Customer Paper

Chongqing Rail Transit Line 2

Summary of Anti-Corrosion Coating Technology for PC Track Beam Finger Plate Assemblies

Railway Beam Bridge System Research Center

July 3, 2009

　　Chongqing Rail Transit Line 2

　　Summary of Anti-Corrosion Coating Technology for PC Track Beam Finger Plate Assemblies

　　The main line of Chongqing Rail Transit Line 2 has a total length of 18.586 km and comprises a total of 2,196 track beams of various types (including PC track beams, RC track beams, continuous beams, steel track beams, and connecting beams), of which 2,023 are PC beams. Each PC track beam is equipped with a total of 12 finger-plate assemblies at both ends, consisting of finger plates, plate seats, finger-plate bolts, and other components.

　　The track beams on the PC track are interconnected via finger plates that cross each other at their fingertips, thereby forming a continuous track line that ensures safe train operations. Consequently, the installation quality of the finger plate assemblies—including the quality of their anti-corrosion coating—directly affects the smooth operation of the track line and the safety of train operations. However, since the second line began operating, various factors have led to severe rusting of the metal structural components of the track beams, particularly the finger plate assemblies, putting the integrity of the track line and the safety of train operations at risk. To address this issue, the Track Beam Bridge System Research Center of Chongqing Rail Transit Corporation, in collaboration with relevant organizations, has undertaken technical research to tackle the corrosion prevention challenges associated with the finger plate assemblies. After systematic analysis, technical studies, and multiple rounds of repeated testing, we have now preliminarily mastered the technology for preventing corrosion in these finger plate assemblies. The results of the tests are summarized below:

　　1. Historical Overview of Corrosion Protection for Finger-Plate Components

　　1.1 Introduction to the Finger Plate Assembly

　　Both the finger plates and plate seats are metallic structural components. The finger plates are made of Q235 ordinary steel plate and are produced through mechanical machining. The plate seats are pre-embedded in the track beams and are made of QT400-25 ductile iron; their contact surfaces with the finger plates have been precisely machined. In total, Line 2 has 1,977 × 12 = 23,724 finger plates and plate seats of various types—251 × 12 = 3,012 units in tunnel sections and 1,726 × 12 = 20,712 units in elevated sections. There are a total of 142,344 finger plate bolts, including 47,448 bolts for the running surfaces and 94,896 bolts for the side surfaces.

　　1.2 Introduction to Historical Paint Schemes

　　In April 2004, due to the successive occurrence of corrosion damage on the finger plates of the newly installed track beams for Phase I of Chongqing Rail Transit Line 2, our company’s track beam bridge project commissioned Chongqing Three Gorges Paint Co., Ltd. and Chongqing Waijian Baixin Co., Ltd. to carry out anti-corrosion treatments on the finger plates along the sections from Jiaochangkou to Daping and from Daping to Dayan. In July 2004, both companies completed their painting work one after another.

　　In April 2005, due to the reappearance of rust damage on the finger plates that had undergone secondary painting as part of Phase I of Chongqing’s Metro Line 2, the section between Jiaochangkou and Daping showed particularly severe rusting. Consequently, the Line Facilities Department requested that Chongqing Three Gorges Paint and Chongqing Waijian Baixin Company take advantage of the completion of precise line adjustments to carry out painting of the weld seams on the wedge-lock blocks. They were also asked to provide a quality warranty service, involving a third round of anti-corrosion treatment for the rusted finger plates. The painting work was completed in August 2005. However, even after the third coating, the rusting condition of the finger plate assemblies remained unresolved, and rust has now become a significant quality defect affecting the monorail line.

　　1.3 Selection of Coating Materials for Line 2

　　1.3.1 The anti-corrosion processes required by the design drawings for Phase I of Line 2 are shown in the table below:

　　Table 1.1: Bottom Surface of the Finger-Plate Bracket

Table 1.2: Bottom Surface of the Finger Plate and Top Surface of the Plate Base

Table 1.3: Top Surface of the Finger Plate

　　1.3.2 Actual Materials and Processes Used for the First Coating of the No. 2 Line Project

　　In the actual coating of the finger-shaped slabs for Line 2, materials and processes such as Kansai Paint, hot-dip galvanizing, Dacromet, and zinc-plus were employed. In particular, Phase II of the project also adopted a thermal spray aluminum-magnesium coating process.

