Introduction: Seeing Rust Spots and Blaming the Supplier – You Might Be Wrong
When rust spots appear on steel grating during procurement or use, the first reaction is often “quality problem” or “galvanizing failure”, followed by an immediate demand for replacement or refund. However, this situation is far more complex than you might think – certain “rust spots” are not product defects and do not mean the steel grating is about to fail. In particular, the white or grey powdery deposits that appear on hot-dip galvanized (hot dip galvanizing) steel grating are explicitly permitted by international standards and industry codes.
Clause 6.1 of GB/T 13912-2002 (Metallic coatings – Hot dip galvanized coatings on fabricated iron and steel articles – Specifications and test methods) states: “For galvanized workpieces stored under wet conditions, white rust (a white or grey corrosion product mainly composed of basic zinc oxide) is allowed to exist on the surface.” Both GB/T 13912-2002 and GB/T 5267.3-2008 point out that white rust only affects appearance and generally has no impact on corrosion protection performance. This is not a uniquely “relaxed” clause in Chinese standards. European standard ISO 1461-1999 clearly states: “White rust produced during storage under wet conditions shall not be a reason for rejection.” Singapore Standard SS 363-2014 similarly adopts a pragmatic approach to the appearance of galvanized coatings: as long as the coating thickness meets the specified requirements, dark or light grey areas of uneven colour are permitted on the surface. White rust is allowed on the surface of galvanized steel grating stored under wet conditions, provided the coating thickness passes the required tests.
This article will help buyers and engineers in the Singapore market clarify: which rust spots are merely superficial problems (white rust and cut‑edge coating loss), and which are serious rust (red rust) that truly threatens structural safety. We will also provide simple identification methods and on‑site remedial measures.
Chapter 1: White Rust – A Common Misunderstanding and the Standard’s Answer
1.1 What is White Rust?
White rust, also known in the industry as “wet storage stain” , is a white or grey powdery corrosion product formed on the surface of a galvanized coating under specific environmental conditions. Its main chemical component is basic zinc oxide. When galvanized steel grating is tightly stacked and moisture or humid air exists between the layers, the zinc coating reacts with oxygen and water to form this layer of white corrosion product.
Chinese industry standards describe this very clearly: as long as the zinc coating thickness is greater than the specified value, white rust is permitted on the surface of galvanized steel grating stored under wet conditions – the thickness remains the same. Industry literature also clearly states: “White rust is allowed on the surface of galvanized steel grating stored under wet conditions; a small amount of white rust does not cause significant damage to the product.”
1.2 Causes of White Rust
White rust is most common during winter, the rainy season, or inside shipping containers. The reasons are exactly the humid environment, high moisture and poor ventilation at those times. Other contributing factors include:
Tight stacking: Grating panels are packed closely together during transport and storage, making it difficult for moisture between layers to evaporate.
Poor ventilation: Packaging or stacking methods block air circulation.
Condensation due to temperature changes: During sea shipment from the factory to Singapore, temperature differences cause condensation.
Inadequate protective treatment: Some grating is shipped without passivation treatment to inhibit white rust.
1.3 Is White Rust the “Culprit”? The Answer May Be the Opposite
Many users worry that white rust means a shortened product life. However, white rust formation is accompanied by the development of a dense protective film of zinc (basic zinc carbonate). Once the environment becomes dry, this product actually acts as a barrier against further corrosion and can be beneficial to subsequent corrosion resistance.
More importantly, acceptance of white rust on compliant hot‑dip galvanized steel grating is subject to two hard preconditions:
The coating thickness must be above the minimum specified value (Singapore and international standards generally require ≥85 μm average, ≥70 μm local).
The white rust must not have reduced the coating thickness below the specification.
When white rust is particularly severe, temporary physical removal (e.g., with a stiff brush) may be carried out, but wire brushes are prohibited – they scratch the zinc surface and damage the remaining protection. After removing white rust, the coating thickness at the affected areas must be checked to ensure sufficient zinc remains to provide protection. For severe corrosion, grinding followed by application of zinc‑rich paint can be considered; for those with the resources, re‑galvanizing is also an option.
1.4 Preventive Measures for White Rust
Storage and transport: Store grating in a well‑ventilated indoor area or under cover, away from rain and moisture.
Stacking method: Place spacers (e.g., foam strips) between layers to create ventilation space and prevent moisture being trapped between tightly stacked galvanized surfaces.
Passivation treatment: Request that the supplier perform chromate passivation before shipment to form a dense protective film on the zinc surface.
Oil coating: Apply oil to the galvanized grating to prevent direct contact between air and zinc; this is effective against condensation and water immersion.
Sea freight packaging: For shipments to Singapore, use waterproof paper, anti‑rust paper or vacuum packaging, and place desiccants inside the container.
