MS vs SS vs GI for Solar MMS – Properties, Parameters & Standards

The Module Mounting Structure (MMS) is one of the most important structural components of a ground-mounted or rooftop solar power plant. It supports the PV modules, maintains the required tilt angle and transfers dead load, wind load and other forces safely to the foundations.

Selecting the correct material for an MMS is therefore not simply a matter of choosing the cheapest steel section. The designer must consider structural strength, section thickness, corrosion resistance, environmental exposure, galvanizing quality, fabrication method, fastener compatibility, expected project life and applicable standards.

Before selecting a section, the overall table geometry must already be understood. If you are following the OneCalcApp MMS series, start with the Solar MMS Table Dimension Calculation and then use this material guide before final member design.

The three terms most commonly encountered in solar projects are:

• MS – Mild Steel
• GI – Galvanized Iron / Galvanized Steel
• SS – Stainless Steel

One important engineering distinction must be understood first:

GI used in solar MMS is normally not a separate structural material. It is usually structural carbon steel or mild steel protected by a zinc coating.

Therefore, the structural strength of a GI member comes mainly from its base steel, while the zinc coating primarily provides corrosion protection.

1. What Is Mild Steel (MS)?

Mild Steel is a low-carbon structural steel widely used in general fabrication and structural construction.

For Indian solar projects, structural members may commonly be specified using grades under IS 2062, depending on the project specification and required strength.

Typical MMS components fabricated from structural steel can include:

• Columns / posts
• Rafters
• Purlins
• Bracing members
• Base plates
• Cleats
• Connection plates
• Module support rails
• Foundation connection members

Main advantages of MS

MS provides:

• Good structural strength
• Easy fabrication
• Good weldability
• Wide availability
• Relatively low cost
• Easy forming and cutting
• Large range of sections

The major disadvantage is corrosion.

Bare mild steel exposed to rain, humidity, soil contaminants and industrial or coastal atmospheres can corrode rapidly unless an appropriate protection system is provided.

For this reason, bare MS is generally not preferred for long-life outdoor solar MMS without galvanizing or an engineered protective coating system.

2. What Is GI in Solar MMS?

GI is commonly called Galvanized Iron, but for structural solar applications a more technically accurate description is usually Hot-Dip Galvanized Structural Steel – HDG Steel.

The fabricated steel member is protected with zinc. A typical sequence is:

• Steel fabrication
• Cleaning
• Pickling
• Fluxing
• Hot-dip galvanizing
• Inspection

The fabricated steel member is immersed in molten zinc, producing a metallurgically bonded zinc coating.

For Indian projects, IS 4759 is commonly referenced for hot-dip zinc coatings on structural steel and allied products, while IS 2629 provides recommended practice for hot-dip galvanizing of iron and steel.

International projects may reference ISO 1461 or ASTM A123/A123M, subject to the client specification.

3. What Is Stainless Steel (SS)?

Stainless Steel contains sufficient chromium to form a protective passive oxide layer at the surface.

Unlike galvanized steel, its corrosion resistance does not depend on an externally applied zinc layer.

SS 304

SS 304 is commonly considered for many normal atmospheric applications such as:

• Fasteners
• Washers
• Clamps
• Small brackets
• Cable-support components

SS 316

SS 316 contains molybdenum and generally provides improved resistance to chloride-related corrosion compared with conventional 304 stainless steel.

It is therefore commonly considered for:

• Coastal sites
• Marine environments
• High-humidity environments
• Chloride exposure
• Critical fasteners

For stainless flat products, project specifications may reference applicable Indian or international stainless steel standards such as IS 6911 or ASTM A240/A240M, depending on the product and project requirement.

4. MS vs GI vs SS – Basic Comparison

5. Important Engineering Point: GI Strength Is the Strength of the Base Steel

A common mistake is assuming that galvanizing increases the structural capacity of a member.

It normally does not.

Consider an MMS member specified as:

IS 2062 E250 structural steel + Hot-Dip Galvanizing

Its structural design is based primarily on parameters such as:

• Yield strength of the base steel
• Ultimate tensile strength
• Section area
• Moment of inertia
• Section modulus
• Thickness
• Buckling length
• Connection strength

The zinc coating is primarily considered for:

• Corrosion protection
• Coating thickness
• Coating mass
• Uniformity
• Adhesion
• Service durability

Base steel controls structural capacity; galvanizing controls corrosion protection.

After material selection, dead load can be calculated from the member geometry and density using the Solar MMS Dead Load Calculation Guide.

6. Important Material Parameters for Solar MMS Design

An MMS designer should not simply write:

Material: GI

That description is incomplete.

A complete specification should identify the structural grade, section, thickness, design standard, corrosion protection and fastener requirement.

6.1 Material Grade

Example:

Structural Steel: IS 2062 E250

or another project-approved grade.

