No. 307
September
2003
Published monthly by
Public Relations Center
General Administration Div.
Nippon Steel Corporation
More about Nippon Steel
http://www.nsc.co.jp WWW
Steel-Canvas Series by Hiroji Noda
“Taiwa—Aki-no-Kaori
(Dialog—The Scent of Autumn) ”
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(Born in 1952, his art combines abstract painting with
other materials. He was awarded the 2001 Newcomer Prize
by the Minister of Education, Culture, Sports, Science
and Technology.)
In this issue
Feature Story
The Genesis of Product Making
Efforts to Combat Rust (Two-part Series: 1)
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No. 307 September 2003
Feature Story
Efforts to Combat Rust
(Two-part Series: 1)
Why does the steel rust? Twenty-one percent of the air
is oxygen. This is why it is virtually impossible for any
metal to exist in its pure form. Metals combine with the
oxygen in the air to form oxides. Iron exists as iron
ore, an oxide, in its natural state, and steel is produced
by reducing the iron ore by using coke. The resulting
steel tends to react again with the oxygen in the air to
create rust and return to its natural state (Fig. 1).
Nippon Steel has addressed the long-standing challenge of preventing rust by developing new metallic
coating and numerous other technological innovations. This two-part series (issue nos. 307 and 308)
highlights coating mechanisms, now the mainstream
approach to rust prevention, and Nippon Steel's worldclass technologies in this field.
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No. 307 September 2003
Feature Story
Metallic Coatings to Protect Steel from Rust
History of Metallic Coatings—"Makeup" for Steel
In order to protect steel from rusting, metallic
coatings serve as "makeup" for the surface of
steel material. The most typical metallic coating
is galvanizing, or zinc coating, and goes back to
the early 1740s. This was when the high-volume
production of zinc ingots became possible in the
United Kingdom owing to improvements in zinc
smelting process and the galvanizing method
was invented in France. Steel by nature tends to
return to an oxide in the air. An iron oxide film
forms on the steel surface before the steel
reaches the coating process. This makes it difficult to deposit molten zinc on the surface. To
solve this problem, a flux (salt) was applied to
the surface before the steel materials were immersed in molten zinc. This hot-dip galvanizing
(flux) method was invented in 1837 and is the archetype of today's hot-dip continuous galvanizing.
The flux method is suited to sheet-by-sheet
galvanizing, but does not lend itself to continuous production. A new method was devised in
1931 whereby cold-rolled coils were continuously
heated at high temperature and reduced by hydrogen to clean the surfaces. This innovative
technique is known as continuous hot-dip galvanizing, or the Sendzimir process. Nippon Steel
introduced this method from 1953 to 1954.
Electrochemical reaction
with water and oxygen
H2O Water
H2O
O2 Oxygen
2e−
Fe2+
(Metallic ion)
Dissolution of
metallic ion
Fe Iron
Electrochemical reaction occurs between iron, water and oxygen, resulting in the dissolution of iron ions.
H2O
O2
FeOOH
Rust
Fe Iron
As the reaction proceeds, iron oxide (FeOOH) forms at the
part. This is the rust.
Beginning of Coating in Japan
The use of metallic coating in Japan is said to
have originated in gilding of cupreous Buddhist
images during the Asuka Era (593-686). In the
Asuka Era, an alloy (amalgam) of mercury and
gold was applied at room temperature over a
Buddhist image. The entire coating was then hea-
Fig. 1 Corrosion Mechanism of Steel
ted so as to evaporate only mercury, which left the
image covered solely in gold. This is said to be
the start of metallic coating in Japan.
Thereafter, this method of gilding was used extensively in the construction of shrines and Buddhist temples in the Heijo capital (modern day
Nara) of the eighth century, and 450 kg of gold
was used for gilding activities of the great image at
the Todaiji Temple. Some say that the relocation of
the capital from Heijo to Heian (Kyoto) was attributed to the health damage of the local residents
caused at that time by the evaporation of mercury.
