【Rust was not degradation but technology: Metals protected by rust, and metals killed by rust】Rust was not degradation but technology: Metals protected by rust, and metals killed by rust
Introduction: The Eternal Dialogue Between Iron and Oxygen
It is no exaggeration to say that iron, the most fundamental material supporting modern civilization, begins its march toward death the moment it is born. Steelmaking is a process of investing vast amounts of energy to forcibly strip oxygen from iron oxide (iron ore), which exists in a stable state in nature, and reduce it to metallic iron, which is thermodynamically unstable. Therefore, the phenomenon of iron combining with oxygen and moisture in the environment to return to a stable iron oxide—that is, "corrosion (rust)"—is nothing more than a return to nature in accordance with the law of increasing entropy.
However, in this process of degradation that seems irreversible, humanity has long found a counterintuitive idea, much like "fighting fire with fire." It is a technological system that uses controlled rust to prevent uncontrollable rust. While "red rust," which is generally avoided, eats away at metal and leads to the collapse of infrastructure, special oxide films called "black rust" or "stable rust" cover the surface of iron like a sturdy suit of armor, protecting the base material for decades or even centuries.
This report focuses on this duality of iron rust—"rust as degradation" and "rust as technology." It comprehensively discusses everything from the elucidation of material science mechanisms to the empirical wisdom found in traditional Japanese crafts, and the strategic utilization of weathering steel in the latest infrastructure maintenance. Furthermore, it clarifies the reality of corrosion costs, estimated to reach 5 trillion yen annually in Japan, and the current state of infrastructure crises based on inspection data of road bridges, tunnels, etc., thereby unraveling the correlation between science and economics surrounding rust.
Thermodynamics and Mechanisms of Corrosion: The Crossroads of Red and Black
To understand the phenomenon of iron rusting, it is first necessary to understand its chemical mechanism and the differences in the properties of the generated oxides at a microscopic level. Rust exhibits completely different physical properties depending on its generation environment and chemical composition.
2.1 Red Rust: Expansion and Porosity Leading to Destruction
What is generally recognized as "rust" is red rust, which exhibits a reddish-brown color. This is mainly composed of hydrated iron oxide and hematite, and occurs naturally in room-temperature atmospheric environments where water and oxygen are abundantly supplied. The greatest feature and greatest drawback of red rust lie in its "volume expansion" and "coarse structure."
According to data on the maintenance of piping and structures, when iron turns into red rust, its volume expands to about 8 to 10 times that of the original base metal. This dramatic expansion pressure is a major factor in reinforced concrete structures that destroys the concrete around the reinforcing bars from the inside, causing cracks and spalling (bursting). Furthermore, the red rust layer is extremely porous, with countless minute holes. Due to this structural defect, the rust layer itself acts like a sponge that guides moisture and oxygen to the inside, and corrosion does not stop at the surface but continues to progress deep into the material. Because it is highly water-soluble and brittle, it leaches out as red water in water pipes, also becoming a cause of water pollution.
2.2 Black Rust: Density and Magnetism Providing Protection
In contrast, magnetite, known as "black rust," exhibits behavior completely different from destructive red rust. Black rust is an oxide generated when iron is in a high-temperature environment or under specific conditions where oxygen supply is limited, and its crystal structure is extremely dense.
(Data source: Organized based on the description in 1)
As shown in the table, the volume of black rust hardly changes compared to the original iron, being about 1 time, and it adheres firmly to the base metal. This dense layer functions as a "passivation film" that blocks the permeation of oxygen and water, physically stopping the progression of further corrosion. Also, because black rust is a good conductor and has ferromagnetism, it has the property of being attracted to magnets. This characteristic is used as an important indicator when diagnosing the state of rust inside pipes non-destructively or when confirming the effectiveness of anti-rust treatments.
In this way, although they are the same iron oxide, due to differences in the binding ratio with oxygen and crystal structure, one leads the metal to death, while the other becomes a shield that protects the metal forever. Controlling this duality is the core of anti-corrosion technology.