The material grade determines important mechanical properties including:

• Yield stress
• Tensile strength
• Elongation
• Bend performance
• Chemical composition

A Material Test Certificate or MTC should be checked against the specified material standard.

6.2 Yield Strength – fy

Yield strength represents the stress level at which permanent deformation begins.

For example, an E250 grade designation corresponds to a nominal minimum yield level of approximately 250 MPa, subject to the applicable thickness, quality designation and provisions of the governing standard.

Higher-strength grades may allow smaller or lighter structural members, but strength alone should never be used to reduce thickness without checking:

• Local buckling
• Distortional buckling
• Deflection
• Connection capacity
• Hole bearing
• Corrosion allowance
• Fabrication tolerance

6.3 Ultimate Tensile Strength – fu

Ultimate tensile strength is important for:

• Bolted connection checks
• Net-section rupture
• Connection design
• Material verification

The exact value should be taken from the applicable material standard and MTC.

6.4 Modulus of Elasticity – E

For normal structural carbon steel, a typical engineering value is approximately:

E ≈ 200 GPa

Stainless steels typically have a similar but somewhat lower modulus depending on grade.

The modulus influences:

• Deflection
• Member stiffness
• Buckling
• Structural deformation

6.5 Density

A typical density used for structural carbon steel is approximately:

7,850 kg/m³

Stainless steels are usually slightly heavier, commonly around:

7,900–8,000 kg/m³, depending on grade.

Density is important because:

which contributes to the structure dead load.

6.6 Section Thickness

For solar MMS, thickness is extremely important.

Typical members may be fabricated from:

• Cold-formed C sections
• Hat sections
• Z sections
• Channels
• Angles
• Hollow sections
• Plates

Two sections having the same overall depth do not necessarily have the same capacity.

The designer must verify actual thickness, manufacturing tolerance, effective section, local buckling, hole location, bending capacity, shear capacity and compression capacity.

Cold-formed light-gauge members require particular attention because thin sections are sensitive to local and distortional buckling. Where applicable, project design may reference IS 801 and related cold-formed steel provisions.

7. Section Properties Required for MMS Structural Analysis

When designing an MMS member, the following parameters may be required:

These values directly influence bending strength, compression strength, buckling, deflection and wind-load response.

For hot-rolled structural sections, IS 808 is commonly referenced for dimensions, mass and sectional properties.

The next structural checks should be coordinated with the Solar MMS Wind Load Calculation and Solar MMS Load Combination Guide.

8. Galvanizing Parameters for GI Solar MMS

For GI / HDG MMS, structural analysis alone is not sufficient. The galvanizing specification is equally important.

Major parameters include:

Zinc coating mass

Usually expressed in:

g/m²

Zinc coating thickness

Usually expressed in:

µm – micrometres

A useful approximate engineering conversion is:

1 µm of zinc ≈ 7.1 g/m²

Therefore:

610 g/m² ≈ 85 µm

approximately.

However, this does not mean every MMS member must automatically be specified as 85 µm.

Coating requirements vary according to factors such as:

• Steel thickness
• Product type
• Governing standard
• Project specification
• Environmental exposure

The final requirement must always be checked against the latest applicable standard and approved project specification.

9. Common Solar Project Requirement – 610 g/m²

In many Indian infrastructure and solar specifications, you may encounter a requirement such as:

Hot-Dip Galvanizing: 610 g/m²

This corresponds approximately to:

85 µm zinc thickness

and is frequently specified for outdoor structural steel.

However:

610 g/m² should not automatically be copied into every MMS specification.

The correct coating requirement must be selected based on:

Material thickness + relevant standard + environmental condition + client/project specification.

Thin cold-formed members and thicker fabricated plates may be governed by different coating requirements.

10. Galvanizing Quality Tests

A proper GI MMS inspection should not rely only on visual appearance.

Important checks can include:

Coating Mass

Confirms the amount of zinc deposited on the steel.

Coating Thickness

Measured using suitable calibrated coating-thickness equipment.

Uniformity

Ensures adequate coating across the component.

Adhesion

Checks whether the galvanized layer is properly bonded.

Visual Condition

Look for:

• Uncoated areas
• Excess zinc
• Rough deposits
• Flux residue
• Ash deposits
• Bare edges
• Damage during transport
• Damaged drilled holes
• Damaged field-cut surfaces

For site inspection and acceptance checks, also refer to the Solar MMS Quality Control Guide.

11. Why Fabrication Should Normally Be Completed Before Hot-Dip Galvanizing

For fabricated MMS members, the preferred sequence is generally:

Cut → Punch / drill → Bend → Weld → Clean → Hot-dip galvanize

rather than:

Galvanize → Weld → Drill → Cut

Drilling, grinding or welding after galvanizing can locally remove the protective zinc layer and expose the underlying steel.

