Corrosion-Resistant Steel with Tin Added Is Newly Developed
－ High-Tensile Steel Plates with High Salt Resistance －

Sumitomo Metal Industries, Ltd. (Sumitomo Metals) has developed a new corrosion-resistant steel with tin (Sn) added that can be used in salt-containing environments, such as at the seaside or in cold climates where anti-freezing agents are sprayed. Bridges made of this new steel is expected to lengthen the intervals between repaints and reduce repainting workload.

1. Background for development
In general, steel bridges are painted for rust prevention. As rust tends to spread from a scratch in the coating or a tip of materials, regular repainting is required. Lengthening the intervals between repaints or lightening repainting workload will result in reduction in maintenance and management costs for bridges.

2. Details of newly-developed steel
Sumitomo Metals has found out the nature of the corrosion mechanism of steel coating in high salt-containing environments. We found out that in such environments, adding a trace of tin (Sn) can significantly improve corrosion resistance, and that this effect can be produced even if rust cannot be removed sufficiently during the repainting work. We were then able to develop this new corrosion-resistant steel with tin added, which at the same time satisfies more than similar basic performance properties(*) as conventional steel needed for bridges such as strength and weldability.

The use of this new steel enables to lengthen the intervals between periodic repaints, and to lighten repainting workload.

3. Future development
As the new corrosion-resistant steel has high corrosion-resistant performance even with no coating, its application for bridges with no coating is also studied.

Sumitomo Metals will carry out further studies on the longer duration between repaints and maintenance and the simplification of repainting work. We will thus contribute to cost reduction over the lifetime of steel bridges and savings in public work investment.

Note*) Performance properties that are described in the JIS standard and the Specification for Highway Bridges (technical standards, such as structural strength, that are required for bridges and highway roads, based on Japan’s Road Act and related government ordinance.)

 

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Examples of prevention of the rusting and coating-delamination after a corrosion test.

	40cycles	80cycles	120cycles
conventional steel
new corrosion-resistant steel with tin added

Steel specimens coated by a general epoxy paint of 180 μm in thickness were tested by an accelerated corrosion test in accordance with SAE J2334* with scratches. The figure shows the appearance of specimens after the test, and the delamination regions around scratches of coating are removed in order to evaluate the delamination.

 

*SAE J2334 is a cyclic dip in saltwater and dry test established by Society of Automotive Engineers.
It is reported that the test simulates the severe atmospheric corrosion environments with high chloride content.
The average corrosion loss for conventional steels after 80 cycles of the SAE J2334 test is 0.8 -1.0 mm in thickness, which corresponds approximately to 2-year exposure of Miyako island, Okinawa pref. Japan.

Ref). H. Nagano, M. Yamashita, H. Uchida: "Environmental Materials Science-Corrosion and Protection Engineering Relevant to Global Environment Preservation-" (in Japanese)