The Origin of "Rust Control" Seen in Traditional Japanese Crafts
Long before modern science elucidated the oxidation mechanism of iron, Japanese craftsmen skillfully utilized the protective function of this "black rust" based on empirical rules. The pinnacle of this can be said to be the surface treatment technology in "Nambu Tekki" (Nambu ironware), a traditional craft of Iwate Prefecture.
3.1 "Kama-yaki" (Pot Firing): Anti-rust Technology Born from Chance and Necessity
In the manufacturing process of Nambu ironware teapots, there is a unique step called "kama-yaki" (metal-scent stopping) performed as a finishing touch after casting. This process is significant precisely because of the temperature range of approximately 900°C. When the iron surface is oxidized in a high-temperature environment, a dense, blue-black oxide film (magnetite layer) is formed instead of ordinary red rust. This artificially generated black rust layer coats the inside of the teapot, preventing the formation of red rust when water is added, and also suppresses the excessive elution of iron into the hot water, thereby serving to stop the "metallic scent."
There is an interesting legend regarding the origin of this technique. It is said that when a major fire occurred in Morioka during the Meiji era, the only items found in the ruins of a burnt-down workshop were iron teapots that, for some reason, were not rusted and did not emit a metallic scent. It is said that craftsmen of the time focused on this phenomenon, discovered that the intense heat of the fire had given the teapots an anti-rust effect, and established it as the "kama-yaki" technique. This is a testament to the craftsmen's powers of observation and spirit of inquiry, as they read the scientific truth of "passivation through high-temperature oxidation" from the accidental event of a disaster.
3.2 Multi-layer defense system using lacquer and oil
The anti-rust technology of Nambu ironware does not end with kama-yaki. For the surface that has turned gray due to kama-yaki, techniques such as baking on lacquer (urushi) or "oil seasoning," where edible charcoal powder is added to plant-derived oil and applied while heating, are performed. Lacquer is a natural thermosetting resin that undergoes a polymerization reaction when heated, forming an extremely hard and highly water-resistant film. On the other hand, the oil film formed by oil seasoning fills the fine pores of the iron along with carbon particles, complementing any slight defects in the black rust layer.
Layer 1: Black rust (iron oxide) film via kama-yaki - Chemical stability
Layer 2: Organic film via lacquer or carbonized oil - Physical barrier and water repellency
Due to this hybrid multi-layer structure of inorganic and organic materials, Nambu ironware achieves both durability as a practical item and aesthetic value (wabi-sabi) that increases in charm with use. It is worth noting that even if a piece of the film that has undergone these treatments were to accidentally enter the mouth, it is harmless to the human body. Unlike modern synthetic chemical paints, this anti-rust system, composed solely of natural materials, can be called a highly advanced technology from the perspective of safety and environmental compatibility.
Modern Alchemy: The Science of "Stable Rust" in Weathering Steel
The wisdom of traditional crafts underwent a dramatic evolution in the 20th-century industrial world in the form of "Weathering Steel." This is an alloy steel that pursues maintenance-free performance by allowing the iron itself to create rust that stops further corrosion, without relying on paint.
4.1 Qualitative transformation of rust through alloy design
While ordinary carbon steel (mild steel) produces coarse, easily peeling red rust when it rusts in the atmosphere, weathering steel exhibits different behavior. Weathering steel contains trace amounts of alloying elements such as copper (Cu), chromium (Cr), nickel (Ni), and phosphorus (P). These elements play a decisive role in the initial stages of rust formation.
When weathering steel is exposed to the atmosphere, it initially appears to generate red rust just like ordinary steel. However, in the process of undergoing repeated dry and wet cycles over several to a dozen years, a change occurs in the structure of the rust layer. Beneath the surface rust layer, at the interface with the base metal, an extremely dense and amorphous nano-level rust layer is formed. This is called "Protective Rust."