Any site modification therefore requires an approved repair procedure.

This becomes particularly important at:

• Slotted holes
• Field-drilled holes
• Cut edges
• Welded joints
• Damaged transport areas

12. Drain and Vent Holes for Galvanized Hollow Members

If hollow steel sections are hot-dip galvanized, appropriate venting and drainage are essential.

The molten zinc must:

• Enter the member
• Displace air
• Flow through the section
• Drain safely

Improper vent or drain design can create entrapped air, incomplete galvanizing, dangerous pressure build-up, zinc accumulation or uneven coating.

Therefore galvanizing requirements should be considered during fabrication design, not after the MMS is already manufactured.

13. MS Painted Structure vs Hot-Dip Galvanized Structure

Painted MS can provide adequate corrosion protection when a properly engineered coating system is applied.

A typical protection system may include:

• Surface preparation
• Zinc-rich primer
• Intermediate coat
• Top coat

However, painted systems may require periodic inspection, touch-up, recoating and surface preparation.

For long-life outdoor solar structures, hot-dip galvanizing is popular because zinc provides both barrier protection and sacrificial protection.

Where painted steel is selected, the project may refer to the ISO 12944 series or another approved protective-coating specification appropriate to the exposure environment.

14. Stainless Steel 304 vs 316 in Solar MMS

Using “SS” alone is also incomplete. The grade must be specified.

SS 304

Typical selection for:

• General outdoor fasteners
• Clamps
• Washers
• Small fittings
• Non-aggressive environments

SS 316

Better suited when chloride exposure is high, such as:

• Coastal solar plants
• Marine areas
• High-salinity environments
• Certain industrial atmospheres

However, SS 316 is more expensive.

For a large utility-scale plant, making every structural member from stainless steel may not be economically justified.

A common engineering approach is therefore:

HDG structural steel MMS + selected SS fasteners / components

subject to galvanic compatibility and project requirements.

For connection-specific guidance, see the Solar MMS Bolts, Nuts and Washers Guide.

15. Stainless Steel Fasteners

Fasteners are critical components of an MMS.

They transfer forces between:

• Modules
• Purlins
• Rafters
• Bracing
• Columns
• Cleats
• Base plates

Stainless fasteners should be specified by both material grade and property class, rather than simply saying “SS bolt”.

The ISO 3506 series is commonly referenced for mechanical properties of corrosion-resistant stainless steel fasteners.

Typical project descriptions may refer to grades such as A2 stainless or A4 stainless, depending on exposure and project specification.

16. Galvanic Corrosion Between SS and GI

Another important MMS issue is dissimilar metal contact.

For example:

SS fastener + galvanized steel member

can create a galvanic couple when moisture or another electrolyte is present.

This does not automatically mean the combination cannot be used.

It means the designer should consider:

• Area ratio
• Moisture exposure
• Chloride concentration
• Insulating washers
• Surface coatings
• Drainage
• Connection geometry
• Long-term zinc consumption

This issue becomes particularly important in coastal solar installations.

17. Major Standards for Solar MMS Material Selection

Engineering note: Always verify the latest revision, amendment, client specification and statutory requirement before using a standard for project approval or procurement.

18. MMS Material Selection by Environment

Normal Inland Environment

A common economical solution is:

Structural steel + Hot-Dip Galvanizing

Advantages include good structural strength, reasonable cost, low maintenance and long outdoor service potential.

Industrial or High-Humidity Environment

Consider:

• Increased corrosion-protection requirement
• Detailed coating specification
• Improved drainage
• Inspection provisions
• Appropriate fastener materials

Coastal Environment

Coastal conditions require greater attention because chlorides accelerate corrosion.

Possible strategies include:

• Enhanced HDG specification
• Duplex system – galvanizing plus protective paint
• SS316 fasteners
• Suitable material isolation
• More frequent inspection
• Full stainless construction only where lifecycle and project economics justify it

19. Which Is Best for Solar MMS?

There is no single material that is automatically best for every project.

MS is best when:

• Low initial cost is important
• Fabrication is extensive
• A proper corrosion-protection system will be applied

GI / HDG Steel is best when:

• Outdoor durability is required
• Cost must remain reasonable
• Long-term maintenance needs to be reduced

For many ground-mounted solar projects, hot-dip galvanized structural steel provides one of the best balances between structural performance, durability and cost.

SS is best when:

• Corrosion exposure is severe
• Component size is relatively small
• Maintenance access is difficult
• High lifecycle durability justifies higher initial cost

SS is therefore widely useful for fasteners, clamps, washers, special brackets and coastal critical components rather than necessarily making the entire MMS from stainless steel.

20. Recommended MMS Material Specification Format

Instead of writing:

Material: GI

a more professional specification could state:

This is much more technically meaningful than specifying only “GI structure”.

