AASHTO T 213M/T 213-2023
Mass [Weight] of Coating on Aluminum-Coated Iron or Steel Articles

Standard No.

AASHTO T 213M/T 213-2023

Release Date

2023

Published By

American Association of State Highway and Transportation Officials

Latest

AASHTO T 213M/T 213-2023

Introduction

Standard Overview and Technical Background

Standard T 213M/T 213-231, published by the American Association of State Highway and Transportation Officials (AASHTO), is the definitive test method for evaluating the coating quality of aluminum-coated steel products. This standard, which underwent technical revisions in 2023 to maintain technical consistency with ASTM A428/A428M-21, is maintained by Subcommittee 4f (Metallic Materials).

Aluminum coatings provide corrosion protection for steel substrates by forming a relatively inert barrier. Different applications have varying requirements for coating quality. This standard provides three chemical stripping methods to accurately measure coating quality, providing a technical basis for engineering material selection and quality control.

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Test Scope and Applicability Analysis

Test Object	Minimum Specimen Area	Recommended Dimensions	Unit Selection
Aluminum Coated Sheet	2000 mm² [3 in.²]	3300 mm² [5 in.²]	SI or Imperial Units
Aluminum Coated Wire	Length>300 mm [12 in.]	Approximately 600 mm [24 in.]	Based on Material Specifications
Other aluminum coated products	1935 mm² [3 in.²]	Based on actual shape	Uniform surface area

The standard specifically emphasizes that for threaded products such as bolts and screws, testing should be performed on parts that do not contain threads to ensure the accuracy of the measurement results. The test results can be expressed as the total coating mass on both sides or the coating mass on each side as required.

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Chemical Reagents and Safety Specifications

A variety of chemical reagents are involved in the testing process. The standard has made clear provisions on the purity and preparation requirements of the reagents:

Reagent Name	Preparation Requirements	Concentration Specifications	Safety Precautions
Antimony trichloride solution	200g SbCl₃ dissolved in 1000mL concentrated HCl	Analytical grade	May produce highly toxic gas SbH₃
Sodium hydroxide solution	20% mass concentration	ACS reagent grade	Strong alkalinity, avoid skin contact
Stannic Chloride Solution	100g SnCl₂·2H₂O dissolved in 1000mL concentrated HCl	Add tin particles for protection	Acidic and corrosive
Hydrochloric Acid Solution (1+1)	Mix equal volumes of concentrated HCl and reagent water	Cool to room temperature	Provide adequate ventilation; avoid inhalation

Important Safety Note: The hydrochloric acid-antimony trichloride-stannous chloride method may generate a small amount of highly toxic antimony hydrogen (SbH₃) gas. Hydrochloric acid fumes and hydrogen are also released during the stripping process. Testing should be performed under adequate ventilation. A fume hood is strongly recommended for large sample batches or long-term testing.

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Test Method Technical Comparison

Method Type	Chemical System	Applicable Coatings	Operating Characteristics	Temperature Control
Method A	Sodium Hydroxide-HCl Cycle	All Coating Types	Multiple Cycles	NaOH Heated to 90°C
Method B	HCl-Antimony Trichloride-Stannic Chloride	Commonly Used for Plates	Single Immersion	≤38°C [100°F]
Method C	Dilute Hydrochloric Acid (1+1)	Coatings with Low Silicon Content	Solution Can Be Reused	≤38°C [100°F]

Method A Instructions

Immerse the specimen in a hot sodium hydroxide solution at approximately 90°C until the reaction ceases, but not for more than a few minutes. Remove and scrub with a cellulose sponge under running water to remove any loose deposits formed in the NaOH solution. After blotting most of the water with a towel, immerse the specimen in room temperature concentrated hydrochloric acid for no more than 3 seconds on one side. Scrub again and re-immerse in the hot NaOH solution. Repeat this cycle until no visible reaction is observed from the hydrochloric acid immersion. This cycle will typically require 1-3 or more cycles, depending on the type and quality of the coating.

Method B Key Points

Mix 100mL each of antimony trichloride solution and stannous chloride solution. Immerse individual specimens in the mixture until hydrogen evolution ceases, typically 1-4 minutes or longer, depending on coating thickness and silicon content. Use fresh solution for each test, and the stripping solution temperature must not exceed 38°C. After stripping, rinse with water and scrub with a soft cloth, using a small amount of abrasive cleaning powder if necessary.