　　1.3.3 Selection of Anti-corrosion Materials for the Second Coating of Line 2

　　The second coating was undertaken by Baixin Company and Three Gorges Paint respectively. Based on the differences in the original primer coatings of the fingerboard substrates, the two companies selected Dacromet and epoxy-rich zinc fluorocarbon paint for corrosion protection. The material selections made by the two companies are shown below:

　　(1) Baixin Company

　　a. Secondary coating of the Kansai lacquer-based surface

　　b. Hot-dip galvanizing, Dacromet, and secondary coating with zinc-based materials

　　(2) Three Gorges Paint Co., Ltd.

　　a. Coating compatibility solutions for workpieces coated with zinc plating and Dacromet processes

　　b. Coating system for workpieces originally coated with Kansai lacquer paint

　　1.4 Current Status of Corrosion Protection for the Finger-Plate Assembly on Line 2

　　Currently, whether it’s the initial coating applied during the construction of Line 2—using Kansai paint, hot-dip galvanizing, Dacromet, or zinc-plus coatings—or the secondary and tertiary coatings—Dacromet or epoxy-rich zinc-fluorocarbon paints—all these corrosion protection technologies fail to meet the corrosion protection requirements for the steel structure of track beams under Chongqing’s climatic and operational conditions. Specifically, the finger-plate assemblies suffer from extremely severe rusting, with numerous finger plates exhibiting defects such as blistering, paint chipping, and rusting. In some sections, the finger-plate bolts have experienced serious quality issues, including rust-induced fracture and severe adhesion between the bolts and the bolt-hole seats (see Table 1.1 “Rusting Conditions of Finger Plates in Various Sections” and Appendix 1 below). Therefore, it is imperative to select and explore other reliable technologies that can adequately meet the operational needs of the light rail system.

　　Table of Corrosion Status of Finger Plates in Each Section Table 1.1

　　2. Adopt zinc-nickel diffusion coating technology to address the corrosion issues in finger-plate assemblies.

　　Given that the anti-corrosion technologies currently employed on Line 2—such as Kansai paint, hot-dip galvanizing, and Dacromet—cannot meet the anti-corrosion requirements for the steel structures of Chongqing’s rail transit beams, the Rail Beam Bridge System Research Center of Chongqing Rail Transit Corporation, in collaboration with Chongqing Dayou Surface Technology Co., Ltd., has undertaken intensive technical research and systematic studies on the corrosion protection of finger-plate components. After numerous batches of experiments and multiple cycles of testing, a wealth of experimental data has been collected, demonstrating that “the application of zinc-nickel diffusion coating technology can effectively address the rusting problems affecting finger-plate components.” The details of the zinc-nickel diffusion coating technology trials are presented below:

　　2.1 Principle of Zinc-Nickel Diffusion Coating Anti-Corrosion Technology

　　Zinc-nickel diffusion coating technology is a chemical heat treatment process conducted in the ferritic state. Under specific conditions, steel parts are heated while being thoroughly brought into contact with multiple elements, including zinc, nickel, and rare earths. This allows zinc, nickel, and other alloying elements to uniformly diffuse into the surface of the steel product, forming an alloy diffusion layer that achieves the purpose of corrosion protection for metal surfaces. This technology represents an advancement based on vacuum zinc diffusion and zinc-aluminum co-diffusion processes. A comparison of its performance with that of common metal corrosion protection technologies is shown in Table 1.2 below.

　　Comparison of the Corrosion-Resistant Performance of Zinc-Nickel Diffusion Coatings with Common Metal Anti-Corrosion Technologies – Table 1.2

　　2.2 Corrosion Resistance Test Procedure for Zinc-Nickel Diffusion Coatings

　　This test was conducted in three phases: The first phase was a preliminary (single-point) test phase; the second phase was a comprehensive verification phase on the test track; and the third phase was a field application test phase on the mainline. The test conditions for each phase are as follows:

　　2.2.1 Phase 1: Early (Single-Point) Testing Phase

　　2.2.1.1 Test Procedure

　　On March 22, 2007, led by the Research Center for Rail Beam Bridge Systems and in collaboration with companies including Dayou Corporation, the first trial of zinc-nickel diffusion-coated anti-corrosion finger plates was conducted on track beam J3-31 at the base test line. To determine the corrosion-resistant performance of this technology, the trial adopted a comparative testing approach: two zinc-nickel diffusion-coated running-surface finger plates provided by Chongqing Dayou Corporation were installed at positions #1 and #2 on track beam J3-31, while two hot-sprayed aluminum-magnesium treated running-surface finger plates retrieved from the warehouse were installed at positions #8 and #9 on the same track beam. The two sets of finger plates were subjected to a simultaneous comparison test under identical environmental conditions, as shown in the figure below.