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No. 307 September 2003
Feature Story
Surface Treatment Mechanisms
Types of Treatments
Coating is one of surface treatments to prevent
corrosion of materials. There are four major types
of surface treatments—metallic coating, inorganic
coating, organic coating and conversion treatment. Typical metallic coatings used to protect
steel against rust are hot-dip coating and electrolytic coating.
The films available for coating can be divided
into two major types: "sacrificial protective film"
(Fig. 2) and "barrier-type protective film" (Fig. 3). In
the former case, steel materials are coated with
zinc, aluminum or another metal that more readily
oxidizes and dissolves into water than does iron.
The metallic coating dissolves selectively to the
base steel, thereby inhibiting corrosion of steel. In
the latter case, lead, tin or another metal that corrodes less easily than iron is used to coat the steel
surface, thereby isolating it from water and oxygen. In the case of barrier protective films, scratches and other defects in the protective film can
cause red rust of the base steel. Strict quality control of the coated film is a primary consideration.
Types of Surface Coating
Fig. 2 Mechanism of Sacrificial Protection
Metallic coating
Zn2+
(Metallic ion)
Hot-dip coating (Zn, Al, Pb, Zn-Fe, Al-Zn, Sn etc.)
Electrolytic coating (Zn, Ni, Cr, Cu, Sn, Au, Zn-X etc.)
Electroless plating (Cu, Ni, Sn etc.)
Zn Zinc
Water+O2
Zn2+
Coating layer
Dry coating (PVD [Vapor-deposition, IP, SP], CVD)
Spraying
Fe Iron
Cementation
Inorganic coating
Ceramic coating
Glass lining
Steel material is coated with the metal (zinc, aluminum etc.)
that oxidizes and dissolves more readily than iron. The metallic
coating dissolves before the base steel does, thereby protecting the steel from corrosion.
Enameling
Organic coating
Fig. 3 Mechanism of Barrier-type Protection
O2 Oxygen
Painting
Laminating
H2O Water
Resin lining
Conversion treatment
Chemical treatments (phosphate, chromate, oxidizing)
Metallic coating
Coating layer
Anodic oxidation (Al, stainless steel, Ti)
Fe Iron
The metal (lead, tin etc.) that corrodes with more difficulty than
iron is applied on the surface of the steel material, thereby isolating it from water and oxygen.
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No. 307 September 2003
Feature Story
Major Reasons for the Extensive Use of Galvanizing
The following major elements are arranged in order of the ease with which they dissolve in water
(or their tendency to ionize): potassium (K), calcium (Ca), sodium (Na), magnesium (Mg), aluminum (Al), zinc (Zn), iron (Fe), nickel (Ni), tin (Sn),
lead (Pb), hydrogen (H), copper (Cu), mercury
(Hg), silver (Ag), platinum (Pt) and gold (Au).
While the elements preceding hydrogen—from
potassium to lead—are more readily soluble in
water (or easier to oxidize), the elements that follow hydrogen—from copper to gold—are more
stable than hydrogen and more difficult to oxidize. Iron belongs to the group that more readily
oxidizes (Fig. 4).
A major reason why galvanizing is now widely
used as a protective coating for steel is that zinc
dissolves into water faster than iron and protects
steel by what is known as "sacrificial action."
Since zinc's rate of corrosion is smaller than that
of iron, a small amount of zinc can protect iron
over an extended period. In addition, zinc's low
melting point (419 degrees C, compared to aluminum's 660 degrees C) permits hot-dip galvanizing to be performed with less energy. In the
case of electrolytic coating, aluminum and magnesium, which are more soluble than iron, cannot become metals because of excessive
electrolysis of water and cannot be used as coating materials. In light of this, too, zinc is suited as
a coating material.
Fig. 4 Comparison of Ionization
Tendency among Elements
Smaller ionization tendency
Au Gold
Cu
Copper is more stable than hydrogen.
Being stable, copper is hard to be oxidized.
Copper
H2 Hydrogen Standard
Fe Iron
Iron ionizes more readily
than hydrogen.
Iron is easy to be oxidized.