According to technical documents from Nippon Steel and JFE Steel, this stable rust layer not only blocks the permeation of corrosion factors such as moisture and oxygen, but also has the effect of refining and aggregating the rust particles themselves. In particular, Cr and Cu are thought to act as catalysts that inhibit the crystal growth of rust and promote the formation of a dense amorphous layer. Once this layer is completed, the corrosion rate drops to the absolute minimum, and the appearance changes from the initial bright brown to a calm dark brown, allowing it to blend into the landscape.
4.2 Life Cycle Cost (LCC) and infrastructure maintenance
The greatest merit of weathering steel is its economic rationality due to the fact that painting is unnecessary (unpainted use). In conventional steel bridges, heavy-duty anti-corrosion painting is applied for protection, but since the paint film inevitably peels off due to ultraviolet rays and deterioration, repainting is required approximately every 10 to 15 years. This maintenance cost is enormous, and when viewed over the entire service life of the bridge (for example, 100 years), it far exceeds the initial construction cost.
On the other hand, although weathering steel bridges have slightly higher material costs due to alloy additions, they do not require a painting process, and subsequent repainting costs do not occur. This makes it possible to significantly reduce life cycle costs (LCC). According to estimates by the Ministry of Land, Infrastructure, Transport and Tourism, maintenance and renewal costs for social infrastructure are projected to increase to about 1.2 times the current level by around 2050. In Japan, where the working population is declining, the adoption of weathering steel, which can save on maintenance labor, is a technology directly linked to solving national issues. In fact, weathering steel is used in more than about 20% of newly constructed steel bridges, and that ratio is increasing year by year.
4.3 Limits of the technology: The battle against saline environments
However, the technology of "protecting with rust" is not omnipotent. Its success or failure is strictly governed by the "salt concentration" in the environment. Chloride ions contained in sea salt particles are the greatest enemy that inhibits the formation of stable rust and causes abnormal corrosion.
Through research by various steel manufacturers and public institutions, a clear boundary line of "airborne salt content of 0.05 mdd" has been set as the environmental standard for using weathering steel without painting.
Airborne salt content of 0.05 mdd: The amount of salt (in terms of NaCl) adhering to a sensing surface of 100 square centimeters (1 dm²) per day is 0.05 mg or less.
In environments that exceed this value even slightly, such as areas near the coastline or road bridges in mountainous regions where large amounts of anti-freezing agents (sodium chloride, etc.) are scattered, highly protective amorphous stable rust is not formed. Instead, flaky, crumbling rust (lamellar peeling rust) occurs, and corrosion progresses rapidly. This is because chloride ions promote the crystallization of rust and prevent the formation of a dense layer.
Japan is an island nation surrounded by the sea on all four sides, and sea salt particles are carried deep inland by seasonal winds, so this "0.05 mdd" constraint is strict. Therefore, Japanese steel manufacturers have developed "coastal weathering steel" and "high-Ni weathering steel" with significantly increased nickel (Ni) content (about 1% to 3%) to cope with harsher salt damage environments. Ni has high resistance to chloride ions and works to help the formation of stable rust even in high-salt environments. As a result, it has become possible to construct unpainted bridges in coastal areas where it was previously considered inapplicable, and the scope of application of the technology is steadily expanding.
The Economics of Collapse: The Huge Losses Caused by Infrastructure Corrosion
While the technology of "protective rust" continues to evolve, the damage caused by "destructive rust" is quietly but surely eroding national assets. Japan's social infrastructure, which was intensively developed during the period of high economic growth, has passed 50 years since construction and is simultaneously reaching a period of aging. Corrosion is no longer just a material degradation, but a massive risk factor that affects the macroeconomy.
5.1 Corrosion Cost Estimation: 5.25 Trillion Yen Lost
In a country's economic activity, the total amount of wealth lost due to corrosion is called the "Cost of Corrosion." In Japan, the Japan Society of Corrosion Engineering (JSCE) and the Japan Association of Corrosion Control (JACC) have taken the lead in conducting large-scale research projects.