　　2.2.1.2 Test Results

　　After installing the test finger plates, we observed their corrosion resistance under identical conditions for two different treatment methods, checking them once a month. One month after the test began, the finger plates treated with hot-sprayed aluminum-magnesium coating started to discolor; two months later, rust spots appeared (as shown in the figure below). By May, extensive surface rusting had set in. In contrast, the zinc-nickel diffusion-coated finger plates remained intact and showed no discoloration whatsoever. As of July 2, 2009, these test finger plates had not exhibited any quality defects such as discoloration, flaking, paint loss, or rusting. The experimental results clearly demonstrate that the zinc-nickel diffusion coating offers superior anti-corrosion performance compared to anti-corrosion processes like hot-sprayed aluminum-magnesium, effectively enhancing the surface corrosion resistance of steel structures.

　　2.2.1.3 The comparison results are shown in Table 1.3 and Appendix 2, Appendix 2-1.

　　Comparison Test Table Table 1.3

　　2.2.2 Phase 2: Complete Verification Phase on the Test Track:

　　2.2.2.1 Test Procedure

　　Given that the single-point tests conducted in the first phase demonstrated the advanced nature and excellent corrosion-resistant performance of the zinc-nickel diffusion coating technology, the Rail Beam Bridge System Center and Dayou Company jointly carried out the second-phase testing on May 28, 2007, as shown in the figure to the right.

　　The second-phase trial was conducted on the test track SDK-28 at the base. This trial had three objectives: First, to comprehensively verify the corrosion resistance of the zinc-nickel diffusion coating; second, to explore a comprehensive on-site treatment solution for the already installed plate seats; and third, to test the corrosion resistance of finger-plate bolts coated with the zinc-nickel diffusion layer. As in the previous trial, this one also employed a comparative testing approach: Eleven finger plates treated with the zinc-nickel diffusion coating—provided by Chongqing Dayou Company—were installed at the following positions on the SDK-28 rail beams: Nos. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 12. The No. 11 finger plate continued to be made using the hot-dip galvanizing process. Both sets of finger plates were subjected to comparative testing under identical conditions and in the same environment. At the same time, the finger-plate bolts were also treated with the same corrosion-resistant process and tested concurrently.

　　2.2.2.2 Test Results

　　To evaluate the coating performance of finger plates, our department conducts an inspection of the test finger plates once a month to observe the corrosion protection effect of the finger plates treated with zinc-nickel diffusion layers. Additionally, to assess the effectiveness of zinc-plated bolts, countersunk holes sealed with sealant, and the application of release agents on screw threads and the bottom surfaces of the finger plates, we organized two special inspections on the coated finger plate seats of Pier No. 28 on the base test track—SDK-28—on July 6, 2007, and February 2008 (as shown in the figure below).

　　The experimental results show:

　　(1) The finger-plate system with zinc-nickel diffusion coatings used in the complete set of tests demonstrates strong corrosion resistance; no quality defects such as discoloration, blistering, or rusting have been observed on either the finger plates or the bolts, as shown in the figure below.

　　(2) This technology’s corrosion resistance is significantly superior to that of finger plates treated by processes such as hot-dip galvanizing.

　　(3) Finger-plate bolts treated with a zinc-nickel diffusion coating also exhibit excellent corrosion resistance, with no signs of rust or other quality defects appearing.

　　2.2.2.3 Records of the test monitoring status are shown in Table 1.4 and Appendix 3 below.

　　Complete Test Table Table 1.4

　　2.2.3 Phase 3: In-Service Application Trial Phase:

　　2.2.3.1 Test Procedure

　　Given that the results of both the single-point tests in Phase I and the full-system tests in Phase II similarly demonstrated the advanced nature and excellent corrosion protection performance of the zinc-nickel diffusion coating technology, the Rail Beam Bridge System Center and Dayou Company jointly conducted the third-phase field application tests from December 10 to 21, 2007. In this phase of testing, a total of 120 finger plates from 10 joints—spanning from piers D212-28 to D212-37 on the down line between Yang and Dong stations on Line 2—were selected for corrosion protection testing.
The objectives of this test are as follows: First, to evaluate the corrosion resistance of the zinc-nickel diffusion-layer finger-plate assembly under the wheel-rail contact conditions on the mainline; second, to explore an integrated treatment solution for the plate seats on the mainline; and third, to test the corrosion resistance of the finger-plate bolts coated with a zinc-nickel diffusion layer.

