Titanium is Resistant to Hot Spring Corrosion｜Explaining the Problem of Metal Rusting and Dissolving in Hot Springs with Supporting Data

Why metals rust in hot springs (The essence of hot spring corrosion)

The phenomenon of metal rusting in hot springs cannot be explained simply by being outdoors.
Corrosion in hot spring areas (hot spring corrosion) is accelerated primarily by the simultaneous action of the following three factors:

• Strong acidity: The lower the pH, the easier it is for the protective film on the metal surface to break down.
• High Temperature: Chemical reactions progress faster as temperature rises, increasing corrosion rates.
• Chloride ions: A factor that readily induces pitting corrosion (pinhole-like localized corrosion) in stainless steel.

In other words, for materials, hot springs are places where acid, heat, and salt (chloride) converge.
Under these conditions, even metals that are generally considered highly corrosion-resistant can deteriorate much faster than expected.
As a result, the frequency of component replacement increases, leading to accumulated costs for scaffolding, shutdowns, safety measures, and landscape repairs.

Environmental conditions of hot springs: Strong acidity, high temperature, chloride (e.g., Kawayu Hot Spring / Noboribetsu Hot Spring)

The below table summarizes conditions for representative hot spring areas.
The crucial point is that the type of harshness varies by hot spring location (strong acid type, high temperature type, etc.).

*Source: Key points extracted from materials provided by Company N (March 19, 2024) (numbers/temperatures).

A pH of 1.7 indicates strong acidity, and temperatures exceeding 90 deg. C represent extremely harsh conditions for architectural hardware.
Rather than addressing hot spring corrosion after it occurs, eliminating corrosion scenarios at the material selection stage is the most rational approach for long-term operation.

Test Result: The corrosion rate of pure titanium is 0.00 mm/year (Difference compared to grade 304 stainless steel and aluminum)

Below is the result of metal weight loss testing using Kawayu hot spring water.
Test temperature: 77 deg. C, Test duration: 3 hours
Under these conditions, the difference between materials is clearly evident in the figures.

The key point here is not to limit the comparison to material costs alone.
In hot spring environments, if the material fails prematurely, it triggers a chain of subsequent costs.
Scaffolding, safety management, shutdowns, landscape restoration, and complaint handling—when these are totaled, titanium often delivers the best overall cost performance in the long run, even if the initial cost is slightly higher.

Examples of short-term deterioration occurring on-site: Immersion exposure and bolts

(1) Immersion exposure at Noboribetsu hot spring source: Grade 304 stainless steel and aluminum dissolved

According to the result of immersion exposure testing on grade 304 stainless steel, titanium, and aluminum alloy at Noboribetsu Hot Spring source, the aluminum and grade 304 stainless steel dissolved after four months of exposure.
This clearly demonstrates that hot spring corrosion is faster than imagined.

(2) Comparison Using Bolts: Zinc-Plated Steel Corrodes Rapidly, Titanium Bolts Remain Stable

The problem of metal rusting in hot springs often manifests first in bolts, nuts, and fasteners rather than in plates or main structures.
Immersion exposure using Kawayu hot spring water showed that while zinc-plated steel bolts corroded significantly within days to weeks, titanium bolts maintained their appearance even after three weeks.

If bolts deteriorate first, it’s not just a simple replacement.
It leads to reduced fastening strength, safety risks, potential component detachment, and increased effort for regular inspections and repairs.
In hot spring resort construction, the approach of designing from the smallest structural and safety-critical components (bolts) is effective.

Conclusion on design judgment: Why titanium offers the best cost-performance ratio (30-year TCO)

The impression that titanium is expensive applies only when looking at the material cost alone.
In hot spring areas, material replacement becomes the core of operational costs.
Here, we simplify this by organizing it as the 30-year TCO (Total Cost of Ownership).

Test results suggest pure titanium has a corrosion rate of 0.00 mm/year, while grade 304 stainless steel and aluminum show significantly higher corrosion rates.
In other words, when considering hot spring corrosion, titanium is a material highly likely to dramatically reduce replacement frequency.

This translates to the highest cost performance when considering long-term durability and safety.
For architectural decision-making, it is rational to judge not based solely on material unit cost, but as a specification design that includes renewal plans (replacement frequency).

Recommended applications in hot spring areas (grating, bolts, roofs, signs, etc.)

The number of cases where titanium is being adopted and used in trial installations at hot spring areas is increasing.
The key is to prioritize areas most susceptible to hot spring corrosion damage.

There is no need to convert everything to titanium at once.
A practical approach is to gradually expand the application of titanium in this order: close to the source, difficult to replace, safety-critical, and directly impacting the landscape.

Important notes regarding specifications: Dissimilar metal contact (galvanic corrosion) and installation details

While titanium is highly resistant to hot spring corrosion, proper fitting (interfaces) is crucial for architectural hardware.
Particular attention should be paid to galvanic corrosion, where dissimilar metals come into contact and corrosion progresses due to potential differences.
Hot spring areas tend to be rich in electrolytes (ions), making the risk of galvanic corrosion more likely to manifest.

We can provide documentation outlining considerations on specifications tailored to project conditions (water quality, temperature, chloride content, installation location) and specific components (bolts, gratings, roofs, signs, etc.). Aligning specifications during the design phase reduces operational uncertainty.

Frequently Asked Questions (FAQ)

Q1. Isn’t stainless steel (grade 304) sufficient for hot spring areas?

When hot spring corrosion conditions (strong acidity, high temperature, chloride) overlap, even grade 304 stainless steel can experience pitting corrosion and wall thinning.
A test result indicates a corrosion rate of 1.27 mm/year (pitting corrosion onset) for grade 304 stainless steel.
The harsher the conditions, the greater the advantage of titanium.

Q2. Where should I prioritize to stop metal rusting in hot springs?

The recommended priority order is: #1 Areas near the source or steam, #2 Safety-critical fastening points (bolts), #3 Areas difficult to replace (requiring scaffolding), #4 Areas with high aesthetic value (signage, etc.). Even partial optimization yields benefits.

Q3. Isn’t titanium expensive?

While the material cost per unit tends to be high, in hot spring areas, renewal costs (scaffolding, shutdowns, safety management) often become the dominant expense.
Materials that reduce replacement frequency often prove cheaper over a 30-year TCO, which is the essence of maximum cost performance.”

Q4. Is it true that titanium doesn’t rust?

We cannot guarantee 100% corrosion resistance (as it varies by spring quality and conditions), but test results suggest pure titanium has a corrosion rate of 0.00mm/year.
At the very least, there is supporting data showing it outperforms grade 304 stainless steel and aluminum under the same conditions, making it a highly rational material choice.

Conclusion｜Hot Springs × Titanium: Simultaneously Designing Durability and Operational Efficiency

Hot spring corrosion, where metals rust in hot springs, is a problem that directly impacts not only the appearance of materials but also safety, maintenance, and landscape value.
Test results suggest pure titanium exhibits a corrosion rate of 0.00 mm/year, clearly outperforming grade 304 stainless steel and aluminum.

Therefore, titanium becomes the most cost-effective choice (minimizing TCO) from the perspectives of long-term durability, safety, and reducing replacement costs.