The investigation committee organized in 1999 calculated Japan's corrosion cost based on fiscal year 1997 data as follows. This survey used two different statistical methods.
Uhlig Method (Bottom-up method): A method of accumulating direct costs incurred for corrosion countermeasures, such as anti-corrosion painting, surface treatment, and costs for changing to corrosion-resistant materials, for each item.
Estimated amount: 3.938 trillion yen (0.77% of GNP)
Hoar Method (Industry-based estimation method): A method of estimating and summing up the loss amount and countermeasure costs due to corrosion for each industrial sector (construction, transportation, electric power, etc.).
Estimated amount: 5.258 trillion yen (1.02% of GNP)
This amount of 5.258 trillion yen is equivalent to approximately 1% of Japan's GNP (Gross National Product) at the time. Compared to the previous survey conducted in 1974, it has increased by approximately 1.54 times in monetary terms. Although the ratio to GNP has decreased slightly, this is an apparent change due to the expansion of the economic scale, and the absolute loss amount due to corrosion continues to increase.
More importantly, these figures are merely "Direct Costs." If "Indirect Costs" such as plant shutdowns caused by corrosion, product loss due to pipe leaks, traffic congestion due to bridge closures, and accident response are included, some analyses suggest that the total loss amount swells to two to four times the direct cost. From a global perspective, the NACE (National Association of Corrosion Engineers) IMPACT study estimates that the global cost of corrosion reaches 2.5 trillion US dollars, or approximately 3.4% of the world's total GDP. This figure suggests that corrosion is an economic threat that rivals or exceeds natural disasters such as typhoons and earthquakes.
5.2 Infrastructure "Health Status" as Told by Ministry of Land, Infrastructure, Transport and Tourism Inspection Data
Supporting this macroeconomic loss, inspection data at the site level also shows a serious situation. The Road Maintenance Annual Report (2nd round inspection results) published by the Ministry of Land, Infrastructure, Transport and Tourism, conducted from fiscal year 2019 to 2023, presents the reality of Japan's infrastructure with cold, hard numbers.
The table below summarizes the inspection results of road bridges, tunnels, and road accessories nationwide.
(Data source: Excerpted and organized from the Ministry of Land, Infrastructure, Transport and Tourism Road Maintenance Annual Report, Reiwa 5, page 15) Note: The above data refers to figures centered on those managed by the Ministry of Land, Infrastructure, Transport and Tourism and expressway companies. The situation tends to be even more severe when including those managed by local governments.
What should be noted here is the percentage of "Judgment Category III (Early Action Stage)." This is defined as a "state where there is a possibility that the function of the structure will be hindered, and measures should be taken at an early stage."
For bridges, about 10% of the total are at this stage. One of the main causes of Judgment Category III in steel bridges is serious corrosion. In particular, in girder ends where water easily accumulates and in splice joints (around bolts and rivets) that connect members, cases are frequently occurring where salt and moisture are locally concentrated, breaking through the paint and causing severe corrosion to progress.
Also, it cannot be overlooked that about 26% of tunnels and about 16% of road accessories are in Judgment Category III. In particular, accessories such as road lighting poles and sign poles are severely corroded at the ground contact point due to the influence of dog urine, herbicides, and snow melting agents, and there is no end to accidents where they rot from the inside and collapse even if there is no problem in appearance. These can be called "invisible bombs" right next to our living spaces.
The Battle Inside the Pipes: Stopping Red Water and Extending Lifespan
The problem of infrastructure corrosion is not limited to visible bridges and roads. Even inside water supply pipes and air conditioning pipes running inside buildings, an endless battle between metal and water is unfolding.
6.1 Red Water Problem and Magnetite Method
Sometimes reddish-brown water comes out of the water pipes of old buildings and houses first thing in the morning. This is "red water," which is red rust (etc.) generated inside the pipes that has peeled off and dissolved due to the water flow. As mentioned above, red rust is porous and brittle, and because it involves volume expansion, if left unattended, it can block the inner diameter of the pipe (rust nodules) or penetrate the pipe wall, causing water leakage accidents.