　　2.2.3.2 Test Results

　　To evaluate the effectiveness of zinc-impregnated bolts and painted finger plates on the main line, the Line Facilities Department conducts monthly monitoring inspections on 10 finger plates. The inspection process combines record-keeping with photographic documentation. Meanwhile, on March 18, 2008, and March 17, 2009, a joint inspection team from the Bridge Research System Center and Chongqing Dayou Surface Technology Co., Ltd. conducted two dismantling and reassembly inspections to assess the corrosion protection status. The inspections confirmed that the finger plates, finger plate bolts, and spring washers treated with zinc-nickel diffusion coatings showed no signs of rust. The test results indicate:

　　(1) The finger-plate system with zinc-nickel diffusion coatings used in the mainline tests demonstrates strong corrosion resistance; no quality defects such as discoloration, blistering, or rusting have been observed on either the finger plates or the bolts.

　　(2) Under the impact and friction of train wheelsets, the surface coating of the finger-shaped plates shows no signs of wear, rust, or other quality defects, and the anti-corrosion performance is evident. As shown in the figure below:

　　(3) After more than 550 cumulative days of impact and friction from train wheelsets, no wear or thinning of the coating thickness was observed, demonstrating that this anti-corrosion technology is viable and can thoroughly address the rusting problem affecting the finger-plate assemblies.
 
　　2.2.3.3 Records of the test monitoring status are shown in Table 1.5 and Appendix 4 below.

　　Complete Test Table

　　Inspection Date: 2009.3.17 Table 1.5

　　3. Test Summary

　　3.1 The comparison of tests at each stage is shown in Table 1.6.

　　Comparison Table of Trials at Each Stage (as of July 2, 2009) Table 1.6

　　3.3 Conclusion

　　(1) After multiple-stage and multi-batch tests, the finger-plate system with zinc-nickel diffusion layers showed no quality defects such as discoloration, delamination, or rusting, demonstrating strong corrosion resistance.

　　(2) Although the various corrosion-resistant indicators measured in the laboratory are comparable, other corrosion-resistant technologies fail to meet the technical requirements for corrosion resistance, wear resistance, and skid resistance under the conditions of Chongqing’s acid rain environment as well as the impact and friction exerted by train wheelsets. However, after long-term testing, the zinc-nickel diffusion coating technology has proven capable of simultaneously satisfying all three requirements: corrosion resistance, wear resistance, and skid resistance.

　　(3) Finger-plate bolts treated with a zinc-nickel diffusion coating also exhibit excellent corrosion resistance, effectively preventing quality defects such as rusting, fracture, and rust-induced adhesion.

　　(4) The corrosion protection technology using zinc-nickel diffusion layers can meet the corrosion protection requirements for monorail finger plate assemblies and effectively address the corrosion challenges faced by the finger plate bases of already-completed and operational monorail systems.

　　4. Recommendations

　　Given the strong corrosion resistance of zinc-nickel plating technology, as well as its ability to meet the technical requirements for anti-skid and wear resistance necessary for the safe operation of trains, enhance the aesthetic appeal of railway lines, effectively address the severe rusting issues affecting finger plates and plate seats, reduce operational and maintenance costs, and ensure the safe operation of railway lines, we recommend the following:

　　(1) During the design process for Phase I and Phase II of Line 3, as well as the extension sections of Lines 2 and 3, the design institute immediately incorporated into the design drawings the requirement to use zinc-nickel alloy co-diffusion corrosion protection technology for the finger-plate components.

　　(2) The Monorail Company shall, as soon as possible, take measures to adopt a joint approach with Dayou Company to perform zinc-nickel diffusion coating for the uninstalled finger plates and plate seat assemblies on the under-construction lines (including Phase I and Phase II of Line 3, as well as the extension sections of Lines 2 and 3).

　　(3) For the installed track beams, the design institute and the monorail company, in collaboration with Dayou Company, will carry out secondary processing.

Railway Beam Bridge System Research Center

July 3, 2009