Marine exposure and what to specify
Marine service is not one environment. A casting under a cowling sees humid, salt-laden air but stays mostly dry; a bracket on deck sees direct spray and wet-dry cycling; a part below the waterline sees permanent immersion and galvanic couples with whatever it is bolted to. One drawing cannot serve all of them, and "marine grade" is not an environment.
Write into the RFQ: splash-zone, sheltered or immersed; washdown chemistry; which mating parts are a different metal; and what is being bought — alloy and temper, as-cast condition, machining stock, coating system and masking, leak requirement, acceptance tests and sampling.
For structural marine parts, A356.2 with T6 heat treatment is the common starting point, and the full callout is set out in the A356-T6 casting specification guide. CEX casts A356 and A356.2 in squeeze casting and performs T6 heat treatment in-house. Reference values for squeeze-cast A356-T6 are 260–320 MPa tensile and 220–260 MPa yield (company-reported, not acceptance criteria); your drawing states the properties that govern acceptance.
Neutral salt spray to ISO 9227: the buyer sets duration and acceptance
ISO 9227 is a method. It describes the apparatus, the solution and how exposure is run; it does not say how long to expose a part or what counts as a pass. Those decisions belong to the buyer's specification, written before samples are made.
Your specification names the duration, whether the specimen is a coated panel or a production part, whether a scribe is cut through the coating, and which rating scheme applies — ASTM D1654 and ISO 4628 are the usual references by name. Where the end customer's standard governs, attach it. A supplier cannot infer a duration from the phrase "salt spray tested".
CEX does not claim in-house salt spray testing. Exposure can be arranged with your nominated or an accredited third-party laboratory against the duration and criteria in your specification, with the report filed in the first-article package. The trap: a system can pass on a flat coupon and still fail at an edge, a thread or a masked face.
Coating systems, pretreatment and anodize
Coating families for marine aluminium castings differ in cost, coverage and repair strategy.
- Conversion coat plus powder or liquid paint. The pretreatment is the load-bearing layer — a properly applied conversion coat gives the organic coating something to bond to. Skipped or under-rinsed pretreatment is the usual root cause of blistering and creepback from a scribe.
- Electrocoat. Good coverage of complex geometry and internal surfaces, as a primer under powder or a standalone finish. It needs a conductive substrate and a planned masking scheme, so casting skin quality and racking design matter.
- Anodize, where the alloy allows. Anodizing high-silicon casting alloys such as A356 gives a darker, less uniform finish than wrought alloys, so it is usually specified for wear or dielectric function rather than appearance. Agree a sample and dye requirement first if appearance matters.
ISO 12944 is a useful reference by name when a buyer wants to state a corrosivity category and a durability expectation for the system; the actual system, dry film thickness and test schedule belong on the drawing. Mask threads, mating faces, grounding points and press-fit bores — re-coating a masked face after assembly is expensive and rarely inspected.
If a substitution is proposed to suit an anodizing line or a cheaper melt, treat it as an engineering change and record the consequences in the alloy equivalent substitution approval checklist before any tooling moves.
Leak testing: helium, pressure decay, immersion
Pressure tightness on marine castings — pump housings, water-jacketed engine parts, sealed enclosures — is often written in one line on a drawing and causes the most argument at acceptance. Fix the method, the medium, the pressure, the allowable leak rate, the stabilization period and the pass/fail instrument before quoting.
- Helium leak testing. Tracer gas gives the finest sensitivity of the methods here; meaningful only with a fixture, a sealed reference and a calibrated leak standard. CEX has helium leak testing equipment for parts that require pressure tightness.
- Pressure decay. Cheaper per part, but sensitive to fixture volume, temperature drift and part elasticity. Specify the stabilization period and the reference part.
- Immersion / bubble test. Visual and operator-dependent; suitable for coarse acceptance or for locating a leak another method has flagged.
State whether every part is tested or the test is sampled, whether parts are tested dry or wet, and what happens to a failed part — rework, re-test, scrap or return for root cause. Internal soundness checked by X-ray is a separate callout from tightness; both are agreed against acceptance criteria.
Marine part selection: route considerations
Route choice is driven by wall thickness, pressure tightness, machined features, tooling budget and volume. The table is a starting point for the conversation, not a rule.
| Part | Typical duty | Squeeze casting | HPDC | Sand casting |
|---|---|---|---|---|
| Outboard / engine housing | Structure, water jacket, tightness | Where tightness and ductility both matter; T6 in-house | Volume and thin walls; tightness proven by test | Low volume, thicker walls, more machining stock |
| Steering / helm bracket | Safety-related structural | Preferred where elongation and fatigue matter | Simpler geometry; property scatter to manage | Workable, heavier, more machining |
| Pump housing | Pressure boundary | Good candidate; pair with helium or pressure-decay test | Volume parts; tightness risk tested out | Common at low volume, thick sections |
| Deck hardware bracket | Clamp load, coating durability | Where load and thin sections combine | Cost-effective at volume | Prototype and legacy parts |
Squeeze casting at CEX runs on machines from 300 to 1,100 tonnes, covers parts from 50 g to 20 kg and envelopes up to 1.2 × 0.8 m, with ±0.2 mm on the as-cast blank (company-reported); aluminium HPDC runs from 160 to 1,100 tonnes. Tooling, T6 heat treatment and 25 CNC machining machines are in-house. Sand casting appears above for comparison; the in-house routes at CEX are squeeze casting and high-pressure die casting.
Galvanic and dissimilar-metal notes
Many failures blamed on the alloy are really assembly failures. Aluminium is anodic to stainless steel, bronze, copper-nickel and graphite, and a small anode against a large cathode in a wet, conductive environment loses material quickly. The rules are old and still routinely ignored:
- Keep aluminium away from copper alloys and from stagnant water traps; drain and vent closed volumes.
- Isolate faying surfaces with sealant, a gasket or a coated washer, and specify fastener finish, not only fastener grade.
- Use insulating or plated washers and sleeves where a stainless fastener passes through an aluminium lug, and avoid large stainless-to-small-aluminium area ratios.
- Do not rely on coating alone at a joint — damage under a bolt head is normal, so the joint needs its own defence.
- Specify thread inserts, their installation, and re-coating after installation where the joint will be serviced afloat.
Alloy selection and joint design interact. If a substitution is proposed for a corrosion reason — different casting alloy, different temper, sand to squeeze casting — the approval path is the same: documented comparison, buyer sign-off, and a first article that re-tests the affected characteristics.
A confirmed marine case
CEX publishes a boat part squeeze cast in A356.2-T6 that replaced a conventional aluminium high-pressure die casting, with at least 50% higher strength (company-reported). It shows where the route earns its cost: the geometry stayed a casting, and the gain came from how the metal solidified and was heat treated rather than from a different manufacturing family.
Read published case results as directional. A strength gain is only meaningful with the test method, sampling, temper verification and the drawing's acceptance criteria alongside it. Specify on your drawing the minimum properties you will accept, not the properties a supplier reports.
What to put in the RFQ
CEX operates as a combined factory and trading company: the factory is Qingdao Dechengxin Metal Products Co., Ltd., which holds the quality certificates in its own name — ISO 9001:2015, IATF 16949:2016 with product design excluded from scope, ISO 14001:2015 and ISO 45001:2018 — and the trading company is Qingdao Chengenxin, under the same owner. For a marine buyer that means one commercial contact and one factory accountable for casting, heat treatment, machining and inspection planning.
Send the environment, the drawing, the coating specification, the test schedule and the acceptance criteria as one package. The casting RFQ checklist lists what a foundry needs before it can quote a marine part without guessing.