Replacing (renewing) pipes requires a large amount of money and large-scale construction that breaks walls and floors. Therefore, "rehabilitation technology" that extends the life of pipes through a chemical approach has become popular in recent years. A representative example is the technology that converts red rust into black rust. In this construction method (called the magnetite method, etc.), the environment inside the pipe is chemically manipulated using special reaction devices and rust inhibitors.
Reaction mechanism: By applying a reducing action to unstable and easily soluble "red rust (Fe2O3 / FeOOH)," the crystal structure is rearranged into chemically stable and hard "black rust (Fe3O4)."
Effect: The black-rusted layer adheres densely to the inner surface of the pipe and becomes a protective film that blocks further contact with oxygen and water. As a result, the generation of red water stops, and the reduction in pipe wall thickness is also suppressed.
What Nambu ironware craftsmen used to do using the power of fire, modern facility engineers are reproducing inside pipes filled with water using the power of chemical reactions. This is a shift in philosophy from "removing rust" to "changing the nature of rust and coexisting with it."
Comprehensive Discussion: He Who Controls Rust Controls the Future
What has become clear from the above investigation is the profound duality inherent in the phenomenon of rust, and the history of human wisdom surrounding it.
7.1 Mismatch between "Environment" and "Material" Creates Corrosion
The limit value of "0.05 mdd" in weathering steel, and the formation conditions for "" in Nambu ironware. These indicate that whether rust becomes benign or malignant depends on extremely delicate environmental conditions. Much of the infrastructure corrosion in Japan is caused by a lack of assessment of these environmental conditions, or by changes in the environment. For example, there are cases where the massive spraying of anti-freezing agents not anticipated at the time of construction, or the intensification of typhoons and increased salt air due to climate change, have exceeded the permissible limits of weathering steel, transforming what was supposed to be "protective rust" into "destructive rust." Research on weathering steel bridges in Canada also points to the risk that road salt and climate change (rising temperatures, changes in precipitation patterns) accelerate the corrosion process, which is a common challenge worldwide.
7.2 Paradigm Shift Toward Prioritizing LCC
The annual corrosion cost exceeding 5 trillion yen is a burden that cannot be ignored by the Japanese economy. However, this should not be viewed merely as a loss, but as an "opportunity to change the quality of investment." Given the current situation where past infrastructure development that pursued only low initial construction costs has resulted in enormous running costs, we should not spare investment in "rust-protecting" technology from now on.
Appropriately incorporating technologies such as weathering steel, high-durability coatings, or cathodic protection from the construction stage. And, performing preventive maintenance to discover signs of "malignant rust" early through inspections and returning it to a "benign state" before it is too late. This is the only solution for infrastructure maintenance in a depopulating society.
7.3 Fusion of Technology and Tradition
The "kama-yaki" (pot firing) of Nambu ironware, the latest "weathering steel," and the "magnetite method" for piping all share the same underlying principle. It is a technique like judo that utilizes the laws of nature, "converting the energy that iron tries to use to oxidize into the energy for forming a barrier that protects the iron itself."
Even in the modern age where science and technology have advanced, we may not have fully unlocked the potential of the material known as iron. By dissecting the empirical rules sleeping within traditional crafts with the scalpel of modern science and applying them to nanotechnology and alloy design, there is a possibility that even more advanced "rust control technology" will be born.
Conclusion
"Rust was not degradation but technology."
This proposition, set forth in the title of this report, has turned into a conviction now that the investigation is complete.
Understanding Duality: There exists porous "red rust" that destroys metal and dense "black rust (and stable rust)" that protects metal. This difference is merely a slight variation in atomic arrangement, but its engineering value is as vast as heaven and earth.
Genealogy of Technology: The wisdom of corrosion prevention discovered by Nambu ironware craftsmen in the flames has been passed down steadily to modern weathering steel and infrastructure life-extension technologies. This is a manifestation of the spirit of craftsmanship that Japanese people have possessed since ancient times: "to dialogue with materials and coexist with them."
