Galvanized Steel (Coated): Properties and Key Applications

Galvanized steel, commonly referred to as coated steel, is a type of steel that has been coated with a layer of zinc to enhance its corrosion resistance. This steel grade is primarily classified as low-carbon mild steel, which is known for its ductility and weldability. The galvanization process involves either hot-dip galvanizing or electro-galvanizing, both of which provide a protective zinc layer that prevents rust and extends the lifespan of the steel.

Comprehensive Overview

Galvanized steel is characterized by its excellent corrosion resistance, which is primarily due to the protective zinc coating that acts as a barrier against environmental factors. The primary alloying element in galvanized steel is zinc, which, when exposed to moisture, forms a protective layer of zinc oxide that prevents further corrosion of the underlying steel.

Key Characteristics:
- Corrosion Resistance: The zinc coating provides a sacrificial layer that protects the steel from rusting.
- Ductility and Weldability: Low-carbon content allows for easy fabrication and welding.
- Surface Finish: Galvanized steel has a shiny, aesthetically pleasing finish that is often desirable in architectural applications.

Advantages:
- Extended Lifespan: Galvanized steel can last for decades without significant corrosion.
- Cost-Effective: The initial investment in galvanized steel can be offset by its longevity and reduced maintenance costs.
- Versatility: Suitable for a wide range of applications, from construction to automotive.

Limitations:
- Temperature Sensitivity: The zinc coating can degrade at high temperatures, limiting its use in high-heat applications.
- Surface Damage: Scratches or abrasions can expose the underlying steel, leading to localized corrosion.
- Weight: The added zinc coating increases the weight of the steel, which may be a consideration in certain applications.

Historically, galvanized steel has played a significant role in various industries, particularly in construction and automotive sectors, where its durability and resistance to environmental factors are paramount.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	G90	USA	Closest equivalent for corrosion resistance
ASTM	A653/A653M	USA	Specification for hot-dip galvanized steel
EN	10346	Europe	European standard for continuously galvanized steel
JIS	G3302	Japan	Japanese standard for hot-dip galvanized steel
ISO	1461	International	Standard for hot-dip galvanized coatings

The differences between equivalent grades can affect performance, particularly in terms of corrosion resistance and mechanical properties. For instance, G90 offers a thicker zinc coating compared to G60, which may be critical in environments with high corrosion potential.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.05 - 0.25
Mn (Manganese)	0.30 - 0.60
P (Phosphorus)	≤ 0.04
S (Sulfur)	≤ 0.05
Zn (Zinc)	90 - 98 (coating)

The primary role of zinc in galvanized steel is to provide corrosion resistance. Carbon and manganese contribute to the steel's strength and ductility, while phosphorus and sulfur are controlled to maintain weldability and reduce brittleness.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Annealed	270 - 450 MPa	39 - 65 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	200 - 350 MPa	29 - 51 ksi	ASTM E8
Elongation	Annealed	20 - 30%	20 - 30%	ASTM E8
Hardness (Brinell)	Annealed	120 - 180 HB	120 - 180 HB	ASTM E10
Impact Strength (Charpy)	-20°C	30 - 50 J	22 - 37 ft-lbf	ASTM E23

The combination of these mechanical properties makes galvanized steel suitable for applications requiring good strength and ductility, such as structural components and automotive parts.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	-	7.85 g/cm³	0.284 lb/in³
Melting Point	-	1425 - 1540 °C	2600 - 2800 °F
Thermal Conductivity	20 °C	50 W/m·K	34.5 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	-	0.49 kJ/kg·K	0.12 BTU/lb·°F
Electrical Resistivity	-	0.0000017 Ω·m	0.0000017 Ω·in

Key physical properties such as density and thermal conductivity are crucial in applications where weight and heat dissipation are factors, such as in HVAC systems and automotive components.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Saltwater	3.5	25/77	Fair	Risk of pitting
Acetic Acid	5	20/68	Poor	Susceptible to SCC
Sulfuric Acid	10	25/77	Not Recommended	Rapid corrosion