Core conclusion: White rust is not a reason to reject the product; as long as the zinc coating thickness meets the standard, white rust is completely normal.
Chapter 2: Cut Edges – The Most Easily Overlooked “Legitimate Rust”
2.1 An Unavoidable Reality: Cutting Destroys the Galvanized Coating
During installation, on‑site cutting, drilling or welding of steel grating is almost unavoidable. These operations destroy the original hot‑dip galvanized coating and expose the bare steel substrate. If a protective layer is not applied promptly, the exposed steel will begin to rust within hours in humid air.
However, this is not a quality problem with the steel grating – it is a corrosion‑protection blind spot left by the on‑site construction process. Industry maintenance guidelines clearly state that galvanized coating damage caused by on‑site cutting, drilling or impact must be treated within 24‑48 hours using zinc‑rich repair paint with a dry film zinc content ≥90%, applied to the cut section and the surrounding 5 cm.
2.2 Why Cut Edges Must Be Repaired Within 24 Hours
After cutting, the exposed steel loses both the sacrificial anode protection and the physical barrier of the zinc coating. If no corrosion protection is applied within 24 hours, red rust (meaning the base steel has already begun to corrode) will start to form on the cut edge. Moisture and chloride ions will then penetrate along the interface between the galvanized coating and the steel, causing the coating to blister and flake over large areas. A power plant that incorporated this into its standard operating procedure reported no new rust spots for five years, significantly reducing maintenance frequency.
2.3 Proper Repair Method for Cut Edges
The most common way to repair damaged galvanized coatings is to use zinc‑rich paint. Zinc‑rich paint provides cathodic protection, extending the protective mechanism of hot‑dip galvanizing. Apply 2‑3 coats to achieve the required dry film thickness. Singapore’s humid and hot environment imposes higher demands on repair materials; choose products with proven resistance to humidity and heat.
Before repair: Use a wire brush or file to completely remove rust and welding slag from the cut surface until bare metal is exposed.
Application: After the first coat has dried to a certain extent, apply the second coat, and then the third coat in the same manner.
Material selection: Must use zinc‑rich primer or cold galvanizing coating. High zinc content provides cathodic protection, extending the protection mechanism of hot‑dip galvanizing. Do not use ordinary anti‑rust paint – it does not provide long‑term protection nor cathodic protection, and may actually seal moisture inside and accelerate corrosion.
Important tip: Never apply paint directly over rust and slag without removing them first; this will only trap moisture and corrosion underneath and accelerate failure.
Chapter 3: Red Rust – The Real “Alarm Signal”
3.1 What Does Red Rust Mean?
Red rust is fundamentally different from white rust. White rust is a corrosion product on the zinc surface, consumes very little zinc and has minimal effect on the structure. Red rust (brown‑red iron rust, i.e., hydrated ferric oxide Fe₂O₃·nH₂O) means that the zinc coating has been completely consumed and the underlying steel is being corroded – the structural load capacity is irreversibly decreasing. Once red rust appears, the zinc layer is gone and real damage is happening. This is why we hot‑dip galvanize: to use zinc to protect steel. When true red rust appears, the zinc has vanished and the material is undergoing substantial attack.
3.2 Conditions for Red Rust Formation
Coating thickness insufficient or the presence of uncoated areas, leading to rapid consumption in a corrosive environment.
Long‑term use of steel grating in a severe corrosive environment where the zinc coating is completely sacrificed and the steel is directly exposed.
Cut edges left unrepaired for a long time, allowing red rust to propagate into the panel interior.
Use of carbon steel clips to fix 316L stainless steel grating – galvanic corrosion dissolves the carbon steel clips, producing iron rust while accelerating connection failure.
3.3 Quick Identification Table for White Rust vs Red Rust
| Comparison Dimension | White Rust | Red Rust |
|---|---|---|
| Appearance colour | White or grey powder | Red‑brown, flaky, layered |
| Corroded material | Zinc coating surface (zinc corrosion product) | Steel substrate |
| Effect on structure | Minimal, does not affect structural strength | Severe – section loss, strength reduction |
| Finger wipe test | Can be wiped off, revealing sound zinc underneath | Cannot be easily wiped off; pits beneath |
| Acceptability | Allowed by standards (as long as coating thickness is met) | Absolutely unacceptable |
| Remedial action | Remove with stiff brush, passivate or keep dry | Must grind to remove rust → apply zinc‑rich paint, or re‑galvanize |
| Urgency | Low – can be part of routine maintenance | High – evaluate and treat immediately |
3.4 Failure Progression of Galvanized Coating
Stage 1 (white rust only) : Zinc coating forms white corrosion products in humid or salt‑spray environments. Coating thickness is still adequate and protection is not impaired. Zinc corrosion rate is about 1‑15 μm/year, while unprotected steel corrodes at 25‑100 μm/year or more – several times to an order of magnitude faster.
