
20
10
Eutectic = lowest
possible freeze point
a deicer can give
0
Liquid is saturated
23% NaCl @ -6 0F
-10
-10
0
10
20
% NaCl by Weight
30
40
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50
10
Some chemicals can lower the freezing
point more than others
Deicer Eutectic Temperatures
20
magnesium calcium
chloride
chloride
salt
Potassium
acetate
0
20
-20
-40
-60
-80
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Not all deicers are created equal
salt
CaCl2
Ice Melting Capacity
8
7
6
5
4
3
2
1
0
25
15
5
-5
Temperature (degrees Fahrenheit)
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-15
-25
12
4
Fact: deicers work by lowering the
freezing point of water
• Myth: the lower the freezing point of a liquid
deicer, the better it will work at low
temperatures.
• From wikipedia: “Deicing fluids work best
(surprisingly) when they are diluted with
water.”
• An anti-freeze will work best when diluted,
but not an ice-melter.
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+
1 tablespoon salt melts ~ ½ cup ice
+
+
Dissolve that salt in ½ cup water first,
And the “liquid deicer” can’t melt any more ice.
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14
+
So – we have to add more salt to our liquid
deicer to make it melt any ice
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5
+
+
The more salt we add, the more ice it can melt
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Calcium Chloride So lution
60
40
20
0
-20
Best working concentration
= 30%?
-40
-60
0
10
20
30
40
50
% Calcium Chl oride
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Ice Melting Capacities 15 degrees F
After 60 Minutes
2
Highest concentration provides
most ice melting capacity
1.5
Eutectic
dilution
1
0.5
0
44.5
35
29.6
% Calcium Chloride
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20
10
18
6
How do we understand this?
• Need to be careful with freezing point charts!
• Important thing is freezing point of water (to
form ice)
• Above eutectic concentration, the “freezing” is
not of water
• It is excess deicer chemical crystallizing
because liquid is saturated!
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Liquid NaCl
40
30
20
Freezing point of
water drops as more
salt is dissolved
10
“freezing” is excess
salt crystallizing from
saturated liquid
0
Liquid is saturated
no more salt can dissolve
f.p. of water no
longer changing
-10
-10
0
10
20
% NaCl by Weight
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30
40
50
20
So what happens if we put “too
concentrated” liquid on cold road?
Excess solid deicing chemical crystallizes out
Will solid deicer still melt ice? Of course!
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7
Deicer dilution can have some value
• Better low temperature storage stability
– 26.4% salt brine “freezes” at 32 0F
– 23.3% salt brine stable to – 6 0F
• But higher concentrations will always give
more ice melting capacity
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Fact: certain additives can lower the
freezing point of chloride deicer brines
Myth: this will make the brine melt
ice to lower temperatures
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Freezing Point
Ice Melting Capacity
31% MgCl2 Brine + Organic Liquid Additive
1.4
10
1.2
0
1
-10
0.8
-20
0.6
-30
0.4
-40
0.2
-10
0
10
20
30
40
50
60
% Organic Liquid Additive
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8
Why didn’t it work??
• Some organic additives (e.g. sugars, glycols,
certain proteins) behave as “cryoprotectants”
• Cryoprotectants inhibit the freezing of water
Used in ice cream to prevent ice crystals, they occur
naturally in arctic fish, plants, etc.
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Cryoprotectants do not work the same
way as deicers
• Cryoprotectants inhibit freezing by slowing it
down immensely
• They do not cause a true drop in the actual
freezing point of water
• They have a “glass transition temperature”
rather than a freezing point
• They can inhibit freezing, but can’t melt ice
• They can “fool” the freezing point test
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How do we measure freezing points?
Standard method for deicers is
ASTM D1177
Cool liquid at a steady rate
Look for break in cooling curve
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9
Plain Deicer Brine
No Additive
70
60
50
40
Well defined freezingg
point = 19 0F
30
20
10
-5
0
5
10
15
20
25
30
Time (minutes)
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plain brine
brine + 25% organic liquid
ASTM D1177 Cooling Curves
80
60
freezing point = 19 0F
40
20
0
-20
freezing point = -35 0F??