Galvanized steel exhibits excellent resistance to atmospheric corrosion and saltwater, making it ideal for outdoor applications. However, it is susceptible to corrosion in acidic environments, which can lead to rapid degradation. Compared to stainless steel, galvanized steel is less resistant to corrosive agents, particularly in aggressive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	200	392	Beyond this, zinc begins to oxidize
Max Intermittent Service Temp	300	572	Short-term exposure is acceptable
Scaling Temperature	400	752	Risk of zinc loss at higher temps

At elevated temperatures, galvanized steel can lose its protective zinc coating, which compromises its corrosion resistance. This makes it unsuitable for high-temperature applications without additional protective measures.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG	ER70S-6	Argon/CO2	Good for thin sections
TIG	ER70S-2	Argon	Provides clean welds
Stick	E7018	-	Requires preheat for thick sections

Galvanized steel can be welded using various methods, but care must be taken to avoid overheating, which can lead to zinc vaporization and weld defects. Preheating may be necessary for thicker sections to ensure proper fusion.

Machinability

Machining Parameter	Galvanized Steel	AISI 1212	Notes/Tips
Relative Machinability Index	60	100	More difficult due to coating
Typical Cutting Speed (turning)	30 m/min	50 m/min	Adjust for coating thickness

Machining galvanized steel can be challenging due to the zinc coating, which can wear down cutting tools more quickly than uncoated steel. Using sharp tools and appropriate cutting speeds can mitigate these issues.

Formability

Galvanized steel exhibits good formability, making it suitable for cold and hot forming processes. However, the zinc coating can affect the bending radius, and care must be taken to avoid cracking during forming operations.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	600 - 700 / 1112 - 1292	1 - 2 hours	Air	Improve ductility and reduce hardness
Normalizing	800 - 900 / 1472 - 1652	1 - 2 hours	Air	Refine grain structure

Heat treatment processes can significantly alter the microstructure of galvanized steel, enhancing its mechanical properties. Annealing, for instance, can improve ductility, making it easier to work with in fabrication.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Construction	Roofing Sheets	Corrosion Resistance, Ductility	Long-lasting outdoor use
Automotive	Body Panels	Strength, Formability	Lightweight and durable
Agriculture	Fencing	Corrosion Resistance, Weldability	Exposure to harsh environments
HVAC	Ductwork	Thermal Conductivity, Corrosion Resistance	Efficient heat transfer

Other applications include:
- Electrical Enclosures: Protection against moisture and corrosion.
- Furniture: Aesthetic appeal and durability.
- Pipes and Tubing: Resistance to rust in plumbing applications.

Galvanized steel is chosen for these applications due to its balance of strength, corrosion resistance, and cost-effectiveness, making it ideal for environments where exposure to moisture is a concern.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	Galvanized Steel	Stainless Steel	Carbon Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	Moderate Strength	High Strength	Variable	Galvanized is less strong than stainless
Key Corrosion Aspect	Good	Excellent	Poor	Galvanized is suitable for moderate exposure
Weldability	Good	Excellent	Good	Galvanized requires care to avoid defects
Machinability	Moderate	Good	High	Coating can wear tools faster
Formability	Good	Moderate	Good	Zinc coating affects bending
Approx. Relative Cost	Low	High	Low	Galvanized is cost-effective
Typical Availability	High	Moderate	High	Widely available in various forms

When selecting galvanized steel, considerations include cost-effectiveness, availability, and specific application requirements. Its balance of properties makes it a versatile choice for many industries, though its limitations in high-temperature and acidic environments must be acknowledged. Additionally, safety considerations regarding the handling of galvanized materials, particularly during welding, should be taken into account to prevent exposure to zinc fumes.

In summary, galvanized steel is a highly valuable material in modern engineering and construction, offering a unique combination of properties that make it suitable for a wide range of applications. Its historical significance and continued relevance in various sectors underscore its importance in material science and engineering.