Stage 2 (local red rust begins to appear) : Zinc coating has been consumed at local pitting sites; steel is exposed and red rust begins to form. At this stage, timely local repair by grinding to remove rust followed by application of zinc‑rich paint is recommended. Epoxy zinc‑rich primer is suitable for steel surfaces in industries such as mining, docks and steel structures, and can also be used as an anti‑corrosion primer for galvanized surfaces.
Stage 3 (widespread red rust) : Zinc coating has largely failed; the steel substrate continues to corrode, cross‑section is reduced and stiffness declines. This stage means the product’s service life is nearing its end; overall replacement or complete refurbishment should be considered.
Chapter 4: Q&A – Common Questions About Rust on Steel Grating
Q1: How can I quickly determine whether white rust on received steel grating needs treatment?
A: Follow these steps:
Step 1 – Visual inspection: Is the rust white or grey powder? If yes, it is initially judged as white rust (not a serious defect). According to GB/T 13912-2002 and ISO 1461, white rust is not a reason for rejection as long as coating thickness meets requirements.
Step 2 – Wipe test: Gently wipe the white area with a finger or dry cloth. If it can be easily wiped off to reveal the silver‑grey zinc layer underneath, the white rust is very light and has no substantial effect on the product. When the corrosive environment is removed, white rust will spontaneously convert into a dense zinc oxide protective film that continues to protect the steel.
Step 3 – Thickness verification: Measure the coating thickness in the white rust area with a magnetic thickness gauge. If the average thickness is ≥85 μm (≥100 μm recommended for Singapore coastal areas) and the local minimum thickness is ≥70 μm, the remaining zinc coating is adequate and the product is fully compliant – no rejection needed.
Step 4 – Check for pitting: If you can feel pits under the white rust with a fingernail, the white rust has consumed the zinc layer and local pitting has occurred. In this case, grinding and application of zinc‑rich paint are required.
Q2: Large areas of white rust appeared on steel grating during sea shipment in a container – is this a supplier quality problem?
A: The unique “temperature difference condensation” during container shipping is a major trigger for white rust. Day‑night temperature differences cause condensation on the inside walls of the container, and water droplets fall directly onto the galvanized surface. In the nearly zero‑ventilation conditions of sea freight, this environment is a classic recipe for white rust formation (high moisture, poor ventilation). White rust is permitted by GB/T 13912-2002 and ISO 1461 and is not a reason for rejection or scrapping. Ways to avoid white rust include: using foam spacers between grating panels to create ventilation space; requesting the supplier to perform passivation treatment before shipment; for high‑value projects, placing desiccants inside the packaging or wrapping with anti‑rust paper.
Q3: When stainless steel and hot‑dip galvanized carbon steel are used together, why do abnormal rust spots appear?
A: This is a hidden and very dangerous corrosion risk. Never allow carbon steel clips to come into direct contact with stainless steel grating, especially in Singapore’s high‑humidity environment. When a carbon steel clip is fixed to a 316L stainless steel grating, the two dissimilar metals in a moist, electrolytic environment form a galvanic cell – carbon steel as the anode, stainless steel as the cathode – dramatically accelerating the corrosion of the carbon steel, with corrosion depth increasing 3‑5 times. In a coastal power plant, carbon steel clips used with 304 stainless steel grating caused galvanic corrosion, resulting in 40% section loss within three years. Therefore, stainless steel grating must be used with stainless steel grating clips of the same grade, and the potential difference must be controlled.
Q4: What specific corrosion resistance requirements does Singapore BCA have for imported building materials?
A: Singapore’s tropical maritime climate, with high salt content in the air, makes building materials highly susceptible to corrosion. The Building and Construction Authority (BCA) requires chemical building materials to pass 500‑hour neutral salt spray testing with a rust area less than 5%. Although this clause primarily addresses chemical building materials, it represents the baseline corrosion resistance benchmark for building materials in Singapore. For structural steel grating used in coastal or marine environments, buyers should proactively request salt spray test reports (at least 500 hours, rust area ≤5%) from suppliers, which is crucial for passing BCA certification spot checks.
Q5: Are the “dark grey markings” on the surface of steel grating a quality problem?
A: Not necessarily. Galvanised workpieces are permitted to have dark or light grey areas of uneven colour on the surface. Such dark markings are usually natural growth stripes formed during zinc crystallisation, or original surface differences from different steel batches (hot‑dip galvanizing does not significantly improve the original surface condition – if the substrate has severe rust pits or scratches, they will still be visible after galvanizing). These dark markings do not affect coating thickness or protective performance and can be used with confidence.