21. Parameters to Check in an MMS Material Approval

Before approving MMS material, the engineer should verify:

• Material grade
• Material Test Certificate
• Chemical composition
• Yield strength
• Tensile strength
• Elongation
• Actual section thickness
• Section dimensions
• Sectional properties
• Member weight per metre
• Galvanizing standard
• Zinc coating mass
• Zinc coating thickness
• Coating uniformity
• Coating adhesion
• Surface defects
• Fastener material
• Bolt property class
• Weld quality
• Hole dimensions
• Fabrication tolerances
• Corrosion environment
• Galvanic compatibility
• Design life
• Project specification compliance

22. Common Mistakes in Solar MMS Material Selection

Mistake 1 – Writing only “GI”

GI does not identify the structural grade.

Always specify base steel + grade + galvanizing requirement.

Mistake 2 – Assuming zinc increases structural strength

The zinc layer is a corrosion-protection system, not the primary load-carrying material.

Mistake 3 – Using one galvanizing thickness for every member

Coating requirements vary with product type, material thickness and governing specification.

Mistake 4 – Ignoring cold-formed section buckling

A thin high-strength section can still fail by local buckling, distortional buckling or overall buckling.

Mistake 5 – Welding after galvanizing

This damages the zinc coating unless an approved repair procedure is followed.

Mistake 6 – Ignoring coastal exposure

Material that performs well at an inland plant may not provide the same service life near the sea.

Mistake 7 – Specifying only “SS bolt”

Always define the stainless grade, property class and applicable standard.

23. Final Recommendation

For a typical long-life solar MMS, material selection should be treated as two separate engineering decisions.

Structural Design

Determine:

Steel grade + section + thickness + structural capacity

Corrosion Protection

Determine:

Galvanizing / coating system + thickness / mass + environment + design life

A well-designed solar MMS therefore cannot be defined simply as MS / GI / SS.

Instead, it should be defined by a complete combination of:

Material grade + mechanical properties + section properties + structural design standard + corrosion-protection standard + fastener specification + environmental exposure.

For many Indian ground-mounted solar power plants, hot-dip galvanized structural steel remains an effective balance between strength, corrosion protection, fabrication practicality and project cost.

Stainless steel becomes especially valuable for fasteners and components exposed to aggressive environments, while unprotected mild steel should generally be avoided for long-term outdoor service.

For a complete design sequence, continue through Solar MMS Dead Load Calculation, Wind Load Calculation, Load Combinations, member design and the Solar MMS Foundation Guide.

Frequently Asked Questions

Is GI stronger than MS?

Not necessarily. The structural capacity of GI generally comes from the steel underneath the zinc coating. Galvanizing primarily improves corrosion resistance.

What is the best material for solar MMS?

For many utility-scale solar plants, hot-dip galvanized structural steel offers a practical combination of strength, durability and cost. Final selection depends on site environment, structural design and design life.

Is 80 or 85 micron galvanizing compulsory for every solar structure?

No. Zinc-coating requirements depend on material thickness, product type, governing standard, exposure conditions and project specification.

Why is SS316 preferred near the coast?

SS316 generally provides better resistance to chloride-related corrosion than standard SS304, making it useful for severe coastal exposure.

Which Indian Standard is commonly used for structural steel in MMS?

IS 2062 is commonly referenced for structural steel material, while structural design is generally carried out using applicable provisions such as IS 800 and wind loading using IS 875 Part 3.

Which standard covers galvanizing of structural steel?

For Indian projects, IS 4759 is commonly referenced for hot-dip zinc coatings on structural steel and allied products, while IS 2629 provides recommended hot-dip galvanizing practice. International projects may reference ISO 1461 or ASTM A123/A123M.

Related Solar MMS Guides and Tools

Continue the OneCalcApp MMS design series with these related resources:

• Solar MMS Material Selection Guide – broader procurement, storage and material quality considerations.
• Solar MMS Table Dimension Calculation – establish the 2P × 28 table geometry before structural design.
• Solar MMS Dead Load Calculation – calculate module and steel self-weight.
• Solar MMS Wind Load Calculation – calculate wind pressure and structural loading.
• Solar MMS Load Combinations – combine dead load and wind actions for design.
• Solar MMS Bolts, Nuts and Washers Guide – connection hardware and installation checks.
• Solar MMS Quality Control Guide – material and erection inspection checkpoints.
• Solar MMS Design Calculator – preliminary MMS geometry, wind pressure, support reactions and steel take-off.
• Wind Load on Module Calculator – quick wind-load estimation for a solar module table.

Standards Note

Standards and project specifications are periodically revised. Before using any material grade, coating requirement, fastener class or design value for procurement, fabrication or statutory approval, verify the latest applicable revision, amendments, client specification and local authority requirements.