-40
-5
0
5
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15
Time (minutes)
20
25
30
29
Freezing Point vs. “Glass Transition”
Normal
f.p. curve is
“V” shaped
“glass transition”
Curve has no “V”
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10
MgCl2 Brine + 25% Organic Liquid
MgCl2 Brine + No Additive
Deicer Freezing Point Curves
40
20
0
Plain MgCl2 brine
shows
well defined eutectic
“V”
shaped curve
-20
Addition of organic
liquid causes typical
glass transition curve
-40
-60
0
5
10
15
20
% Magnesium Chloride
25
30
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31
Potential Organic Additive Value
•
•
•
•
Improved residual effect
Corrosion inhibition
Leaching inhibition
Lower temperature ice melting???
– Unlikelyy
– Must be verified by a performance test
– Lower liquid “freezing point” does not indicate
lower temp ice melting
– ASTM D1177 is not accurate for f.p. of liquids
containing cryoprotectant type additives
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“Exothermic Deicers”
• Calcium chloride is a well known “hot”
deicer
• How does it work?
• How important
p
is the heat released for
ice melting?
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11
When chemicals dissolve, they either release
heat (get hot)
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Or they absorb heat (get cold)
+
urea
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Heat Released by 1 Pound Solid Deicer
800
Magnesium
Chloride (pure)
600
400
Calcium
Chloride
(pure)
200
Magnesium
Potassium Chloride
Acetate
(flake)
salt
urea
0
-200
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12
How Important is the Heat Released?
Ice Melting Capacity at 25 degrees F
8
salt
7
6
Calcium
Chloride
5
4
3
2
0
10
20
30
40
50
60
70
Time (minutes)
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Heat Released in Perspective
• Calcium chloride is an excellent ice melter
• The “hot” contribution is relatively unimportant
• Remember – the environment is an enormous heat
sink
– Like trying
y g to heat an entire house with a candle!
– Heat released by deicer dissipates very quickly!
• Also – liquid deicers have no heat to release
– Once dry chemical dissolves, heat is quickly lost
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Fact: corrosion inhibitors reduce the
corrosiveness of chloride deicers
• Caution: this must also be understood in
perspective
• Important
I
t t to
t understand
d t d corrosion
i tests
t t
• At best corrosion tests provide a relative
ranking of corrosiveness for a specific
exposure condition
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13
Corrosion Rates of Deicer Solutions (0.513 M)
PNS Test Method
70
60
50
40
30
20
10
0
H2O
1
NaCl
2
CaCl2
3
MgCl2
4
KCl
5
urea
6
KAc
7
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Current Industry Corrosion Standard
• PNS Test Method
• Simulates a moderately aggressive exposure
condition
– 10 minutes wet/50 minutes “aeration” cycle
– Simulates repeated deicer brine “splash”
splash on
exposed mild steel
– Cannot be extrapolated to other metals
– Cannot be extrapolated to concrete rebar
• Standard spec is at least 70% less corrosive than salt
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PNS Corrosion Tester
ProvidingCustom
ersW
ithDeicingSolutions
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14
PNS Corrosion Tester
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Distinguishing PNS Categories 1,2, and 4
• PNS categories 1 and 2 are for liquid
magnesium and calcium chloride.
– These are commonly used to pre-wet salt
• PNS category
g y 4 is for corrosion inhibited solid
salt
• Pre-wetting liquid specs generally require
meeting PNS specs 1 or 2
• Blending a PNS 1 liquid with salt does not
necessarily make a PNS 4 deicer!
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PNS Corrosion Test
Two Commercial MgCl2 + OBPE Products
100
Meets PNS
Spec
Meets PNS
Spec
80
60
Fails PNS
Spec
Fails
F
il PNS
Spec
40
20
0
Liquid
Treated
Salt
Liquid
Treated
Salt
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“Simulated Equipment Corrosion” Test
• We have developed our own test to estimate
equipment corrosion
• Steel coupons directly immersed in piles of
treated salt
• Salt pre-wetted with corrosion inhibited liquid
~ 70% less corrosive than salt wet with plain
salt brine
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Corrosion Conclusions
• All chlorides will be aggressive to mild steel –
not a huge difference between them
• Corrosion inhibited chlorides probably always
more corrosive than non-chlorides
• Corrosion
C
i iinhibited
hibit d d
deicers
i
offer
ff a middle
iddl
ground option
• Corrosion protection of standard pre-wetted
products is probably primarily restricted to
equipment
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Questions?
• Feel free to contact me with any
technical deicer questions
• Scott Koefod, Principal Scientist
• Cargill
g Deicing
g Technology
gy
• Phone: 952-742-5670
• Email: scott_koefod@cargill.com
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