Zn Zinc
Al Aluminum
Mg Magnesium
Mn+
(Metallic ion)
Greater ionization tendency
Hot-dip and Electrolytic Coatings
There are two metallic coating methods: hot-dip
coating and electrolytic coating.
In the hot-dip coating, a metal coating is deposited on the surface of the base material by
immersing it in a molten coating metal. The hotdip process not only performs surface cleaning
and coating but also serves to fine tune the
mechanical and other properties of the base material by means of heat treatments conducted prior to hot-dipping. Because this process has the
advantage of permitting the heavy coating of
metals, it has been applied in the manufacture of
coated steel products that will be used in highly
corrosive environments (Fig. 5). Ordinarily, coat-
ings range in thickness from 7 to 40 microns.
By contrast, the electrolytic coating method is
suited for lighter coating requirements. After the
mechanical properties are controlled in the continuous annealing, the coating is applied on the
surface of the steel sheet by passing it through
an aqueous solution that contains the ion of the
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No. 307 September 2003
Feature Story
coating metal. In this solution, the steel surface
serves as the cathode while electrodes positioned on the both sides of the sheet serve as
anodes (Fig. 6).
The thickness of electrolytic tin coating on tinplate for beverage cans and containers is around
0.4 microns (2.8 g/m2 in coating mass). At first, it
was difficult to achieve uniform coating. This was
because the coatings tended to become heavier
at locations where ions readily become metals
on the atomic level when electric discharges occur between two electrodes. But, an improved
treatment solved this problem. Reheating the tin
to a temperature greater than its melting point
(232 degrees C) after coating enabled uniformity.
Tin-free steel is another indispensable material
for beverage cans. It contains no tin in its coatings, which are extremely thin and range from 10
to 25 nm. (One nm is one billionth of one meter.)
Presently, hot-dip coating and electrolytic coating are properly used according to the intended
application of the product. Hot-dip coated products are used primarily in applications exposed
to corrosive environments and in applications requiring long-term durabilty. Such applications in-
clude auto-bodies, fuel tanks and building
materials. Electrolytically coated products are
used mainly in applications amenable to lighter
Fig. 5 Arrangement for Hot-dip Coating
Hot-dip coating layer
Cooling
coatings, including beverage cans and indoor
home electric appliances.
Fig. 6 Arrangement for Electrolytic Coating
Electric
current
Rectifier
Metallic coating layer
Coating-mass control
by gas wiping
(Wiping nozzle)
Coating solution
Annealing furnace
Anode
Mn+
Steel sheet
Steel sheet
Mn+
Cathode
Anode
Molten zinc
Mn+
(Metallic ion)
Electric
current
Metallic coating is deposited on the surface of steel sheets
when immersed in a solution of molten coating metal. This
method is adopted for coating steel sheets intended for applications such as automotive steel sheets and building materials
that are used in highly corrosive environments.
Rectifier
Coating is applied to the surfaces of a steel sheet by passing
the sheet through an aqueous solution containing coating ions.
In this solution, the metallic ion in the solution is reduced to the
metal by the electric current applied. This method is suited for
the lighter coatings such as of the tinplate for beverage cans.
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No. 307 September 2003
Feature Story
Alloying Treatments in Pursuit of
Higher Performance and Greater Ease of Use
Towards Faster and More Uniform Hot-dip Coating
The following discussion uses corrosion-resistant
steel sheets for auto-bodies to exemplify technical
factors in hot-dip coating that, thanks to heavy
coating, provide exceptional rust prevention.
Any attempt to prevent rust in automobiles using a single technology would involve huge
costs. It is common practice to deal with this task
by combining the use of paint, sealants and wax
for partial rust prevention with coated steel
sheets and structural design aimed at avoiding
water condensation. Since coated steel sheets
are particularly effective in preventing rust in
hard-to-paint automotive components and in outer panels that are visible to consumers, they are
finding rapid acceptance as materials for auto-
motive use.
