「No.4」Let us talk about the anti-rust technology of metal products used in our daily life.
“HOUROU” is our subject for this time.
“HOUROU” which “Vitreous enamel” called in the United Kingdom and “Porcelain enamel” called in the United States, are the technology used in traditional crafts “SHIPPOHYAKI” called in Japan, to put the glaze (mainly composed of glass) on the metal surface and bake so that shiny colors can be given on the metal surface.
Currently, it is called “SHIPPOHYAKI” which uses gold, silver, copper, etc. for the body metal, and it is a name used mainly for art goods and crafts.
On the other hand, it is commonly referred to as “HOUROU”, which uses iron as the base material, for daily necessities and industrial products.
There are pots, kettles, and storage containers around the kitchen, if you say “HOUROU” which is close to you. It is also used for building materials because it is hard to be damaged, and it is excellent in weather resistance.
The method of solidifying and cementing the glaze of glass to the metal can be seen in the products excavated from 1400 BC stuffs as example of the oldest ones.
The technique of “SHIPPOHYAKI” is used on the mask of Tutankhamun of the ancient Egypt, and it is said that the powder of lapis lazuli is burnt on the blue decoration part.
As the glazing materials, Natural stone, ash, and minerals are also commonly used for ceramic products including “SHIPPOHYAKI”.
Although “SHIPPOHYAKI” using natural raw materials had high durability from the properties of the glass on its surface, cracking and peeling due to impact are likely occurred, and those with poor adhesion property to the bodies will soon peel off and loose rust-preventive effect.
It is not the one that can withstand the use as daily necessities, and therefore the use as the art products are main because of beautiful luster and the color peculiar to the glaze.
The technology of “SHIPPOHYAKI” began to be used as a practical product as the current “HOUROU”, it became the Meiji era onwards in Japan.
It is said that the anti-rust performance of HOUROU has been dramatically improved by the development of steel sheets for HOUROU and titanium glazes etc. in those days, and by the undercoating for increasing the adhesion property with the iron, and by overcoating for the high abrasion resistance.
The steel sheets for HOUROU improve the adhesion property to the glass layer by the presence of precipitates and inclusions in the steels such as moderate size metal oxides.
○ Manufacturing process of “HOUROU”
The manufacturing process of “HOUROU” can be simply divided into three steps.
(1) Manufacture of glaze mainly composed of glass etc.
・ Melt the components, such as glass etc. and then ground and mix after drying.
・ Stretch them by water and glue etc. and use as glaze.
(2) Manufacture of metal products as body
・ Mold with press steel sheet for “HOUROU”.
・ Treat the surface by degreasing, pickling, nickel, to neutralize.
(3) Bake the glaze to the metal products
・Apply the glaze and let them dry.
・Fire for 5 ~10 minutes at 800 ~ 850 °C.
Repeat these operations, in the order of undercoating, overcoating of the glaze, and painting.
○ Features and disadvantages of “HOUROU”
Excellent features of “HOUROU”
(1) Heat resistance
The high heat resistance products can withstand against temperatures of about 500 °C, and it is also possible to use in the oven etc. for the daily necessities.
(2) Acid resistance
Because glass can withstand strong acids, it is industrially used for reaction tanks and piping of chemicals.
(3) Abrasion resistance
The hardness of glass is high, and it has strong properties against scratches and rubbing. Therefore, the coating of the glass layer is hard to wear and can protect the body for a long time.
Disadvantages of “HOUROU”
(1) Vulnerable to shock
Even if the hardness is high, because it lacks the ability to extend, the body easily cracks if it gets a strong impact. If the body is exposed by cracks and chipping, rust will occur from there.
(2) Weak to large heat changes
In general, the glass is weak to sudden heat change and may cause cracks due to rapid quenching or rapid heat-up.
From the viewpoint of “Anti-rust technology”, “HOUROU” is classified as “glass lining” as one of the inorganic lining, which is to cover against the corrosion-prone metals with the inorganic (ceramics, etc.), which do not cause corrosion.
The anti-rust performance of the lining depends on the corrosion resistance property of the coating itself and the adhesion property between the coating layer and the body.
Therefore, “HOUROU” for the purpose of rust prevention is important to improve the adhesion property between the body metal and the glass layer, the surface treatment of the metal, and the adhesion performance of the glaze.
We talked about “rust prevention technology” which is used for “HOUROU”.