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Calculation system and unit conversion

The standard provides a complete calculation system, covering different specimen types and unit systems:

Calculation of plate coating mass

SI unit system: When measuring the area of a single side of the plate, the coating mass calculation formula is:

C = (W₁ - W₂) × K / A

Where C is the coating mass (g/m²), W₁ is the original mass (g), W₂ is the mass after stripping (g), A is the single-side area (mm² or in.²), and K is a constant (A is 1×10⁶ when in mm² and 1.55×10³ when in.²).

Imperial Units: Coating weight is calculated as follows:

C = (W₁ - W₂) × K / A

Where C is the coating weight (oz/ft²) and K is a constant (A is 3.28×10³ in mm² and 5.08 in.²).

Special Size Optimization

The standard specifically states that if the specimen is prepared according to the dimensions required by Note 2 (area 3330 mm² or 5.08 in.²), the coating weight values have a direct conversion relationship between SI and imperial units: the mass loss in grams is numerically equivalent to the coating weight in ounces per square foot.

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Precision Analysis and Quality Control

Based on an interlaboratory study conducted in 2008, 14 laboratories tested 5 different materials and established standard reproducibility limits:

Material Type	Average Upper Coating Weight (oz/ft²)	Reproducibility Limit R	Average Lower Coating Weight (oz/ft²)	Reproducibility Limit R
Material A	0.224	0.054	0.205	0.064
Material B	0.279	0.082	0.216	0.073
Material C	0.292	0.098	0.203	0.079
Material D	0.233	0.051	0.183	0.053
Material E	0.219	0.063	0.188	0.079

The accuracy of the test results depends largely on the professional ability of the operator and the calibration and maintenance of the equipment used. Laboratories that meet the R 18 standard are generally considered to have qualified and objective testing capabilities, but compliance with R 18 alone does not completely guarantee reliable test results.

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Standard Evolution and Technological Development

T 213M/T 213 standard has undergone several technical revisions. The main updates of the 2023 edition include:

• Maintaining full technical consistency with ASTM A428/A428M-21
• Strengthening safety specification requirements and adding Section 1.5 Chemical Hazard Classification
• Improving the unit conversion system to ensure accurate correspondence between SI and English units
• Updated precision analysis data based on the latest inter-laboratory research results

The technological development of the standard reflects the continuous expansion of the application of aluminum coating materials in transportation infrastructure, from traditional highway guardrails to key engineering parts such as bridge components and tunnel linings, which puts forward higher requirements for coating quality control and durability evaluation.

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Implementation Recommendations and Best Practices

Based on standard requirements and engineering experience, the following implementation recommendations are proposed:

Laboratory Configuration Requirements

The testing laboratory should be equipped with an analytical balance with an accuracy of at least 0.01g, a constant temperature water bath, a fume hood system, and pure water preparation equipment that meets ASTM D1193 Type IV or higher. Chemical reagent storage should comply with OSHA's hazard communication standard (29 CFR 1910.1200).

Specimen Preparation Specifications

Plate specimens are recommended to use a standard size of 57.7mm x 57.7mm square or 65.1mm diameter round, ensuring a test area of precisely 3330mm². Wire specimens should avoid bending or deformation and maintain their original surface condition. All specimens should be cleaned with petroleum ether or a suitable solvent and thoroughly dried before testing.

Quality Control System

It is recommended that laboratories establish a quality management system based on the R 18 standard and conduct regular equipment calibration and personnel training. For key engineering projects, inter-laboratory comparison tests should be considered to ensure the comparability and reliability of test results.

Data Reporting Specifications

The test report should clearly indicate the unit system used (SI or Imperial), test method (A, B or C), number of specimens, and statistical processing method. Coating mass values should be reported to the nearest 1g/m² in SI units and to the nearest 0.01oz/ft² in Imperial units. The average value of multiple specimens should be rounded according to the rounding method specified in ASTM E29.

By strictly implementing the requirements of the T 213M/T 213 standard, engineering units can accurately evaluate the anti-corrosion performance of aluminum-coated steel products and provide technical support for the long-term durability of infrastructure.

AASHTO T 213M/T 213-2023 history

• 2023 AASHTO T 213M/T 213-2023 Mass [Weight] of Coating on Aluminum-Coated Iron or Steel Articles

Standard and Specification