A primary goal in hot-dip galvanizing is to realize uniform coatings at higher speeds (presently,
about 9 km/h). Higher speeds ensure higher productivity. In hot-dip galvanizing, the required
mass of the coating is maintained by spraying nitrogen gas against the hot-dip coating as it is deposited. When the speed of this process is
increased, it is difficult to obtain a uniform coating mass longitudinally and transversely. Nippon
Steel overcame these difficulties by developing
an array of technologies to precisely control the
coating mass, including methods for adjusting
the mass of nitrogen gas used and for controlling
gas spraying.
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No. 307 September 2003
Feature Story
Galvannealing: Mainstay among Automotive Steel Sheets
Another technical breakthrough in hot-dip coating is the development of galvannealing. This
process is aimed at improving the versatility of
hot-dip coated steel, thereby providing automobile makers with greater ease of use when working with zinc-alloy coated steel sheets.
Automotive steel sheets are stamped into autobodies and various other shapes. When great
pressure is applied to coated steel sheets, zinc
sticks to the surface of the stamping die which
leads to greater frictional resistance.
Hot-dip zinc-iron alloyed (galvannealed) steel
sheet was developed to improve this situation,
i.e., to make automotive steel sheets more versatile and easier to use. How is this alloyed
sheet produced? Immediately after hot-dip galvanizing, the sheet is reheated, causing iron in
the base metal to diffuse into the molten zinc,
thereby forming a zinc-iron alloy that completely
covers the surface of the sheet. In addition to exceptional stampability, this galvannealed sheet
shows improved weldability because of the alloying of the iron in the base metal with the coating
(Fig. 7). Galvannealed sheet is now the standard
material for Japanese auto-manufacturers.
Fig. 7 Galvannealing Process in Pursuit of
Improved Performance and Greater
Ease of Use (conceptual illustration)
Cold-rolled
base sheet
After
immersion
into molten
zinc
Steel sheet
Fe
Zn
Zn
Fe
Steel sheet
Reheating and
Galvannealing
Molten zinc
Iron at the surface
interface diffuses
into the zinc.
Reheating immediately after galvanizing causes the iron
at the interface to
diffuse, accelerating
the Zn-Fe alloying
process.
Zn-Fe
After
galvannealing
Formation of
zinc-iron
alloyed coating
Steel sheet
The alloying process
progresses further,
resulting in the formation of a complete
zinc-iron alloyed
coating.
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No. 307 September 2003
Feature Story
A look at current worldwide trends in rust-prevention technologies for automobiles shows that
non-alloyed hot-dip galvanized steel is the
choice of the European automakers. Traditionally, in Europe, partial rust-prevention was the
dominant approach to secure auto-body corrosion resistance. However, from the late 1990s it
became difficult for painting or partial rust-prevention approaches to cope with the growing
pressure for a twelve-year guarantee for perforation of auto-body caused by corrosion. This rapidly accelerated the use of galvanized steel
sheets. At first it was mainly electro-galvanized
sheets with thick zinc-coating. Later, in the pursuit of greater economy, hot-dip galvanized steel
sheets became primary. Obscured by this is the
fact that excellent zinc-iron alloyed (galvan-
nealed) sheets were not available in Europe at
that time.
In Japan, meanwhile, galvannealed steel
sheets steadily emerged from a pursuit for greater ease of use and became the standard material of automakers (Fig. 8).
Before this came about, though, a major obstacle blocked its broad application in the automotive
field. During the zinc-iron alloying (galvannealing),
if the iron content of the coating becomes too
high, the coating will harden and become brittle,
which leads to lower adhesion of the coating. How
did Nippon Steel solve this serious technical problem? The next issue will discuss this in detail and
introduce Nippon Steel's high-quality galvannealed sheet.
Fig. 8
GI
Hot-dip galvanized steel sheet
Zinc (Zn) layer
Zn 100%
Iron (Fe) layer
Formation of Zn-Fe alloyed
coating by reheating
GA
Galvannealed steel sheet
(Galvanizing + Annealing)
Zinc-iron
alloyed coating
Zn- 9 ∼12%Fe
(Zn content: 88-91%)
Iron (Fe) layer
The galvannealing process creates a zinc-iron alloyed
coating that is hard, yet highly stampable.
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Thank you. Next issue coming soon.