Applications: Automotive - Brake Hose Hydraulics (2023)

• Abstract
• Introduction
• Trailer Corrosion Tests
• Test Results
• Additional testing
• SUMMARY AND CONCLUSIONS
• bibliographical references

Abstract

For many years the pipes in car brake systems have been made of low carbon steel. One or more surface coatings are applied after welding to protect the steel substrate from corrosion because steel has no inherent resistance to corrosion in the road environment. Although the coating composition has changed since the original hot lead-tin coatings were used, coating defects remain a problem. The addition of zinc-rich paints slightly improved the protection of the pipe. Current aluminum-zinc coatings and added polyvinyl fluoride coatings are still insufficient to fully protect steel pipe.

In a recent series of tests, copper-nickel 90-10 (UNS C70600) pipe was fabricated into standard brake system "shapes," which were then attached to a test trailer and transported through various corrosive and mechanically abusive test track environments. The tests involved holding the tubes in a high humidity chamber for a portion of each 24-hour test cycle. After 40 cycles and every 10 cycles thereafter, individual tubes were required to pass a pressure test of 20,684 kPa (3,000 psi). Candidate pipe materials had to complete 60 cycles to meet the minimum requirement.

Current production steel tubes passed the 60 cycle requirement but failed well before 120 cycles. The 90-10 copper-nickel tubes completed 200 cycles with virtually no reduction in their original burst strength.

Introduction

The brake pipes are in a highly corrosive area. Although many other automotive components operate in the same hostile environment, few are less forgiving in the event of a breakdown. Thus, one of the main considerations in the design of an automotive hydraulic brake system is the integrity of the brake line that distributes the system pressure.

In 1965, an annual safety check of motor vehicles was introduced in Sweden and subsequently in other European countries. This process included inspecting the hydraulic brake lines for rust. At the same time, the Swedish Motor Vehicle Inspection Society began publishing annual reports on the results of these tests.1

In 1969, laboratory tests were reported comparing some inherently corrosion resistant copper alloy pipe materials with then current production materials.2

In the early 1970s, the Swedish Corrosion Institute approached the problem of brake pipe corrosion from the point of view of using a corrosion-resistant material instead of trying to protect the surface.1

The European car industry's initial response to brake pipe corrosion problems was to end the use of the then existing hot-dip ternet metal coating over steel pipe. Laboratory testing in a 6% neutral vapor salt spray test showed that corrosion resistance could be achieved with a 25 micron zinc coating in place of the ternet coating. In the years that followed, it became apparent that the laboratory tests did not accurately reflect the conditions that exist in the real operating environment. Various plastic coatings were then applied over the zinc and some are still used today.1

Figure 1.Copper-nickel brake lines run from 1990 Volvo master cylinder.

The bar graph shown inFigure 2illustrates the percentage of vehicles that fail safety inspections due to defects in the brake systems of eight-year-old Volvo passenger cars. 1970 model cars had terne coated steel pipes. The pipes on the 1971 models were zinc coated. Defects other than rusted pipes are included in these values, but their effect on the data is minimal. The reduction in defects associated with the introduction of 90-10 copper-nickel tube in 1976 is dramatic.

Figure 2. Annual Swedish Vehicle Safety Inspection Results. Bars indicate the percentage of 8-year-old Vovlo passenger cars inspected in the indicated year with brake lines that did not meet inspection requirements. 1976 was the year Volvo introduced 90-10 copper-nickel ("Cunifer Alloy") pipes to its vehicles.

The paper2presented at the SAE Annual Meeting in January 1970, which dealt with the then "state of the art" in pipe coatings. the data presented in this paper are still relevant. Voids, poor adhesion, discontinuities and physical damage in the surface coatings used today can lead to accelerated, localized corrosion attack that renders any intact coating elsewhere in the pipe useless.

An incident reflecting the latter situation was recorded in an SAE paper presented in 1991.3A brake line that should have burst when tested at 1 1 5 832 to 158 579 kPa (1 6 800 - 23 000 psi) actually burst at 4 825 kPa (700 psi). The document states, "This particular section of pipe was at the end, above and behind the rear axle, and had extensive corrosion, perhaps due to impact with gravel."