Proposals for the Future: The key to saving Japan's infrastructure crisis lies not in viewing rust as an enemy, but in managing it. To stop the increase in corrosion costs, appropriate material selection according to the environment (the right material in the right place) and the spread of advanced diagnostic technology to determine the quality of rust are essential.
Iron tries to return to the earth by rusting. We cannot completely stop that will of iron. However, we can rewrite that process from "degradation" to "protection." Finding hope in the name of technology on the other side of reddish-brown rust. That is the responsibility of modern civilization living with iron.
List of Major Reference Data Sources
This report was created based on the following reliable information sources.
9: "Domestic Corrosion Cost Survey Report" (1974/1997 editions) and related papers by the Japan Society of Corrosion Engineering (JSCE) and the Japan Anti-Corrosion Technology Association (JACC).
2: Technical explanations regarding the manufacturing process of Nambu ironware, pot firing (kanake-dome), and lacquer/oil seasoning (Nambu ironware specialty stores, manufacturer websites).
1: Physical and chemical property comparison data of red rust and black rust, technical materials regarding the magnetite method.
14: NACE International "IMPACT Study" (Global Cost of Corrosion).
15: Ministry of Land, Infrastructure, Transport and Tourism "Road Maintenance Annual Report (FY2023)" (inspection results as of March 2024).
5: Technical reports (technical papers) on weathering steel and coastal weathering steel by Nippon Steel, JFE Steel, and the Japan Iron and Steel Federation, papers on airborne salt standards and alloy design.
4: Academic review papers on the performance of weathering steel bridges, the impact of climate change, and maintenance strategies.
References
Countermeasures for Red Rust and Red Water in Water Pipes - TKK Evolution, https://tkk-evo.co.jp/solution/magnetite/rust
What is Kamayaki (Iron Sealing)? - Specialty Store for Nambu Tekki and Iron Kettles - Shinwadenki, https://www.nanbutetsu.jp/smp/freepage_detail.php?cid=2255&fid=3
Technology and Materials | Mugu | Moments in daily life that move the heart., https://mugu.co.jp/nof/
Corrosion Performance of Atmospheric Corrosion Resistant Steel Bridges in the Current Climate: A Performance Review - PMC - NIH, https://pmc.ncbi.nlm.nih.gov/articles/PMC12347768/
High Performance Steels, https://www.jisf.or.jp/info/book/documents/HighPerformanceSteel(ForBridgeConstrunction)en.pdf
3%-Ni Weathering Steel Plate for Uncoated Bridges at High Airborne Salt Environment, https://www.nipponsteel.com/en/tech/report/nsc/pdf/n8706.pdf
Development of New Weathering Steel for High Salinity Environment for Coastal Use, https://www.jfe-steel.co.jp/en/research/report/026/pdf/026-25.pdf
Performances of Coastal Weathering Steel, https://www.nipponsteel.com/en/tech/report/nsc/pdf/8115.pdf
New Publication: Report on Corrosion Costs in Japan, http://www.jacc1.or.jp/fushokukosuto.pdf
Corrosion Cost Survey in Japan, https://www.j-cst.org/main/download.htm?code=C00070500252
Cost of Corrosion in Japan - ResearchGate, https://www.researchgate.net/publication/316782019_Cost_of_Corrosion_in_Japan
Cost of Corrosion in Japan, https://www.j-cst.org/main/download.htm?code=C00010200097
JSCE's report on the cost of corrosion in Japan - ResearchGate, https://www.researchgate.net/publication/288478439_JSCE's_report_on_the_cost_of_corrosion_in_Japan
Economic Impact | Corrosion - NACE IMPACT Report, http://impact.nace.org/economic-impact.aspx
Summary of Inspection Results for Bridges, etc. in Fiscal Year 2023 (Reiwa 5), http://www.qsr.mlit.go.jp/oita/site_files/file/road_info/maintenance/nenpou/r5.pdf