Chapter 5: Quick Reference Table – Types of Rust on Steel Grating and Remedial Measures
| Rust Type | Appearance | Cause | Product Defect? | Recommended Remedial Action |
|---|---|---|---|---|
| White rust (light) | White or grey powder, can be wiped off, sound zinc underneath | Humid storage/transport, poor ventilation, tight stacking | No (permitted by standards) | Keep dry; once the humid environment is removed, white rust self‑passivates. Ensure ventilation and use spacers during storage. |
| White rust (moderate) | Obvious white powder deposit, requires stiff brush to remove | Long‑term high humidity (e.g., inside shipping containers) | Depends on remaining coating thickness | Remove with stiff brush (never wire brush!). After removal, check remaining thickness. If ≥85 μm, acceptable; if local <70 μm, apply zinc‑rich paint. |
| Cut‑edge rust | Red‑brown rust at cut edge, propagating inward | On‑site cutting or drilling without zinc repair within 24 hours | No (construction process issue, not product defect) | Must be treated within 24 hours: remove rust and slag with wire brush or file until bare metal shows; apply 2‑3 coats of zinc‑rich paint (dry film zinc ≥90%) covering cut edge and surrounding 5 cm. |
| Red rust (local, early stage) | Red‑brown flaky rust, small area, palpable pits | Local depletion of zinc coating due to pitting (small area damage or scratch not repaired) | Yes (potential hazard) | Grind to remove rust until bare metal → apply 2‑3 coats of zinc‑rich paint. Inspect every 6 months to prevent spread. |
| Red rust (widespread, advanced) | Large area red‑brown layered rust, zinc coating completely gone | Severely insufficient coating thickness, long‑term exposure to corrosive environment (C4‑C5‑M), or severe cut‑edge rust propagation | Yes (serious defect) | Assess remaining structural strength. Options: replace entirely with 316L stainless steel grating (severe cases); lightly affected: grind and re‑galvanise entire panel (higher cost). |
| Galvanic corrosion rust | Stainless steel grating surface not rusted, but carbon steel clips heavily rusted; or stainless surface shows abnormal rust near contact points | Carbon steel clips in direct contact with stainless steel grating, forming a galvanic cell in moist electrolyte environment | Yes (design/material selection error) | Immediately replace carbon steel clips with same‑grade stainless steel clips. In high‑corrosion environments (C5‑M, e.g., Singapore coastal), must use 316L stainless steel fasteners with potential difference ≤250 mV. |
Chapter 6: Conclusion and bangtu Company’s Technical Commitment
Rust spots are not always a sign that “the steel grating is broken”.
White rust and exposed cut edges are routine phenomena allowed by industry standards. As long as the galvanized coating thickness meets the requirements and on‑site repairs are carried out promptly, they do not affect the product’s corrosion protection performance or structural integrity. What really needs attention is red rust – once it appears, it means the zinc coating has locally failed, the base material is being corroded, and the load capacity and safe life of the structure are being eroded.
During construction and use, follow these three principles:
Identify first: When you see rust, ask “is this white rust or red rust?” A simple wipe test and visual comparison will give you a quick answer.
Repair promptly: After cutting, drilling or welding, the damaged area must be fully repaired within 24 hours using zinc‑rich repair paint with dry film zinc content ≥90%. Do not substitute ordinary anti‑rust paint.
Match the environment: Singapore’s high salt spray, high humidity environment falls under C5‑M marine corrosion class. For coastal areas, 316L stainless steel grating with matching stainless steel fixing clips is strongly recommended.
About bangtu Company
Bangtu Company has specialised in the steel grating field for over two decades. Our products are widely used in industrial platforms, petrochemical facilities, marine engineering, and municipal infrastructure in Singapore and globally. For the Singapore market, we provide:
Full range of hot‑dip galvanized steel grating compliant with SS 363:2014 and GB/T 13912-2020. Each batch is supplied with a third‑party coating thickness test report (≥85 μm, ≥100 μm recommended for Singapore coastal areas).
Matching stainless steel fixing components: We strictly avoid mixing carbon steel clips with stainless steel grating, supplying stainless steel grating clips and grating clamps of the same grade as the base material, with controlled potential difference.
On‑site repair technical support: We provide zinc‑rich repair paint (dry film zinc content ≥94%) and application guidance, helping construction teams complete cut‑edge corrosion repair within 24 hours.
Singapore‑specific packaging solutions: To combat the humid sea freight environment, we offer foam spacers, waterproof paper, desiccants and other white‑rust prevention measures to ensure excellent surface condition upon arrival.
Bilingual (Chinese/English) technical specifications and 500‑hour neutral salt spray test reports to meet Singapore BCA certification and project acceptance requirements.
Choose bangtu – choose full‑process quality assurance from product delivery to on‑site maintenance.
Tel/Whatsapp: +8613363180165
Email: james@bangtuwiremesh.com
Website: www.bangtusteelgrating.com | www.chinawiremesh.ru