Based on the background summarized above, a test program was undertaken by the Copper Development Association Inc., in cooperation with an automobile manufacturer, to thoroughly evaluate the applicability of Copper Alloy C70600, 90-10 copper-nickel pipe, for the line car brake application. The pipe material is described inTable 1.

Table 1. Material requirements for copper alloy C70600 for use in automotive brake line

Composition, maximum percentage, unless shown as a range
In Fe Mn Zn Cu	9,0 - 11,0 1,0 - 1,8 1.0 1.0 Rest of
Mechanical properties
Output power, min. (0.2% offset) Tensile strength, min Extend to 50.8mm (2.0)	110.316 kPa (16.000 psi) 310.179 kPa (45.000 psi) 40-55%
Pressure proof test
Unless otherwise specified, the finished tubing shall withstand a hydrostatic profile test, without evidence of failure, at a pressure p which will subject the material to a fiber stress of 110,316 kPa (16,000 psi). The test pressure is determined by Barlow's equation for thin hollow cylinders under tension: Where: Pi is internal pressure, psi t is pipe wall thickness, in. small is allowable pipe wall tensile strength, psi Hey is pipe O.D., in. No pipe shall be tested in excess of 34,473 kPa (5,000 psi) hydrostatic pressure unless otherwise specified.

Trailer Corrosion Test

The design test procedure generally used today to evaluate the corrosion resistance and integrity of motor vehicle body and chassis components consists of 100 cycles of moisture-controlled soaking and drying, salt spray and mileage accumulation on various road surfaces with test specimens mounted on a trailer. The sequence of events of the test cycle is listed inTable 2.

Beginning with the 40th cycle and at 10 cycle intervals thereafter, each tube is subjected to an internal pressure test of 20,684 kPa (3000 psi). Candidate materials must complete 60 cycles to meet the minimum requirement.Figure 4shows the test equipment on which the hydrostatic pressure tests were performed.

This performance would have been expected even if a measured surface friction had been induced on the pipe as a prerequisite for evaluating the pipe surface.

Test Results

The data inTable 3 4reveal that after 200 test cycles, which exceeds three times the minimum benchmark of 60 cycles, the copper-nickel material retained more than 89% of its original average burst strength.

Also notable is the close difference in burst pressure after the test. This confirms the uniformity of strength and physical properties of copper-nickel, a feature not present in the currently used carbon-coated steel tube.

Additional testing

Figure 5. The 1976 Volvo engine bay features uncorroded copper-nickel brake lines.

The brake pipes were removed from this vehicle and hydrostatic break-in was tested with the following results:

Tube No. 1 111.694,95 kPa (1 6.200 psi)
Tube No. 2 106.868,62 kPa (1 5.500 psi)

This real-world data is a welcome confirmation of the trailer test results.

SUMMARY AND CONCLUSIONS

The automotive industry faces many challenges in the market and on its test tracks. It must produce vehicles that will compete in a global market based on quality, safety, reliability, durability and cost. The test results presented above demonstrate that 90-10 nickel copper pipe is a significantly better choice for automotive brake lines than low carbon steel because:

1. Using an inherently corrosion-resistant material is the best protection against long-term brake pipe corrosion. This has been proven by Volvo, which has been using 90-10 copper-nickel tubes in cars it has produced for the past 15 years.
2. Trailer corrosion test results indicate that copper-nickel 90-10 (UNS 70600) pipe is a superior product compared to the coated steel pipe used for brake lines in today's US-built vehicles.
3. Current double-wrapped, brazed and coated steel pipes are susceptible to weld gaps, coating gaps, poor coating adhesion and discontinuities. These vulnerabilities, combined with accidental service damage, mean that the actual life of brake line materials currently used in US-built vehicles should be considered unacceptable.

It must be recognized that all underbody components, including the brake pipes, will be struck by objects thrown from the tires. Such accidental damage should be considered the most vulnerable link in the chain.

The pipe designer generally specifies the addition of a metal or plastic sleeve to the areas of the pipe believed to be most vulnerable to rock damage. However, an inherently corrosion-resistant copper-nickel tube provides the surest protection against such accidental damage, especially when compared to a coated steel tube.